Darwin's Theory of Natural Selection and Sexual Selection PDF

Darwin's Theory of Natural Selection and Sexual Selection Charles Darwin proposed natural selection to explain the evolution of adaptive traits in species. In On the Origin of Species and The Descent of Man and Selection in Relation to Sex, Darwin introduced the concept of sex...

Darwin's Theory of Natural Selection and Sexual Selection Charles Darwin proposed natural selection to explain the evolution of adaptive traits in species. In On the Origin of Species and The Descent of Man and Selection in Relation to Sex, Darwin introduced the concept of sexual selection, a subset of natural selection focused on reproductive advantages. Primary and Secondary Sexual Characteristics Darwin studied primary sexual characteristics (genitalia differences) and secondary sexual characteristics (traits not involved directly in reproduction) in various species. Secondary sexual characteristics include traits like colorful plumage in male birds or large antlers in male mammals, often used in mating displays. The Puzzling Nature of Secondary Sexual Traits Darwin found secondary sexual traits puzzling as they seemed costly and risky, potentially reducing survival chances due to their energy demands and visibility to predators. He questioned why traits like the male peacock’s tail evolved if they seemingly reduced survival. Sexual Selection as a Reproductive Advantage Darwin theorized that exaggerated male traits may increase reproductive success rather than survival. Sexual selection involves two main processes: ○ Mate competition (intrasexual selection): Males compete for mating opportunities, sometimes through physical combat. ○ Mate choice (intersexual selection): Females choose mates based on attractive traits. These processes drive sexual selection, favoring traits that increase reproductive success. The Evolution of Two Sexes: Anisogamy The existence of two sexes (male and female) is linked to anisogamy, where males produce small motile gametes (sperm) and females produce large nutrient-rich gametes (eggs). Anisogamy evolved from isogamy (similar-sized gametes) due to disruptive selection, favoring: ○ Proto-males: Individuals producing many small gametes.  ○ Proto-females: Individuals producing fewer large gametes. The Role of Anisogamy in Sexual Selection Different gamete sizes established distinct reproductive strategies, setting the stage for sexual selection. Sexual selection, therefore, is grounded in these differences, as it promotes traits that enhance reproductive success across both sexes. Bateman's Hypothesis on Sexual Selection Angus Bateman studied sexual selection using fruit flies, observing differences in male and female reproductive success. Key findings: ○ Males showed higher variation in reproductive success than females, due to male-male competition. ○ Male reproductive success was tied to the number of mates, whereas female success was limited by egg production. ○ This led to intense competition among males for female gametes, intensifying sexual selection on males. Parental Investment Theory by Robert Trivers Trivers expanded Bateman’s hypothesis by introducing parental investment as a crucial factor in sexual selection. Parental investment is any effort by a parent that increases an offspring's survival but reduces the parent's ability to invest in other offspring. Predictions from Trivers' theory: ○ The sex with higher parental investment (often females) will be choosier in mate selection. ○ The other sex (often males), with less parental investment, will face more intense sexual selection. Operational Sex Ratio (OSR) and Sexual Selection Intensity The operational sex ratio (OSR) reflects the difference in sexually receptive males and females at any time, often skewed towards the less invested sex. A skewed OSR results in greater sexual selection pressure on the sex with more individuals ready to mate (typically males). Exaggerated Male Traits and Sexual Selection Due to sexual selection, males often develop exaggerated traits:  ○ Weapons: Morphological traits for male-male competition. ○ Ornaments: Traits used to attract females, such as plumage or courtship displays. Exaggerated traits are common in mammals, birds, and arthropods, showcasing the influence of sexual selection in diverse species Background Information In many beetle species, males possess horn-like projections absent in females. Male dung beetles (Euoniticellus intermedius) use these horns to fight for access to dung piles, where females create tunnels to mate and raise offspring. Research Question Do larger horns in male dung beetles provide an advantage in male-male competition for mating opportunities? Hypothesis Males with larger horns have a higher chance of winning fights for access to females, giving them an advantage in mating success. Methods Experiment staged in artificial nesting arenas with two panes of glass, filled with soil and dung, allowing for observation of male contests. A single female was placed in the arena to dig a tunnel, followed by introducing two males of similar body size but different horn lengths. Body length was measured with calipers; horn length was measured from photographs. Success was defined by which male remained in the tunnel after 24 hours, with winners usually mating with the female. Results In contests between small males, body size or longer horn length predicted success. In contests between large males, horn length alone was the primary predictor of success. The greater the difference in horn length between two males, the more likely the male with the longer horn would win. Conclusion  Long horns in male dung beetles provide a competitive advantage in male-male contests for mates, supporting the idea that sexual selection, driven by competition, maintains these exaggerated traits. Background Information Peacocks (Pavo cristatus) have long, ornate tails with colorful eyespots (ocelli), thought to play a role in female mate choice. Males gather on leks, where they display to females and compete for display sites. Research Question Do tail ornamentation (number of ocelli) and display behaviors in peacocks influence male mating success? Hypothesis Males with more ornamented tails and higher display rates are more successful in securing mates due to female preference and male competition. Methods Researchers observed peacocks at Parc Zoologique de Clères, France. Males were color-banded, and their tail length, body size, and number of ocelli were recorded. Male interactions, display behaviors (tail fanning, vocalizations), and copulations were tracked throughout the breeding season. Results Only 45 of 61 males successfully defended a display site, with larger males and those with longer tails being more successful. Females were selective, with only 12 males achieving copulations; a single male accounted for over one-third of all copulations. Males with the most ocelli and highest display rates obtained the most copulations, regardless of their body size. Conclusion Both male competition for display sites and female mate choice favored the evolution of long, ornamented tails in peacocks.  