Primate Introduction PDF

9. What are Primates? Anthropology 201 Winter 2018  Objectives At the end of lectures students should: Know the defining features of primates Know the basic taxonomy of primates (2 main groups, their infraorders and superfamilies) Be able to identify key trait...

9. What are Primates? Anthropology 201 Winter 2018  Objectives At the end of lectures students should: Know the defining features of primates Know the basic taxonomy of primates (2 main groups, their infraorders and superfamilies) Be able to identify key traits of each living group of primates Know representative species of the living primate groups and their characteristics Reading: Chapter 5 of textbook (up to p. 124) 2 Primates are an Order of Mammals Mammalian synapomorphies: – Warm blooded – Viviparity – Lactation & mammary glands These traits are primitive for primates – Shared with last common ancestor and with all other mammals 3  Distribution of living nonhuman Primates Found in mostly tropical areas: in C. & S. America, Sub-Saharan Africa, parts of Mediterranean Africa, Saudi Arabia, Madagascar, Tropical Asia (up to Himalayan foothills), Japan 4  Primate Characteristics 1. Grasping Hands & Feet 2. Sensory System 3. Large Complex Brains and Associated Behaviour 4. Dental specializations, but generalized skeleton 5 1. Grasping Hands & Feet Opposable thumb and hallux (big toe) Nails, not claws Sensitive tactile pads Power Grip – Squeeze an object strongly between finger pads and palm, allows full strength of the forearm muscles to be applied (tennis racket grip) Precision Grip – Use just the tips of your fingers - for fine control (picking up a grape) 6 2a. Sensory systems - Vision Forward facing eyes – Stereoscopic vision – Depth perception Greater reliance on vision – Elaboration of the visual centers of the brain Colour vision – at least dichromatic (blue, green), many trichromatic (RBG) 7  2a. Cranial Anatomy to protect the eye All primates have a postorbital bar Higher primates (haplorhines) have postorbital closure Non-primate mammals generally only have postorbital process 8 2b. Sensory System - Olfaction Reduced reliance on olfaction Reduction of the snout Reduction of the olfactory centers of the brain 9 3. Large, Complex Brains Large brains relative to body size – Indian elephant = 5,400 cc – Modern human = 1,350 cc; African ape = 275-400 cc – But primates 2x larger than other mammals their size, humans 7-8x – Also many folds - sulci & fissures - to increase surface area Human Rabbit (not to scale!!) 10 3. Large, Complex Brains Learning & socialization very important for survival Greater reliance on learning linked to reduction in reliance upon instinct 11 3a. High Investment in Offspring Fewer offspring, but greater investment in rearing them – Usually means longer lived – Typically give birth to a single young (not litters) Infants are relatively altricial – “requiring nourishment” – vs. precocial (e.g. ungulates) Cling to mother, not left in nests (usually) - have grasping hands (even humans) Longer juvenescence (juvenile development period) 12  3b. Tendency Toward Sociality 13  4. Dental Formula Ancestral mammal condition = 3.1.4.3 Primitive primates (most strepsirhines & NWM) = 2.1.3.3 Later primates (OWM, Apes, Humans) = 2.1.2.3 Chimpanzee 2.1.2.3 dental formula (chimpanzee - 32 teeth in total) 14 4. Dental specializations Colobus Gorilla 15  4. Generalized skeleton Generalized limb structure Generalized, flexible morphology Non-specialized physical form By Unkown (Brehms Tierleben, Small Edition 1927) [Public domain], via Wikimedia Commons 16  Two points to remember Not all of these traits characterize all primates (e.g. no grasping toe in humans) None of these traits is unique to primates (e.g. dolphins have relatively large brains too) but together this suite of traits is only found in primates 17  Primates Apes Lemurs & Lorises Tarsiers manigerz34.files.wordpress.com/2009/06/7969-philippine-tarsier-0.jpg Platyrrhine Cercopithecoid Humans Monkeys Monkeys 18  Taxonomy Reminder Our taxonomic classification system is hierarchical: – 7 main levels – Kingdom, Phylum, Class, Order, Family, Genus, Species Animalia, Chordata, Mammalia, Primates, Hominidae, Homo, sapiens – Many sublevels, e.g. superfamily, infraorder, etc… – See p. 117 of textbook (Figure 5.9) Same as Table 5.2 in the 8th and 9th of the textbook and Table 5.3 in the 6th and 7th editions. 