Sexual Selection/Courtship PDF

Sexual selection / courtship  The terms The strategies: Sexual selection The tactics: Courtship, intra-species competition  Sexual selection Quantity: The number of o spring "left behind" is not the only important factor in reproductive success but als...

Sexual selection / courtship  The terms The strategies: Sexual selection The tactics: Courtship, intra-species competition  Sexual selection Quantity: The number of o spring "left behind" is not the only important factor in reproductive success but also the quality (i.e., probable reproductive success) of the o spring. Quality: The "quality" (e.g., health, reproductive potential, etc) of the o spring is highly dependent on the "quality" of the parents. Mate choice and competition for mates become crucial factors in the reproductive success of an individual. ff ff ff  Sexual selection can be: Intra-sexual: Males or females competing with each other. Competition within one sex (bu alo) or both sexes (wolves) for access to the other sex. Inter-sexual: Males or females choosing their mates. Mate choice often involves courtship and courtship rituals between sexes; often involves advertisement by males and choice by females. Example: female deer choose to mate with bucks that control the largest feeding areas; many bird species. ff The original view: Darwin (1871) » Trivers (1972) Males: competition for access (may explain slower maturity in many species, e.g., humans) to resources, including females. Females: selection for choice Why choosy females? In mammals: Limited number of ova Child bearing: costly Lactation: costly Prediction: Ovulating females are limited resources for males. Consequence: Reproductive mistakes are much more costly to females (e.g., mountain gorillas: 3-6 years inter-birth intervals; generally 2 females for every male, on average).  Intra-sexual selection Competition can precede or follow conception/mating: Competition before mating: cervidae, bovidae. Example: male bucks obtain feeding areas prior to mating by ghting with each other. Competition after mating: Examples: Pride take-over by strange male lions and infanticide Bruce e ect in mice (pheromonal abortion of female mice by arrival of strange males). In both cases, receptivity of females and mating immediately follows. ff fi  Male-male competition Social and mating systems often determine the amount of intra- sexual and inter-sexual competition. Male-male (intramale) competition is common in polygynous species, some monogamous species. Means: Aggression ( ghts, ritualized aggression) Sperm competition Kleptogamy = Surreptitious mating: cuckoldry, sneak-mating, etc. fi  In other words Genes = “currency” Independently of competition: Males evaluate/assess each other (competition) Females evaluate/assess each other (competition) Males and females evaluate/assess each other (choice) Assessment of the phenotype, i.e., genotype...  Phenotype Static traits (or relatively static) » examples: Antlers/horns Colours (not in cephalopods though…) Dynamic traits » conscious control? examples: Odours (pheromones) Pupil dilation: arousal to very positive or very negative* * Important in primates and birds: under ANS control Ecological factors: the r and K species r-selected species K-selected species "Opportunistic" species "Sedentary" species species-speci c growth rate; carrying capacity of the habitat Think "Kool" Think "rushed" Typical example: African Elephants Most mouse-like rodents Typical environment: Stable: e.g., tropics Unstable: e.g., mountains Typical climate: Constant or predictable Variable or unpredictable Body size: Large Small Development: Slow Rapid Life span: Long Short fi  K-selected species r-selected species Iteroparity or repeated reproduction Semelparity or single reproduction Reproduction: (young at intervals, successive bouts) (young all at once) Delayed reproduction Early reproduction O spring quantity: Low High O spring quality: High; E ciency Low; Productivity Parental care: Extensive Minimal or nil Parental investment: High Low High, catastrophic, density- Mortality rate: Low, directed, density-dependent independent Intraspeci c competition: High Low Home range or territory: Yes None or less obligatory or de ned. ff ff ffi fi fi  Note: The r and K selection model is based on a continuum and has many exceptions: Extensive and prolonged parental care and parental dependency are common to species depending on scarce and low access food (e.g., large carnivores). Primates have the longest periods of dependency but they also have exible behaviour (plasticity of their behavioural repertoire) and impressive learning abilities. fl  Reproductive success The classical de nition: “The number of surviving o spring produced by an individual” (Alcock, 2005). Is a function of (… and can be measured by …): number of o spring's born number of weaned individuals number of individuals available for mating ff ff fi  Di erent sex priorities Sex di erence (in "priorities") Males: function of how many females are inseminated Females: function of how many eggs are produced (and fertilised!). The variance in copulatory (and therefore reproductive) success is usually higher for males than for females (Bateman e ect) EXCEPT in cases of stable pair bonds (strict monogamy). Mating systems are relevant to many “reproductive” issues. ff ff ff  Basic assumptions Recognition of potential mates of your species (or sub-species) is possible. Interbreeding between species but within a genus: rare. A few exceptions with potentially high reproductive success. In most cases (e.g., duck species) it is better to avoid congenerics that are not your species. Individual (incl. kin, to avoid “incest” or inbreeding) and sexual recognition (sexual imprinting) is important. Identi cation of “good genes” cues are important: Phenotype fi  The con icts 1. What males want and what females want. 2. What females want and what is actually “ t”. 