High display rates and a large number of ocelli were key traits preferred by females, supporting the role of both mate competition and mate choice in shaping peacock tail evolution. Background Information Male mate choice is less common but can occur in species where males invest heavily in parental care. In sex-role-reversed species like the pipefish (Syngnathus typhle), females compete for males, who carry and care for the offspring. Research Question Does female ornamentation influence male mate choice in deep-snouted pipefish? Hypothesis Male pipefish prefer females with more prominent secondary sexual traits, as these traits may signal health, dominance, and higher reproductive potential. Methods Anders Berglund and Gunilla Rosenqvist studied male mate choice in pipefish using a simultaneous-choice experiment in aquaria. Males were given a choice between two females over consecutive days. Researchers observed male time spent in courtship and frequency of copulation with each female, focusing on females’ temporary striped body pattern (ornamentation) displayed during courtship. Results Males spent more time and copulated more often with females displaying more pronounced ornamentation. The female ornament likely indicates dominance, health, and body size, with larger females being more desirable due to higher egg production potential. Conclusion Male mate choice in pipefish is influenced by female ornamentation, supporting the role of both mate competition and choice in sexual selection. Female secondary sexual traits evolved in response to male choosiness, illustrating that mate selection can drive elaborate trait evolution across sexes Background Information The sensory bias hypothesis suggests that female mate preferences are byproducts of preexisting biases in their sensory systems, which evolved outside of mating contexts. Males may evolve traits that align with these preexisting biases. Research Question Do guppies exhibit a sensory bias for the color orange, which influences female mate preference? Hypothesis Guppies have an innate preference for orange objects, which could explain why females prefer orange-colored males. Methods Helen Rodd and colleagues conducted experiments in natural guppy populations in Trinidad and in a laboratory setting. Small colored discs (red, orange, green, purple, blue, white, yellow, and black) were placed on a leaf in water; researchers recorded guppy approaches and pecks at each color. Similar tests were conducted in aquariums under controlled conditions. Results Both male and female guppies showed a strong attraction to orange and red discs. The strength of preference for orange correlated with female mating preference for orange-colored males across different guppy populations. Conclusion Male and female guppies display an innate preference for orange, supporting the sensory bias hypothesis. Female preference for orange-colored males likely evolved due to an existing attraction to orange, potentially associated with carotenoid-rich foods Parental Investment Theory: Predicts that the sex investing more time and energy in offspring gains an advantage by being selective in mate choice, often resulting in females being the choosier sex. Benefits of Selectivity:  Direct Material Benefits: Females may select mates who provide tangible resources, like food gifts, access to resource-rich territories, or parental care. Indirect Genetic Benefits: By choosing certain males, females can improve the genetic quality of their offspring, enhancing their fitness. Focus of Section: Examines the direct material benefits that females gain from selective mate choice. Nuptial Gifts in Arthropods In many arthropod species, males provide nuptial gifts (such as prey, carrion, plant material, or glandular secretions) to females before or during mating. These gifts provide nutritional benefits, potentially enhancing female reproductive success. Study on Green-Veined White Butterflies Researchers Nina Wedell and Bengt Karlsson examined whether nuptial gifts from male green-veined white butterflies (Pieris napi) positively affect female reproductive success. Mechanism of the Nuptial Gift Male butterflies provide a protein-rich spermatophore that disintegrates within the female’s abdomen, releasing nutrients she can use for reproduction. The spermatophore serves as an essential protein source since adult butterflies primarily feed on nectar. Methodology Males were fed radioactively labeled leaves, incorporating markers into their spermatophores. After mating, researchers measured radioactivity in the female’s eggs, abdomen, and thorax to assess nutrient transfer from the male. Results Females receiving larger spermatophores (more nutrients) produced more eggs, showing a direct link between the size of the nuptial gift and female fecundity. Conclusion Female butterflies gain a direct reproductive benefit from mating with males that offer larger nuptial gifts, which increase their egg production Background Information Female mate choice can provide direct benefits, such as access to high-quality territories defended by males. In side-blotched lizards (Uta stansburiana), territory quality (measured by rockiness) affects female mate choice and offspring success. Research Question Does territory quality influence female choice and reproductive success in side-blotched lizards? Hypothesis Females prefer males with high-quality territories, as these territories provide better resources for offspring survival. Methods Researchers Ryan Calsbeek and Barry Sinervo studied lizard territory preference by manipulating territory quality. After dominant males claimed rocky territories, the researchers moved rocks from these territories to those of smaller, neighboring males. This manipulation allowed them to separate male quality from territory quality. Following the breeding season, females were captured to lay eggs in a lab setting, and egg-laying date and egg mass were recorded. Results Females preferred improved territories with more rocks, even if occupied by smaller males. Females on higher-quality territories laid eggs sooner and produced larger egg masses. Conclusion Female side-blotched lizards choose mates based on territory quality, gaining direct fitness benefits like earlier egg-laying and larger egg mass. This study highlights that female choice can be driven by access to resources rather than male quality alone. indirect Benefits of Mate Choice Females can obtain indirect genetic benefits by mating with high-quality males, passing advantageous alleles to offspring. Females assess genetic quality indirectly, often through male secondary sexual traits. Fisherian Runaway Hypothesis Proposed by Ronald Fisher (1930): Suggests female preference for male traits and the traits themselves co-evolve, creating increasingly exaggerated male traits. Mechanism: Traits evolve based on (1) a fitness advantage independent of mate choice, and (2) female preference. Runaway Process: Traits continue to evolve until the disadvantage of increased predation outweighs mating benefits. Empirical Evidence: Studies show significant genetic covariance between traits and female preferences in various species, supporting the possibility of runaway selection. Zahavi's Handicap Principle Proposed by Amotz Zahavi (1975): Argues that exaggerated male traits serve as a "handicap," only sustainable by high-quality males. Mechanism: Traits are costly to produce or maintain, indicating genetic quality, as only high-quality males can afford such costs. Good Genes Hypothesis: Traits signal enhanced genetic traits like immune strength, vigor, or fighting ability. Similarities Between Fisherian and Handicap Models Both models indicate that female preference for male traits reflects male vigor and survivorship. Genetic correlations support female preference for traits associated with high male quality (good genes) or male traits themselves (runaway selection). Current Focus in Sexual Selection Research Key research areas include identifying female-preferred traits, understanding how these traits indicate genetic quality, and measuring the fitness benefits of mate choice. Good Genes Hypothesis This hypothesis suggests that females choose mates with costly traits (e.g., vocalizations) to secure genetic benefits for their offspring, enhancing offspring fitness. Study on European Tree Frogs Researchers Julie Jaquiéry and colleagues tested the hypothesis that female European tree frogs (Hyla arborea) select mates based on vocalizations for genetic advantages. Methodology Researchers observed calling males over a 22-night breeding season across four ponds in Switzerland. They captured 15 males, took genetic samples, and documented mating success by collecting and rearing egg masses in the lab. Tadpole growth rate, survival, and genetic analysis were used to assess offspring fitness and male mating success. Results Only 10 of the 15 males sired offspring, indicating female selectivity in mate choice. A positive correlation was found between male attractiveness (mating success) and offspring growth rate, suggesting that females preferred high-quality males whose offspring demonstrated higher fitness. Conclusion Female European tree frogs benefit genetically by choosing males with desirable vocal traits, which results in offspring with greater fitness. This study supports the idea that female mate choice for costly, sexually selected traits can enhance offspring viability, aligning with the good genes hypothesis The Hamilton-Zuk Hypothesis Proposed by William Hamilton and Marlene Zuk, this hypothesis suggests that parasites and pathogens influence sexual selection when secondary sexual traits are both costly and dependent on the male’s health. Males with higher immune response can display more exaggerated traits, signaling better disease resistance. Females benefit by choosing these males, potentially passing immunity-related alleles to their offspring. Study on Field Crickets Researchers Markus Rantala and Raine Kortet tested the Hamilton-Zuk hypothesis using field crickets (Gryllus bimaculatus).  