19 Primates are divided into two groups*: Strepsirrhines Haplorrhines Platyrrhine Cercopithecoid Lemurs Galagos Lorises Tarsiers Monkeys Apes Monkeys Primate Phylogeny * Variably considered as semiorders or suborders  Strepsirrhine Characteristics: Found in Sub-Saharan Africa, Madagascar & Asia 21  Strepsirrhine Characteristics Primate Galago characteristics! Most are nocturnal large eyes Post-orbital bar only Rely on scent marking (olfaction) more so than haplorhines Slow loris 22  Strepsirrhine Characteristics Dental comb Grooming claw – On 2nd digit of the foot Both used in personal grooming Note the canine-like premolar! 23  Lemuriformes Characteristics Strepsirrhines Lemuriformes Lorisiformes Only found on Madagascar & neighbouring Comoros Islands (endemic) Diverse! Because of endemism and lack of competition Small and medium-sized today Diurnal and nocturnal Female dominance 5 families – Know representative examples from the text and film – don’t memorize the five families! 24 Lorisiformes - Characteristics All are nocturnal and small bodied Generally solitary or in small family units Eat insects, gum and nectar, some fruits Two Families: Lorisidae -often immobile Galagidae -slow movers -very active -Africa and Asia -fast movers -Africa only 25 Suborder Haplorrhini - Haplorrhines Order Primates Suborder Strepsirhini Haplorhini Infraorder Lemuriformes Lorisiformes Tarsiiformes Platyrrhini Catarrhini lemurs lorises, galagos tarsiers NW monkeys OW monkeys, apes Tarsiers, monkeys, apes, and humans Three infraorders More ‘derived’ primates 26  Haplorrhine Characteristics Diurnal (except tarsiers & owl monkeys) Reduced reliance on smell & hearing – Flatter faces, shorter snouts Larger, more complex brains – Longer juvenile dependency – Increased parental care/investment – Increased social complexity 27  Enigmatic Tarsiers Infraorder Tarsiiformes Only one living genus (Tarsius) Retain primitive morphology – Same genus applied to fossil from middle Eocene in China (~ 45 mya) Superficially they look like strepsirrhines, so used to be classified in Strepsirrhini – Nocturnal – Small Genetics tells us that they are haplorrhines! 28  Haplorrhini Order Primates Suborder Strepsirhini Haplorhini Infraorder Lemuriformes Lorisiformes Tarsiiformes Platyrrhini Catarrhini lemurs lorises, galagos tarsiers NW monkeys OW monkeys, apes “Higher primates” – can be classified together as Anthropoidea 29 Platyrrhini: broad, outward-facing nostrils 2.1.3.3. dental formula Catarrhini: narrow, downward-facing nostrils 2.1.2.3 dental formula 30  Platyrrhini - Platyrrhine Monkey Characteristics Five families Found in the Americas (Mexico, Central and South America) Arrived from Africa around 35 Ma 31 * Also called the “New World Monkeys”, but many primatologists now avoid that terminology Characteristics of Platyrrhines All have tails Several have prehensile tails All arboreal Smaller body size than cercopithecoid monkeys Most have 2.1.3.3 dental formula Minimal sexual dimorphism Diurnal (except Aotus – the owl monkey) 32  Catarrhine Classification Catarrhini Infraorder Cercopithecoidea Hominoidea Superfamily Cercopithecidae Hylobatidae Pongidae Hominidae Family Cercopithecinae Colobinae Subfamily Two major subdivisions: cercopithecoid monkeys (Cercopithecoidea) & apes (Hominoidea) 33  Superfamily Cercopithecoidea: Found in wide variety of environments – tropical Asia, sub-Saharan Africa, North Africa, Arabian Peninsula All diurnal Single births Some species are terrestrial Larger body size, often sexually dimorphic 34  Two Subfamilies Fruit eaters Leaf eaters Broad Narrow incisors incisors