3. What males want and what is actually “ t”. 4. Females in birds and mammals (and actually most vertebrates) typically invest more energy and time in gametes than males. 5. Females in mammals typically invest more energy and time in the progeny than males, and males in sh typically invest more energy and time in the progeny: internal gestation. fl fi fi fi  Epigametic sexual characteristics Males have important secondary sexual characteristic called epigametic sexual characteristics to in uence female choice. Those characteristics are used in displays to “show-o ” and impress the females. The basic assumption is that females are strongly attracted to epigametic sexual characteristics that are highly correlated with good health (immune system integrity) and good reproductive capability (reproductive system integrity). Examples: bright colours, sophisticated plumage, horns and antlers. fl ff  True for all mating systems? Within a species, when males and females do not look alike we say there is “sexual dimorphism”. Sexual dimorphism is stronger in non-monogamous species. Sexual dimorphism is typically based on epigametic sexual characteristics. Sexual selection is often “stronger” in polygamous species (polyandrous and polygynous), in part because of higher “con icts of interest” between males and females. fl  Dimorphism in Primates Sexual dimorphism in primates is often more pronounced in males Vervet monkeys: Bright blue testicles Howler monkey: beard Orang-utans: size and enlarged facial features Olive baboons (all baboons): long canines Mandrills: blue face/nose Humans: facial hair, musculature, height (size), etc.  Vervet monkey (Chlorocebus pygerythrus) Black howler monkey (Alouatta caraya) Mandrill (Mandrillus sphinx) Bornean orangutan (Pongo pygmaeus) Olive baboon (Papio anubis)  Monomorphism in Primates Prosimians, e.g., Lemurs, aye-ayes, lorises, pottos Gibbons (but: monogamous; more on this link later) Callitrichids (marmosets, tamarins; most are monogamous or polyandrous): sometimes females are larger (consistent with polyandry). Ring-tailed lemur (Lemur catta)  Genetics and mate choice Genes (nature) and environment (nurture) are important. Which genes are chosen: Environment How genes are chosen: Proximate factors, mainly genetics and endocrinology.  Models: 1. Direct bene ts (Darwin, 1871): Survival and/or reproductive value of mate. Sexual selection = mating success. 2. Good genes theory (Hamilton & Zuk, 1984): Good sperm = good genes = good babies... Ornaments are signs of immune integrity, more speci cally, absence of parasites. 3. Runaway selection theory (Fisher, 1958): male traits × female mating preferences. 4. Handicap theory (Zahavi, 1975): ornaments = handicap. Message to females: “I can survive despite this handicap”. 5. Sensory bias theory: Females respond to ancestral male characteristics. fi fi Direct  Direct bene ts: Tangible resources are offered (advertised) by the males. Males have “selling points”. Indicator of the survival and/or reproductive value of the male. This favours choosy/picky females. Females are looking for resources that males can contribute: Food Shelter Assistance with parental care Etc. fi  Territories as resource Hypothesis: Females are not going for the male’s traits as much as its resources. Pied ycatcher (Ficedula hypoleuca) >>> Large territories are held by males that: Are rst at the site ( rst to arrive) Older Blacker BUT Alatato et al. (1986) nds that the choice of females is not correlated with the three factors mentioned above. The choice of females is correlated with the quality of the territory. fi fl fi fi Good  Good genes: good gonads » good sperm » good babies… It is an “indirect bene t” theory: Good genes (genotype) are (hopefully) translated into good phenotype (e.g., good foraging or parenting skills). Epigametic sexual characteristics are important: “good” (attractive) ones may signal “good health”. Assumptions: Females need to be able to identify “good genes”. Females need to be able to identify “cheaters”. Handicap hypothesis or principle (Zahavi): see below Honest advertising: Honest indicators only are getting the attention of females. Honest indicators are COSTLY to produce, therefore, the costlier the trait, the harder it is to fake. fi  Good genes: examples 1 Endoparasites in males (potential indicator): how can they be detected by females? Hypothesis: Trait(s) may be associated (correlated) with endoparasites (e.g., dull colour, less hair, skinny). Hamilton-Zuk hypothesis: Females choose the least parasited males based on those indicators. MHC (major histocompatibility complex): pheromones Proteins produced by MHC genes help the body to identify “foreign” VS “domestic” cells. MHC ≈ ngerprint: each individual has its own MHC Rationale: for a successful “pairing”, match very di erent MHC’s immune system strength fi ff  Good genes: examples 2 Identi able signal (indicator)? In mammals: odour (pheromones) Wedekind studies: MHC matching in humans Women and men will be