Male crickets produce courtship songs with high-frequency (HF) and low-frequency (LF) ticks, a trait females use for mate selection. Methodology Lab-reared, virgin male crickets had their courtship songs recorded and analyzed to measure HF and LF tick production. Immunocompetence was assessed through encapsulation rate, where a nylon filament was implanted to measure immune response. Female mate preference was determined by placing females in a chamber between two speakers playing the songs of males with known immunocompetence. Results Females preferred males with higher HF tick rates and durations, which correlated with higher encapsulation rates (better immune function). Encapsulation ability is heritable, supporting the idea that females gain indirect genetic benefits (higher offspring immunocompetence) by choosing males with pronounced HF ticks. Conclusion The study supports the Hamilton-Zuk hypothesis, showing that females prefer males with strong secondary traits linked to high immune function, allowing them to pass on health benefits to their offspring. Background Information In many species, females may gain indirect fitness benefits by selecting mates with certain qualities. Chemical signals (pheromones) may play a role in mate selection, especially in species where males do not provide direct resources. Study on Spitting Spiders Researchers Teck Hui Koh and colleagues investigated whether pheromones signal male quality in spitting spiders (Scytodes sp.), where males do not provide resources, allowing study of indirect fitness benefits from mate choice. Methodology 1. Mate Choice Experiment:  ○ Phase 1 (Live Male Choice): Females chose between two males behind mesh screens, allowing only chemical but not visual or tactile communication. ○ Phase 2 (Pheromone-Only Choice): Male pheromones were collected on filter paper in Petri dishes, and females chose between filter papers from the previously presented males. ○ Of the 44 females in Phase 1, 42 chose the same male’s pheromones in Phase 2, indicating that females can use pheromones to select a mate. 2. Fitness Impact of Mate Choice: ○ 42 females were split into two groups: one group mated with their preferred male, and the other with a non-preferred male. ○ Female spiders that mated with preferred males produced larger egg sacs, with more and heavier eggs, and had a higher hatch rate. Results Female mate choice based on pheromones resulted in higher reproductive success (larger, healthier egg sacs and higher hatch rates) when females mated with preferred males. Conclusion Female spitting spiders gain indirect fitness benefits by mating with preferred males, likely based on pheromones indicating male quality (e.g., size, health, or fertility). These findings highlight the role of pheromones in mate choice and suggest that indirect fitness benefits from mate selection may improve offspring quality. Mate Guarding Behavior Males in many species engage in mate guarding to prevent their mates from mating with rivals, thus increasing paternity assurance. This behavior is costly, as it reduces a male’s time for seeking other mates or acquiring resources. Study on Black-Throated Blue Warblers Researchers Helen Chuang-Dobbs and colleagues studied the effectiveness of mate guarding in black-throated blue warblers (Dendroica caerulescens) in New Hampshire’s Hubbard Brook Experimental Forest. Observational Study Males and their mates were observed during the female's fertile period.  The distance between the male and female was recorded, along with male-following behavior. Results indicated that males who stayed close to and frequently followed their mates had fewer extra-pair young, suggesting that intense mate guarding reduces the risk of rival mating. Experimental Manipulation To test mate guarding effectiveness, some males were lured away from their mates for one hour during the female’s fertile period using playback of conspecific songs. Eight treatment males were either removed or preoccupied by playback sounds, while eight control males were not manipulated. All nests of removed males contained extra-pair young, while only 50% of control nests did, indicating that reduced mate guarding increased the likelihood of rival mating. Results Males who engaged more in mate guarding had greater paternity assurance, with fewer extra-pair young found in their nests. The experimental data showed that even a short absence during the female’s fertile period significantly increased the chance of extra-pair young. Conclusion Mate guarding is an effective but imperfect strategy to increase paternity, with a trade-off between the benefits of paternity assurance and the costs of lost mating or foraging opportunities. Background Information on Sperm Competition When males cannot continuously guard mates, sperm competition occurs, where sperm from different males competes to fertilize a female’s eggs. Frequent copulation can be a strategy to increase a male's reproductive success by swamping a rival male’s sperm. Study on Tree Swallows Susan Crowe and colleagues tested if frequent copulation increases male fitness in tree swallows (Tachycineta bicolor), a species where extra-pair fertilization is common. Methodology The study was conducted at Queen’s University Biological Station in Ontario, Canada.  Researchers observed 43 pairs for over two weeks, beginning nine days before egg laying, recording copulations which mainly occur at the nest site. Blood samples from adults and offspring were taken for genetic paternity analysis. Results Copulation rate varied among pairs, peaking three days before egg laying, averaging five copulations per hour. 52% of young were sired through extra-pair fertilizations. Males that copulated more frequently with their mate had a higher proportion of young they sired, supporting the hypothesis that frequent mating enhances paternity through sperm competition. Conclusion Frequent copulation in tree swallows reduces paternity loss from sperm competition and may also act as a form of mate guarding by deterring other males. Both increased copulation frequency and mate guarding contribute to higher fertilization success. Additional Insight: Cryptic Female Choice Females may influence fertilization success after mating, favoring the sperm of higher-quality males, a phenomenon known as cryptic female choice. This choice allows females to enhance offspring fitness by favoring unrelated (higher-quality) males over closely related males, reducing inbreeding depression. Background Information on Cryptic Female Choice Cryptic female choice allows females to bias paternity toward preferred males after mating, potentially avoiding inbreeding depression. Female orb spiders (Argiope lobata) have two independent sperm storage organs (spermathecae), allowing storage of sperm from different males. Study on Inbreeding Avoidance in Orb Spiders Researchers Klaas Welke and Jutta Schneider investigated whether female orb spiders use cryptic female choice to avoid inbreeding by selectively storing sperm from non-related males. Methodology Double mating trials were conducted with females mated to: ○ Two sibling males (SS) ○ Two nonsibling males (NN) ○ One sibling and one nonsibling male (SN or NS, with order varied) Males had complementary pedipalps (one right, one left) to control sperm storage in each spermatheca. Researchers measured sperm storage and paternity by counting sperm in spermathecae and using sterilized males to identify fertilization patterns. Results Females stored more sperm from the second male when he was a nonsibling, indicating a preference for non-related males. Paternity analysis showed that nonsibling second males fertilized a higher proportion of eggs (75%) compared to sibling males (48%). Conclusion Female orb spiders demonstrate cryptic female choice by favoring sperm from non-related males, reducing inbreeding within a brood. This study highlights post-mating sexual selection, where females influence sperm competition to enhance offspring fitness. Alternative Reproductive Tactics Not all males can effectively compete for mates through traditional strategies, like winning contests, defending high-quality territories, or displaying exaggerated traits. Alternative reproductive tactics evolve to enhance reproduction in less competitive males, leading to multiple behavioral phenotypes within a population. Common Tactics 1. Bourgeois Tactic: Competitive males (usually larger, older, and healthier) defend nests or territories to monopolize resources for attracting females. 