Low cusps High cusps Cheek pouches No cheek Simple pouches stomach Complex Shorter stomach limbs Long limbs Cercopithecinae Colobinae 35 Subfamily Baboon Cercopithecinae Africa and Asia Baboons, macaques, vervets, guenons…) Wide range of habitats Vervet – Savanna, woodlands, rain forests, deserts, cities, mountains Diet highly variable – Fruits, grasses, roots, tubers, leaves, insects Highly variable social systems – Many multi-male/multi-female groups Mandrill Most are sexually dimorphic More terrestrial species than in any other primate group 36 Subfamily Colobinae Found in Africa and Asia 6-12 kgs Procolobus All arboreal Colobus Specialized folivores – Sacculated stomach, supports bacteria for Semnopithecus digestion of cellulose – High shearing crests on teeth Rhinopithecus Nasalis 37  Superfamily Hominoidea The Apes No tails Larger size and weight Larger brain to body weight ratio More upright posture Longer gestation and maturation 38 Hominoid Distribution 39  Family Hylobatidae: “Lesser Apes” Gibbons & Siamangs Southeast Asia Pair-bonded (“monogamous”) – Sexually monomorphic Siamang Frugivores Move using brachiation – Long strong arms, short legs, elongated hook-like fingers Highly territorial – Singing calls Gibbon 40  Family Hominidae: “Great Apes” Genus Pongo: Orangutans Genus Gorilla: Gorillas Genus Pan: Chimpanzees & Bonobos Genus Homo: Humans Large bodied Suspensory locomotion in trees (NOT brachiation); knuckle- walking or ”fist-walking” on the ground Sexually dimorphic Advanced cognitive abilities (all show tool use) Diverse diets & social systems Most investments in offspring – intense parenting and prolonged juvenile periods 41  Strepsirrhines Haplorrhines Platyrrhine Cercopithecoid Lemurs Galagos Lorises Tarsiers Monkeys Apes Monkeys Primate Phylogeny 10. Primate Ecology Anthropology 201 Fall 2024  Primate Ecology Ecology = interactions between organisms and their environment – physical environment (habitat) – biological environment (other organisms) It’s a broad topic, but we will focus on how primates make a living in their environments and the effect of ecological variables on social systems 2  Objectives Understand and identify the different energy requirements of organisms – (BMR, AMR, Growth and Reproduction) Understand how different primates meet these needs Understand the relationship between diet, range and territoriality – And impact on sociality Identify predation avoidance mechanisms Reading: Chapter 5, p. 125-132 and p. 137-138 3 Making a living as a primate… Two main concerns: – 1) how/what to eat – 2) how to avoid being eaten These two concerns influence sociality 4 1. How and what to eat 5  Organisms need energy Food provides energy (calories) essential for growth, survival and reproduction Total energy required depends on FOUR components: – Basal metabolism – Active metabolism – Growth and growth rate – Reproductive effort 6  1. Basal Metabolism Basal metabolic rate (BMR) = rate at which an animal expends energy at rest, for basic body maintenance – e.g. maintaining body temp Larger animals have absolutely higher BMR, but relatively lower (fewer calories per unit body weight) (Dashed line is 1-to-1 line) 7  2. Active metabolic rate The energy required above and beyond baseline for daily activities – e.g. for locomotion, digestion Depends on size of animal, how far/fast it’s travelling – For a baboon-sized primate = ~2x BMR 8  3. Growth Rate Building new tissue requires energy beyond BMR and AMR – Juveniles/infants have higher energy requirements than predicted for their size 9  4. Reproductive Effort For females, additional cost of reproduction – Late pregnancy: +25% calories – Lactation: +50% calories 10  Nutritional Requirements Diet must satisfy energy requirements & specific nutrients they cannot synthesize themselves – Protein/amino acids for growth, reproduction, regulation of bodily functions We cannot make aa’s ourselves – Fats, oils, & carbohydrates provide energy – Trace vitamins & minerals also important for