more attracted to individuals of the opposite sex with a di erent MHC signature than theirs. Women will even use perfumes that will magnify their MHC signalling. Reusch: questions the basic assumption. And asks, is the point: Very di erent MHC alleles? OR Greater diversity of MHC alleles? fi ff ff  Good genes: examples 3 Symmetry: Fluctuating asymmetry hypothesis Basic idea: deviations (asymmetries) are “bad”. High symmetry = high genetic quality. This theory is in uenced by the developmental stability theories of biology. Symmetry suggests a phenotypic ability to deal with environmental challenges. Symmetry and odour may be associated (Gangestad and Thornhill, 1998). fl Runaway  Male traits × female mating preferences. At least 2 genes (one for each term of the interaction), potentially more. Both genes (or set of genes) are present in both sexes, but expressed only in the appropriate sex. Example: females prefer bright colours and large tails in males peacocks there is a gene for the female high contrast preference the bright coloured and large tail in male peacocks is heritable there is a gene for the male bright coloured and large tail  Result: progeny Males with bright colours and large tails Females liking bright colours and large tails With time, those genes become linked: if the frequency of one gene changes, the frequency of the other will change as well. Run away e ect: stronger male traits (exaggerated) stronger female preference In other words: Male ornaments in uence the choice convention of females >>> Those females are likely to have sons with the same feature(s) (ornaments). ff fl Zahavi’s  The Handicap Theory Not a “general” selection theory per se, but may apply to speci c cases. Honest indicators are COSLTY to produce, therefore, the costlier the trait, the harder it is to fake. Females need to be able to identify “cheaters”. Honest advertising: Honest indicators only are getting the attention of females. This theory is di cult to con rm fi ffi fi  Challenges Example: Barn swallows’ (Hirundo rustica) tail length Møller (1990): Long tails = handicap ↑ parasites = ↓growth of chicks = ↓ tail Long-tailed males suggest parasite resistance for the o spring. Norberg (1994): Puts male swallows in wind tunnels >>> long tails are an aerodynamic advantage. Long tail >>> Direct personal advantage (not a handicap)? Mating advantage? ff Does the mating system matters? Polygamy: More likely to have handicap. Monogamy: Not as likely. If the male is expected to participate in parental care, then a handicap is likely problematic in the future execution of paternal care. Sensory  Sensory exploitation, or sensory drive, or preexisting bias or sensory bias theory*. Choice in females driven by the intensity of the sensory stimulation. Based on sensory predispositions to respond to speci c sensory stimuli or categories of sensory stimuli. Male traits are not important, the detection of tness is not assumed, females are just responsive to that speci c stimulus characteristic (e.g., the colour yellow). This theory explains well the genesis of a preference, not the adaptiveness of the trait. Later, the trait may become selected for as it acquires a sexual function. * Ryan, 1990; Endler & McLellan, 1988; West-Eberhard, 1979, 1983 fi fi fi  In summary: Females respond to male features that are inherited from ANCESTORS and have nothing to do with male quality. Example: Green swordtails (Xiphophorus helleri) >>> extended caudal n (“tails”) or swords. Basolo (1990, 1995): Females prefer males with long tails (swords). Southern Platy sh (X. maculatus) = closely related species to X. helleri but males have no swords at all. BUT: females of X. maculates prefer males with long tails! In other words, females are more attracted to males of a di erent species. = BIAS Sword = Supernormal stimulus? ff fi fi Other  1. Learning (classical and instrumental conditioning) Conditioned stimuli and mating (i.e., not “choice” per se?) 2. Sexual imprinting (as opposed to lial imprinting) 3. Social learning: e.g., mate preference in females can be in uenced by other females Potential for cultural transmission? 4. Ornaments: For the female or for the other males? 5. The extended phenotype fl fi  Learning and imprinting (#1 and #2) are covered in other courses (e.g., 3162, 4140, 4160) Let’s look at # 3, 4, 5  Social learning: Copying Basic idea of this model: Copiers (minority) and choosers (majority) If ↑ copying in females ↑ # males that never mate ↑ # males with very high reproductive success ↑ variance in those males’ reproductive success (males that get attention: ++) A human example? Popular males in bars. Limitation of the “copying” model: It can become maladaptive If too many copiers, copiers copy copiers, not choosers. Reduces genetics variability (the same males are picked over and over) A word on ornaments Ornaments and displays ≟ Male-female assessment? Male-male assessment? Assumption: The choice of females is based on the ability of males to win ghts or dominance contests. Example: Elephant seals >>> 4% of the males reproduce (harems). Females are subject to forced mating BUT, they will call loudly. This may attract/alert a stronger, more dominant male in the vicinity. fi The extended phenotype Sometimes males will try to impress the females with a “construction” and their engineering skills seem to be what females will assess: The “hut” made by the male stickleback The nests made by male Baltimore orioles (Icterus galbula)