2. Parasitic Tactic: Less competitive males employ alternative behaviors to access mates associated with bourgeois males: ○ Satellite Males: Stay near bourgeois males to intercept females attracted to them. ○ Sneaker Males: Stealthily enter a bourgeois male’s territory to fertilize eggs. Explanations for Coexistence of Tactics 1. Conditional Strategy: ○ Individuals adopt a tactic based on their condition. Healthier, more competitive males adopt the bourgeois tactic, while those in poorer condition use the parasitic tactic as a "best of a bad situation" strategy. ○ Males may switch tactics if their condition improves. 2. Evolutionarily Stable Strategy (ESS): ○ Frequency-dependent selection maintains both tactics, where the fitness of each increases as it becomes less common. ○ Parasitic males gain fitness benefits when bourgeois males are more abundant. If parasitic males become too common, their fitness declines due to fewer bourgeois males to parasitize. ○ Predicts equal average fitness for both tactics when coexisting as an ESS. Background Information Male green tree frogs (Hyla cinerea) produce vocalizations to attract females, who prefer loud, high-rate, low-frequency calls. Only large males can produce the lowest-frequency calls, making them more attractive to females. Observed Tactics in Male Green Tree Frogs Bourgeois Calling Tactic: Males call to attract females directly. Satellite Tactic: Silent males (satellites) stay near calling males to intercept females attracted to the caller. Some males alternate between calling and satellite behavior, suggesting a conditional strategy. Hypothesis and Predictions Humfeld’s Hypothesis: Satellite behavior in male tree frogs represents a conditional reproductive strategy. Predictions: 1. Males differ in attractiveness to females; less attractive males are more likely to be satellites. 2. Satellite males and females both prefer low-frequency calls produced by larger, more attractive males. Methodology  Male calls were recorded in the wild, and each male was assigned a role (calling or satellite). Satellite calls were recorded by removing nearby calling males to induce calling in the satellite males. Call preferences of females and satellite males were tested in a lab using synthesized call pairs with different frequencies. Results Male Attractiveness: Calling males were larger, in better condition, and produced lower-frequency calls than satellite males, supporting the first prediction. Call Preference: Both females and satellite males strongly preferred low-frequency calls, supporting the second prediction. Conclusion Male green tree frogs use a conditional strategy based on their size and condition: ○ Larger males call to attract females. ○ Smaller males adopt the satellite tactic, staying close to attractive callers to increase encounters with females. While fertilization success was not measured, data suggest that satellites would be less successful if they attempted to call independently. Background Information Molecular techniques have allowed scientists to study the fitness of different mating tactics, especially in species with alternative reproductive strategies. Study on Pumpkinseed Sunfish Researchers Oscar Rios-Cardenas and Michael Webster examined mating behavior and reproductive success in male pumpkinseed sunfish (Lepomis gibbosus) at Huyck Preserve, New York. Observed Mating Tactics Parentals (Bourgeois Tactic): Males defend nests in breeding territories, where they attract females to spawn and then provide parental care for the eggs. Sneakers (Parasitic Tactic): Smaller, younger males intrude on nests during spawning, attempting to fertilize eggs without defending a territory. Methodology  Over three years, researchers mapped nest locations, observed spawning events, captured involved fish, and took genetic samples from adults and eggs for paternity analysis. They also recorded the weight and age of males, based on scale morphology. Results Parentals were typically older (4+ years) and larger, while sneakers were younger (about 2 years old) and smaller. Parentals sired 85% of the offspring in their nests, while sneakers sired 15%. Although parentals appeared to have higher fitness, sneakers constituted only 15% of the population, balancing reproductive success between the two strategies. Conclusion In this population, the bourgeois and sneaker tactics coexist in an Evolutionarily Stable Strategy (ESS), with equal fitness for both tactics relative to their population proportions. Approximately 15% of males mature early and adopt the sneaker tactic, while the rest mature later to compete for territories as parentals. Hormonal Influence on Tactic Adoption Related research on bluegill sunfish (Lepomis macrochirus) found that parasitic males had higher testosterone and cortisol levels but lower 11-ketotestosterone, suggesting hormones may influence tactic choice. Overall Implication The study on sunfish demonstrates that complex reproductive strategies, such as bourgeois and parasitic tactics, can coexist within a population as an ESS, with physiology potentially influencing the adoption of these strategies. Background Information on Mate Choice Copying Mate choice copying is when an individual’s mate choice is influenced by observing the choices of others. Guppies (Poecilia reticulata), with brightly colored males, provided the first experimental evidence of mate choice copying. Study on Mate Copying in Guppies Researcher Lee Dugatkin tested whether female guppies copy the mate choice of other females. Methodology A focal female was placed in a container in the center of an aquarium with two males (matched for size and coloration) in chambers on either side. The focal female first chose a male by spending time near him. A model female was then introduced near the nonchosen male to simulate her “choosing” him, displaying courtship behaviors visible to the focal female. Afterward, the focal female was released to freely choose between the two males. Results Out of 20 focal females, 17 switched their preference to the male that had been near the model female, indicating they copied the model's mate choice. Conclusion Female guppies exhibit mate choice copying, where their mate preference is influenced by observing the choices of other females. Background Information on Mate Copying Mate choice copying, where females mimic the mate choices of others, is widely documented, especially in vertebrates. Study on Mate Copying in Fruit Flies Researchers Frédéric Mery and colleagues tested mate copying in fruit flies (Drosophila melanogaster). Methodology Males were classified as high- or low-quality based on their diet: high-quality males were raised on a nutrient-rich medium, and low-quality males on a nutrient-poor medium. Experiment Phases: 1. Pretest: Females chose between high- and low-quality males, generally preferring high-quality males. 2. Model Exposure: Half the females observed a model female with the high-quality male, while the other half saw a model with the low-quality male. 3. Posttest: Females chose between high- and low-quality males again. Results Females who observed a model with the high-quality male continued to prefer him.  