specific physiological functions (e.g., iron & copper for hemoglobin synthesis) 11  Nutritional Requirements Diet must also minimize dangerous toxins – Secondary compounds are plant defenses: Alkaloids – can disrupt normal cell processes Tannins – reduce digestibility of plants – Secondary compound concentration is highest in mature leaves, seeds; lower in fruits, flowers, new leaves Caffeine (an alkaloid) 12 Primate Foods Fruit (frugivory) Leaves (folivory) – Young (more easily digested proteins and sugars) – Mature (high cellulose content, requires specific adaptations) Insects (insectivory) – Social insects vs. solitary Also: – Grasses, tubers, corms (enlarged stems) – Gum – Vertebrates (birds, frogs, bats, monkeys) – Bark, fungus, soil (often for traces minerals) For all: Water (directly or through food items) 13 Primate Diets – generalizations Primate diets are diverse, but: 1) Most primates rely on one food type high in protein (P), one high in carbohydrates (CH) Many Strepsirrhines: insects (P) and gum/fruit (CH) Many Monkeys/Apes: insects/young leaves (P) and fruit (CH) 14 Primate Diets – generalizations 2) Primates rely more heavily on some types of foods than on others – e.g. chimps eat ripe fruit preferentially, aye- ayes eat grubs – Gives us the terms: folivores, frugivores, insectivores, etc… 15 We are what we eat Primate diets are reflected in tooth and gut morphology Useful for inferences in the fossil record See Box 5.1 in textbook 16 Primate Diets – generalizations 3) In general, insectivores < frugivores < folivores – BMR scales with body size – Smaller animals require small but high-quality foods that can be processed quickly (e.g. insects), – Larger animals are not constrained by the quality of their food (more by quantity), they can process it more slowly (longer guts, can process leaves) 17 Diet Composition and Body Size Food availability varies in space and time Can be patchy in space and/or time Can be unpredictable Tropical forests may appear lush and fertile, but: - contain many species, only some of which have edible foods - are highly seasonal 19  Temporal Availability Seasonality – In tropics, depends on day length, rainfall During scarcity, may switch to lower quality diet (unripe fruit, mature leaves) and/or reduce energy expenditures (e.g., torpor in dwarf lemurs) Keystone Resources – Fall-back foods during scarce seasons – e.g., Ficus (figs) 20  Spatial distribution Food varies in density: – Most abundant food are leaves/foliage, – Fruits and flowers (also seasonal), – Lowest density = small prey (insects, vertebrates, etc) Primates need to travel to find their food The distance traveled will depend on the density of the food they’re after 21  Ranges Range = the geographical area in which a group (not a species) can be found – Home range: total area used by a group – Day range: area used by an individual on a daily basis Who has the larger home/day range, a folivore or a frugivore? 22  Food distribution influences range size Woolly monkey (Lagothrix) Howler Monkey (Alouatta) Frugivore Folivore Day range:~3km Day range:~100m Home range: >800 Ha Home range: 4-20 Ha 23  Territoriality varies amongst Food Distribution species Some exclusive (e.g. gibbons) – Territory = home range Some home ranges overlap (e.g. capuchin monkeys) Food distribution influences territoriality Resource distribution – Even = not defensible – Clumped, patchy = defensible Capuchin Territories, Barro Colorado Island in Panama  Territoriality Why are some primates territorial? Can have both costs and benefits in terms of an individual’s ability to survive and reproduce (fitness) – Costs: constant vigilance, advertising presence, engage in defense – Benefits: prevent outsiders from exploiting limited resources 25  Territoriality When do benefits outweigh costs? – Depends on the kinds of resources and their impact on fitness: For females = access to food for them and their dependents For males = access to females (mates) Territoriality serves two functions: – Resource defense (food, nesting sites…) – Mate defense 26  How/what to eat - Summary Energy requirements Resource Distribution Diet Range (size) Territoriality 27 2. How to avoid being eaten Predation significant source of mortality among primates Direct evidence hard to obtain 28  Evidence from predators Factors that increase predation risk: – Being terrestrial – Smaller group size 29  How to avoid predators? Alarm calls – e.g. vervets Swarm Associate with other primate species – Share different areas of the canopy LIVE/FORAGE IN GROUPS! 