Those who saw a model with the low-quality male increased their time with that male, showing mate choice copying. Explanation for Mate Copying Mate copying may help females make better choices when it's hard to assess male quality directly. By copying, a female may leverage another female’s potentially better discrimination ability, increasing her chances of choosing a high-quality mate without reducing her own success if the model female’s choice is poor. Background Information on Male Mate Choice Nonindependent mate choice occurs when an individual's mate preference is influenced by observing the mate choices of others. In species with sperm competition, like the eastern mosquitofish (Gambusia holbrooki), males may try to avoid females recently associated with rival males. Study on Mate Choice in Male Mosquitofish Researchers Bob Wong and Miranda McCarthy investigated whether the risk of sperm competition affects mate choice in male mosquitofish. Methodology Three-Stage Experiment: 1. Stage 1: Focal males chose between two randomly selected females, typically preferring the larger female. 2. Stage 2: Focal males were constrained while a rival male was placed next to either the preferred or unpreferred female. 3. Stage 3: The rivals were removed, and focal males could interact with both females again. Results Males maintained their preference for the larger female if they observed a rival near the unpreferred female. If a rival male was seen near the preferred female, focal males spent less time with her in Stage 3, indicating an avoidance of potential sperm competition. Conclusion  Male mosquitofish adjust their mate choice based on observed male-female associations to minimize sperm competition risk. This study highlights that social observations influence mate preferences across various species, including potential implications for humans. Definition of Mating Systems Mating systems describe the social associations and number of sexual partners an individual has during a breeding season. Systems vary widely in social associations, pair bonds, and the number of mates within and across species. Types of Mating Systems 1. Monogamy: One female and one male form a pair bond for the breeding season. 2. Polygyny: One male mates with multiple females. 3. Polyandry: One female mates with multiple males. 4. Polygynandry (Plural Breeding): Multiple males and females form social groups, with mating occurring within these groups. 5. Promiscuity: Multiple males and females mate without specific social associations. Variation in Mating Systems Variation exists within species, populations, and even individuals, with some species displaying multiple mating systems in one population. For example, dunnocks and Milne-Edwards’ sifakas have been observed to exhibit four mating systems within the same population. Evolution of Mating Systems Mating systems evolve based on the reproductive decisions of males and females, influenced by fitness costs and benefits. Environmental conditions and social structure play significant roles in shaping these reproductive strategies. Definition of Mating Systems Mating systems describe the social associations and number of sexual partners an individual has during a breeding season. Systems vary widely in social associations, pair bonds, and the number of mates within and across species. Types of Mating Systems 1. Monogamy: One female and one male form a pair bond for the breeding season. 2. Polygyny: One male mates with multiple females. 3. Polyandry: One female mates with multiple males. 4. Polygynandry (Plural Breeding): Multiple males and females form social groups, with mating occurring within these groups. 5. Promiscuity: Multiple males and females mate without specific social associations. Variation in Mating Systems Variation exists within species, populations, and even individuals, with some species displaying multiple mating systems in one population. For example, dunnocks and Milne-Edwards’ sifakas have been observed to exhibit four mating systems within the same population. Evolution of Mating Systems Mating systems evolve based on the reproductive decisions of males and females, influenced by fitness costs and benefits. Environmental conditions and social structure play significant roles in shaping these reproductive strategies. Key Factors in the Evolution of Mating Systems Emlen and Oring’s Model: Mating systems are influenced by: 1. Sexual Conflict: Different selection pressures on males and females to maximize fitness. 2. Ecological Factors: Resource availability and distribution, which affect the fitness benefits and costs for each sex. Female Fitness and Mating System Selection Female fitness is often limited by resource availability rather than the number of mates. Monogamy: Favored if a male provides significant resources or parental care. Polyandry: Favored if multiple males provide care, benefiting the female’s offspring. Male Fitness and Mating System Selection Male fitness increases with the number of mates, leading to: ○ Polygyny: Favored if males can enhance fitness by mating with multiple females.  ○ Biparental Care and Monogamy: Favored when offspring survival depends on care from both parents, often in resource-poor environments. Types of Polygyny 1. Female Defense Polygyny: Males defend groups of females, often when females aggregate to reduce predation or harassment. 2. Resource Defense Polygyny: Males defend territories rich in resources that attract multiple females. 3. Male Dominance Polygyny (Leks): Males display in fixed locations (leks) to attract females, who benefit from reduced search time for mates. Polyandry and its Evolution Polyandry can evolve when it’s advantageous for females to reduce parental care, leading to male-biased care. High Predation on Offspring: Both sexes benefit if the female can reproduce quickly; males tend to offspring, allowing females to conserve energy for further reproduction. Polygynandry and Promiscuity Polygynandry (Plural Breeding): Multiple males and females mate within social groups (e.g., African lions). Promiscuity: Evolved when the benefits of social living are low, and mate/resource defense is uneconomical, often in high-density or high-predation environments. Summary of Emlen and Oring’s Model Mating systems can be understood by examining: 1. The fitness interests of each sex. 2. Environmental conditions that shape resource defense and mating behaviors. Mating systems and parental care behaviors are interrelated and co-evolve based on these factors. Background on Reed Warbler Mating Systems The Emlen and Oring model predicts that resource availability influences mating systems: in resource-poor environments, monogamy with biparental care is favored, while abundant resources favor polygyny or promiscuity. Study on Reed Warblers  Researchers Bernd Leisler, Hans Winkler, and Michael Wink studied 17 species of acrocephaline reed warblers to examine how habitat quality correlates with mating system and male parental care. Methodology Habitat Quality: Defined as poor, medium, or good based on food biomass and prey size, with poor habitats having low light and small prey and good habitats being more productive with larger prey. Male Parental Care: Categorized as full (equal to female care), reduced, or none, using data from prior studies. Molecular Phylogeny: Created using cytochrome-b gene sequences to analyze evolutionary patterns. Results Habitat Quality and Mating System Correlation: ○ Poor-Quality Habitats: Monogamy and high male care predominated. ○ Medium- and Good-Quality Habitats: Polygyny, promiscuity, and reduced male care were more common. Phylogenetic Analysis: ○ Polygyny evolved independently at least twice in this group. ○ The ancestral species were likely monogamous with high male care. Conclusion Findings support the Emlen and Oring model: resource-poor environments favor monogamy and high male care, while richer habitats favor reduced male care and polygyny or promiscuity. This study highlights how shifts to resource-rich habitats led to changes in mating systems and parental care in reed warblers. The Emlen and Oring Model and Monogamy Prediction: Monogamy is observed when biparental care is essential for offspring survival. Mammals: Monogamy is rare in mammals due to female lactation, which often provides all necessary nutrition for offspring. Exceptions to Monogamy in Mammals Primates: Monogamy is observed in many primates. Rodents: Rare, but occurs in species like the California mouse (Peromyscus californicus). Monogamy in California Mice Adults form exclusive pair bonds, with genetic data confirming no mating outside the pair bond. Pairs defend a territory and share parental responsibilities, with males providing critical care. Male Contributions to Parental Care Males assist in all aspects of care except lactation, including: ○ Huddling: Warming pups through close body contact. ○ Grooming: Maintaining pup hygiene. ○ Transporting: Carrying pups between locations. Key Question Is male care critical for the survival of offspring in California mice? Further research is needed to address this question. Background Information The California mouse (Peromyscus californicus) is a monogamous rodent species in which males contribute significantly to parental care. Male contributions include huddling, grooming, and transporting young, but not lactation. This study investigates whether male parental care impacts offspring survival and reproductive success. Research Question How does male parental care affect offspring survival and reproductive success in California mice? Methods Location: Hastings Natural History Reservation, California. Treatment Groups: 1. Male-Absent Families: Males were removed within three days of the first litter’s birth. 2. Male-Present Families: Males were trapped, handled, and returned to their families. Data Collection: Female body weight changes during pregnancy and post-birth were used to estimate offspring numbers. Offspring survival rates were monitored. Results  No difference in the number of offspring born between male-present and male-absent families. Offspring survival was significantly higher in male-present families compared to male-absent families. Male parental care, such as thermoregulation and protection, positively influenced offspring survival. Conclusion Biparental care, specifically male parental involvement, enhances offspring survival in California mice. The need for biparental care has likely driven the evolution of monogamy in this species Background Information Two poison frog species in Peru, Ranitomeya imitator and R. variabilis, exhibit different mating systems: ○ R. imitator: Monogamous with biparental care; uses small breeding pools (~24 mL). ○ R. variabilis: Promiscuous with male-only care; uses larger breeding pools (~112 mL). Small pools limit resources but reduce predation, requiring parental investment to ensure offspring survival. Research Question Does the size of the breeding pool influence the evolution of parental care and mating systems in these frog species? Methods Transplant Experiment: ○ Tadpoles from both species were placed in large or small pools. ○ Mesh screens prevented additional resources. ○ A control group of R. imitator in small pools received parental feeding. ○ Tadpoles were monitored weekly, and their growth rates and survival were measured. Results Tadpoles without parental care in large pools grew faster than those in small pools. R. imitator tadpoles in small pools with parental feeding (control) also showed high growth rates.  Mortality occurred only in small pools without parental care: ○ 4 of 8 R. variabilis and 3 of 8 R. imitator tadpoles died. Conclusions R. imitator relies on biparental care in small pools to ensure offspring survival and growth, favoring monogamy. Larger pools allow for male-only care in R. variabilis due to greater resource availability. Ecological Constraints: R. imitator likely uses small pools due to poor competition for larger ones, requiring increased parental investment to overcome resource limitations. The evolution of monogamy in R. imitator highlights the critical role of biparental care in resource-limited environments. Background Information Monogamy can evolve in species without biparental care, such as coral reef fishes, some mammals, and crustaceans. Two Hypotheses for Monogamy Without Biparental Care: 1. Territorial Cooperation Hypothesis: Pair formation enhances defense of a critical resource like a burrow or territory. 2. Mate Guarding Hypothesis: Males stay close to a female to monopolize mating opportunities, especially when females are rare or sexually receptive for a short period. Research Question Do territorial cooperation and mate guarding explain monogamy in snapping shrimp (Alpheus angulatus)? Methods 1. Territorial Cooperation Experiment: ○ Compared burrow defense success between single females and female-male pairs against intruders. 2. Mate Guarding Experiment: ○ Allowed males to choose between guarding a high-value (pre-molt) female or a low-value (post-molt) female. 3. Chemical Cue Experiment: ○ Used a Y-shaped maze to test if males are attracted to water containing chemical cues from females at different molting stages. Results Territorial Cooperation: ○ Female-male pairs defended burrows more successfully than solitary females. Mate Guarding: ○ Males preferred guarding pre-molt (high-value) females over post-molt (low-value) females. ○ Males were attracted to water containing chemical cues from pre-molt females in the Y-shaped maze. Conclusion Territorial Cooperation and Mate Guarding: Both factors contribute to the evolution of monogamy in snapping shrimp. ○ Pairs are better at defending territories. ○ Males can assess female receptivity and prioritize guarding females close to sexual readiness. Further Research: Additional studies are needed to explore the role of female behavior and chemical cues in male pairing decisions. Background Information Feral horses (Equus caballas) live in social units called bands, consisting of one male and multiple females. Polygyny is observed, with a dominant male obtaining most or all matings. Bachelor males form separate groups without females. Research Question Is polygyny in feral horses driven by female defense or resource defense? Methods Study Location: Large population of feral horses in New Zealand, studied over three years. Data Collection: ○ Monthly recordings of band locations using natural markings and freeze brands. ○ Home range plotting for each band. ○ Observation of aggressive interactions between males from different bands and bachelor males. Results  Male Behavior: ○ Males aggressively defended females from rival males during encounters with other bands or when bachelor males were nearby. Home Range Analysis: ○ Bands had overlapping home ranges with no exclusive territory defense. ○ Males did not defend specific resources, likely due to the high costs of territory defense. Conclusion Female Defense Polygyny: Males actively defend females rather than resources, as evidenced by overlapping ranges and aggressive male behavior focused on females. Background Information Resource Defense Polygyny: Males defend resource-rich territories to attract multiple females. Polygyny Threshold Model: Predicts that females will choose polygyny if the benefits of accessing high-quality resources outweigh the costs of sharing resources with other females. Research Question Do female red-winged blackbirds (Agelaius phoeniceus) prefer polygynous territories with higher resource quality over monogamous territories with lower quality? Methods Study Site: Marshes in Ontario, Canada. Territory Setup: ○ Adjacent territories (dyads) were manipulated to differ in quality: High-Quality Territories: Nesting platforms placed over water, reducing predation. Low-Quality Territories: Platforms placed on land, increasing predation risk. ○ Females in low-quality territories were removed, leaving a resident male. High-quality territories had a resident male and a female. Observations: Researchers recorded where newly arriving females settled and their nesting success. Results In 12 of 14 dyads, newly arriving females chose the high-quality (polygynous) territories.  Females on high-quality territories built nests, and most laid eggs; those on low-quality territories faced nest abandonment or predation. Fitness calculations showed that nesting over water provided a net benefit of 0.4 extra offspring, despite reduced male provisioning in polygynous territories. Conclusion Support for Polygyny Threshold Model: ○ Females preferred high-quality territories and were willing to mate polygynously due to the survival benefits of reduced predation. ○ Only males defending high-quality territories mated polygynously, demonstrating that resource defense influences mating systems. Implications for Mating Success: ○ Greater variation in male mating success as some males secured multiple mates while others had none. ○ Less variation in female mating success, as most females were able to mate. Background Information Carrion Beetles: Large insects (2–4 cm) that rely on vertebrate carcasses as a rare and clumped resource for their larvae. Reproduction: Females lay eggs near the carcass; larvae feed on it before pupating. Male Aggression: Males fight for carcass access, but females do not show aggression, raising questions about resource defense polygyny. Research Question Do carrion beetles exhibit resource defense polygyny? Methods Species Studied: Ptomaucopus morio (a carrion beetle in Japan). Experiment Setup: ○ Four virgin beetles (2 males and 2 females) were placed in an arena with a piece of meat. ○ Observed male and female behavior, mating patterns, and offspring production. Parentage Analysis: Used DNA fingerprinting to determine parentage of larvae. Results Male Behavior: ○ Large males dominated smaller males, won fights, and actively defended the carcass.  ○ Dominant males obtained multiple matings and sired most offspring. Female Behavior: ○ Females did not exhibit aggression, mated with both males, and shared access to the carcass. Reproductive Success: ○ Large males sired three times more offspring than smaller males. ○ Large and small females had similar reproductive success. Conclusion Resource Defense Polygyny: ○ Dominant males defend carcasses to exclude smaller males and sire the majority of offspring. ○ Females share carcasses without aggression, supporting predictions of the polygyny threshold model. Implications: Male defense of resources, not females, drives polygyny in P. morio Background Information Male Dominance Polygyny: Evolves when resources and females are uniformly distributed, making it too costly to defend them. Leks: Aggregations where males compete to establish dominance hierarchies; high-ranking males obtain most matings, while low-ranking males have limited reproductive success. Research Question Why do low-ranking males aggregate on leks despite low reproductive success? Hypotheses 1. Hotspot Hypothesis: ○ Males aggregate in areas of high female activity (e.g., near food resources). ○ Dominant males occupy prime locations, increasing their encounters with females. ○ Low-ranking males settle farther away to gain some access to females. 2. Hotshot Hypothesis: ○ High-ranking males attract females due to their quality. ○ Low-ranking males aggregate near dominant males to benefit from the increased female visits. Distinguishing the Hypotheses  Predictions on Male Removal: ○ Hotshot Hypothesis: Removing a high-quality (hotshot) male decreases the attractiveness of the lek for females and other males. Low-ranking males move to new territories near another hotshot male. ○ Hotspot Hypothesis: Male removal does not affect female activity at the location. New males settle on vacated sites, and the area remains attractive to females. Experimental Findings Contrasting Results: Experiments testing these hypotheses have yielded mixed conclusions, suggesting that both mechanisms may contribute to lekking behavior Background Information Species: Great snipe (Gallinago media), a lekking shorebird in Europe. Lekking Behavior: Males display on leks during a two-week breeding season, establishing dominance hierarchies. Research Question Does the hotspot hypothesis or hotshot hypothesis explain lekking behavior in the great snipe? Methods Studied three leks with up to 20 males each. Observed male interactions to determine dominance hierarchy: ○ Dominant males won most interactions and achieved the highest mating success. ○ Subordinate males lost most interactions. ○ Subdominant males were of intermediate dominance. Conducted removal experiments: ○ Removed dominant males for one night. ○ Removed subdominant or subordinate males on separate nights. ○ Recorded changes in male and lek behavior. Results Removal of Dominant Males: ○ Vacated territories were left unoccupied. ○ Neighboring males moved away.  Removal of Subdominant or Subordinate Males: ○ Neighbors quickly expanded into vacated spaces. ○ Other males moved closer to the dominant male's territory. Conclusion Hotshot Hypothesis Supported: ○ Removal of dominant males negatively impacted neighboring males, reducing lek attractiveness. ○ Removal of subordinate males caused no significant disruption, with remaining males moving closer to the dominant male. ○ Dominant males (hotshots) are central to the attractiveness and structure of the lek. Background Information Species: Peafowl (Pavo cristatus), a lekking species. Study Site: Large lek at Parc Zoologique de Clères, France, with around 100 individuals. Male Behavior: ○ 75% defended lek sites; 25% were "floaters" without territories. ○ Males near the feeding location displayed more, attracted more females, and had higher mating success. Research Question Do male and female behaviors in peafowl support the hotspot hypothesis or the hotshot hypothesis? Methods Observations: Recorded male–male interactions, display rates, female visitation, and mating success of 29 males. Male Removal Experiment: Removed individual males from lek territories and observed the effects on male replacements and female behavior. Results Male Behavior Post-Removal: ○ Floaters quickly replaced removed males and began displaying. ○ New males displayed at rates similar to previous site owners. Female Behavior Post-Removal: ○ Females continued visiting and mating at the same rates.  ○ Female visitation and mating success were influenced by the proximity of male territories to the feeding site, not male identity. Conclusion Hotspot Hypothesis Supported: ○ Male territories near the feeding site attracted the most females. ○ Male removal had no significant effect on female behavior, as females were drawn to the location, not specific males. Implications: Hotspot and hotshot hypotheses may both explain lek evolution in different species, but further research is needed to evaluate their relative importance across taxa. Background Information Polyandry: A rare mating system where a single female mates with two or more males. Sex-Role Reversal: Common in species like shorebirds (e.g., jacanas) and pipefish, where males provide most parental care, and females are highly territorial and aggressive. Examples: ○ Jacanas: Females defend large territories; males defend smaller sub-territories and provide parental care for clutches. ○ Pipefish: Females lay eggs on males, who brood and care for them. Three-Step Hypothesis for Polyandry Evolution (Andersson 2005) 1. Male-Biased Parental Care: ○ Favored in resource-poor habitats where females require time for energy replenishment to produce eggs. ○ High predation rates on clutches can also favor male-biased care, allowing females to produce replacement clutches. 2. High Female Fecundity: ○ Females evolve the ability to lay more eggs than a single male can care for, leading to constraints on female fitness by the number of available mates. 3. Female Competition: ○ Intense competition for mates favors the evolution of increased fighting ability (e.g., larger body size and aggression) in females. Supporting Evidence High Clutch Predation:  ○ Example: Comb-crested jacana (Irediparra gallinacea) loses ~80% of clutches to predation. Male Care as a Limiting Factor: ○ Shorebirds: Males can incubate only 4–5 eggs, while females can produce 8, requiring two males to care for two clutches. ○ Pipefish: Males can only brood a limited number of eggs, forcing females to mate with multiple males. Resource Evolution: ○ Many polyandrous species now inhabit resource-rich environments but likely evolved from ancestors in resource-poor habitats. Conclusion Polyandry and sex-role reversal evolve through a combination of: ○ Male-biased parental care. ○ High female fecundity exceeding male care capacity. ○ Intense female competition for mates. These traits arise in response to environmental factors like resource availability and predation risk. Background Information Polygynandry: Mating occurs within specific social groups with multiple males and females defending a shared territory. Promiscuity: Mating is not restricted to specific groups and occurs without social associations. Examples of Polygynandry: ○ African lions, banded mongooses, spotted hyenas, chimpanzees, and bonobos. ○ Group defense of territory is more effective than individual defense. Advantages of Polygynandry Group Defense Success: ○ Larger groups, such as lion prides, win over 80% of territorial conflicts. ○ More males in the group enable better defense of females and the territory. Reproductive Patterns: ○ Males in social groups are often related, and multiple males sire offspring. Research: Polygynandry in European Badgers Species: European badgers (Meles meles), which exhibit high social variation. Study Site: Wytham Woods, Oxford, England.  Methods: ○ 17-year study capturing and marking badgers with tattoos. ○ Blood samples were collected for DNA analysis to determine parentage of over 300 cubs. Results Group Composition: ○ Average group size: 12 adults (6 males, 6 females). ○ 27% of males and 31% of females bred successfully each year. Reproductive Behavior: ○ On average, 1.6 females and 1.5 males per group reproduced annually. ○ Up to 5 males and 5 females reproduced within a single group, indicating plural breeding. Mating System: ○ Year-round mating reduces the feasibility of male mate guarding. ○ Multiple males defend females against outsiders, enhancing group defense. Conclusion Evidence for Polygynandry: ○ Multiple males and females reproduce within social groups. ○ Females may mate with multiple males to ensure fertilization, reduce harassment, or increase genetic diversity of offspring. Further Research Needed: To explore the fitness benefits of this system for both sexes. Background Information Promiscuity: Mating system where individuals mate with multiple partners without social associations, often involving uniparental or no parental care. Scramble Competition: Individuals compete indirectly to find and mate with multiple partners. Examples: Seaweed flies (Coelopa frigida) and red squirrels (Tamiasciurus hudsonicus). Seaweed Flies Behavior: ○ Found in decaying seaweed; no courtship occurs. ○ Males attempt to mate with females immediately upon encounter. Reproductive Patterns:  ○ High-density populations can result in males mounting a female every 8 minutes. ○ Females may mate with hundreds of males during their ~3-week lifespan. Red Squirrels Study Details: ○ Location: Near Kluane National Park, Yukon, Canada. ○ Methods: Individuals were tagged, tracked via radio transmitters, and DNA samples were analyzed to determine paternity. Reproductive Behavior: ○ Females are receptive for a single day, attracting multiple males to their territory. ○ Mating chases occur, often resulting in copulations. Mating Outcomes: ○ Females copulated with an average of 5.8 males. ○ 82.5% of litters were sired by multiple males, with an average of 2.3 sires per litter. Evolutionary Drivers of Promiscuity Environmental Constraints: ○ Large territories of both sexes prevent effective mate or resource defense. ○ Female mobility and running behavior make economical defense unlikely. Benefits of Promiscuity: ○ Promotes genetic diversity in offspring. ○ Increases mating opportunities for both sexes in environments where territorial defense is impractical. Conclusion Both seaweed flies and red squirrels demonstrate promiscuity with scramble competition. Promiscuity evolves in species where defending mates or resources is too costly, and females can mate with multiple males to maximize reproductive success Social and Genetic Mating Systems Differ When Extra-Pair Mating Occurs Background Information  Social Mating Systems: Defined by observed pair-bond associations, assuming individuals mate only with social partners. Genetic Mating Systems: Determined by DNA fingerprinting; includes actual reproductive partners. Extra-Pair Copulations (EPCs): Common in socially monogamous species, leading to differences between social and genetic mating systems. Research Questions 1. Why do social and genetic mating systems differ? 2. What factors explain multi-male mating in socially monogamous females? 3. How does the genetic quality hypothesis explain benefits of extra-pair matings? Key Findings Male Perspective: ○ Males engage in EPCs to increase reproductive success. ○ Risk: Loss of paternity if other males mate with their partner. Female Perspective: ○ Risks include losing parental care if paternity is uncertain. ○ Benefits include: Enhanced fertility. Additional protection from predators. Extra resources for offspring if multiple partners provide care. Protection against infanticide by dominant males due to paternity uncertainty. Improved offspring genetic quality. Genetic Quality Hypothesis Premise: Females mate with multiple males to improve offspring fitness through genetic mechanisms. Potential Benefits for Females: 1. Good Genes Hypothesis: Mate with a higher-quality male than the social partner. 2. Heterozygosity Advantage: Increased genetic diversity reduces risks of deleterious recessive alleles being expressed. These mechanisms collectively increase offspring survival and fitness. Conclusion Extra-pair matings allow socially monogamous females to maximize genetic benefits for their offspring. These behaviors are explained by the genetic quality hypothesis, integrating both good genes and heterozygosity advantages. Extra-Pair Mating in Juncos Background Information Genetic Quality Hypothesis Prediction: Extra-pair offspring will have higher lifetime fitness than within-pair offspring. Species Studied: Dark-eyed juncos (Junco hyemalis). Study Context: Long-term research at Mountain Lake Biological Station in Virginia since 1983, with paternity determined through DNA analysis from 1990 onward. Research Question Do extra-pair offspring exhibit higher lifetime fitness than within-pair offspring? Methods Used microsatellite data to assess paternity of over 2,200 nestlings. Analyzed behavioral and demographic data to measure reproductive success across two generations. Results F1 Generation: ○ No difference in fledgling success between extra-pair and within-pair offspring. ○ Extra-pair F1 males and females produced more offspring compared to within-pair F1 individuals. F2 Generation:  ○ Females that engaged in extra-pair matings had nearly twice as many grand-offspring compared to monogamous females. Conclusion Extra-pair mating benefits appear in the F2 generation. Findings strongly support the genetic quality hypothesis: extra-pair mating enhances long-term fitness via increased reproductive success in subsequent generations. Marmot Extra-Pair Mating Background Information Species Studied: Alpine marmots (Marmota marmota). Study Context: Over ten years of research in La Grande Sassière Nature Reserve, French Alps. Social Structure: Socially monogamous groups with a dominant breeding pair and subordinate offspring. Behavior Observed: Extra-pair matings are common. Research Question Do extra-pair young exhibit higher fitness than within-pair young, supporting the genetic quality hypothesis? Methods Marmots were trapped, sexed, and marked with ear tags and transponders for location tracking. Tissue and hair samples were collected for DNA paternity analysis. Social status was assessed through observations of dominance interactions. Fitness metrics included annual survivorship and the probability of attaining dominant status. Results Paternity Analysis: ○ 220 offspring analyzed. ○ 45 were extra-pair young, while 175 were within-pair young. Fitness Comparisons: ○ Survivorship: Extra-pair juveniles: 15% higher survival than within-pair young. Yearlings: 10% higher survival. Two-year-olds: 30% higher survival. ○ Dominance Status: Extra-pair young were 4x more likely to achieve dominant status than within-pair young. Genetic Observations: ○ Extra-pair mates were less genetically similar to the female than her social mate. ○ Higher heterozygosity in extra-pair young was linked to increased fitness. Conclusion Extra-pair mating improves offspring fitness by increasing heterozygosity. Female marmots benefit from extra-pair matings by producing offspring with higher survival rates and greater likelihood of attaining dominant status. These findings support the genetic quality hypothesis and highlight the evolutionary benefits of extra-pair mating in socially monogamous species