30  Sociality and Predation Groups provide safety from predators The three D’s: – Detection (more eyes on the lookout) – Deterrence (swarming/mobbing) – Dilution (“better him than me”) 2 animals = 50% chance of being “chosen” 10 animals = 10% 31  Primate Ecology and Sociality What to eat: How not to be eaten: Resource The three D’s distribution/density – Detection Ranges – Deterrence – Dilution Territoriality Groups are better at defending and controlling Groups improve resources/territories predator avoidance 32  Sociality has costs/benefits Benefits: – Resource control IntERgroup competition – Predator avoidance – Access to mates Costs: – Feeding/mate competition IntRAgroup competition – Disease risk – Cuckoldry, incest, infanticide, etc 33 Trade-offs between predation & food Small groups & solitary animals – Predation risk high, intragroup food competition low Large groups – Predation risk lower, intragroup food competition higher – may lead to fission Balance of these factors may produce optimal group size for a species or population 34  Polyspecific Associations Different species may travel & forage together for extended periods of time Permits greater predator Red Colobus protection without increased competition for food and/or mates Foraging benefits: mutual defense, finding food resources, scrounging Diana Monkeys 35  Summary What primates eat, and how they avoid being eaten, can influence range size, territoriality and group size There are trade-offs (costs/benefits) for territoriality and group size – Competition for defensible resources WITHIN and BETWEEN groups Likely a combination of feeding competition and predation drove the evolution of sociality in primates 36 11. Primate Mating Systems and Sexual Selection Anthropology 201 Fall 2024 Primate Mating Systems (A Closer Look - 5.2) Solitary or “noyau” Orangutans, lorises Pair-bonded (monogamy) Gibbons, owl monkeys Uni-female/multi-male (polyandry) Tamarins [cooperative breeding in text] 2 Primate Mating Systems Uni-male/multi-female (polygyny) Langurs, mountain gorillas Non-resident males may form “bachelor groups” Multi-male/multi-female (polygynandry) Vervets, macaques Special case: fission-fusion (spider monkeys, chimpanzees) 3  Primate Mating Systems How do these mating systems (or groups) influence the reproductive strategies of males and females? 4  Objectives Define a strategy in evolutionary terms Distinguish reproductive strategies of female and male primates – Competition over different resources Define and describe sexual selection – How it differs from natural selection Reading: Chapter 6; Ch 5 p. 133-136 5  The language of adaptation Strategy = set of behaviours occurring in specific functional context – e.g. folivory is a foraging/feeding strategy – e.g. polygyny is a mating strategy à Strategies are products of selection, not conscious plans 6  The language of adaptation Different behaviours have different impacts on fitness* – Beneficial = increases fitness – Costly = decreases fitness What currency should we use to measure costs and benefits? – Best: reproductive success (RS): number of offspring surviving to reproductive age – When RS difficult to measure: use proxies (e.g. foraging efficiency) *Fitness = an individual’s genetic contribution to the next generation 7 The language of adaptation Sets of behaviours (i.e. strategies) that tend to increase fitness will evolve through selection Strategies may be different for males and females, and in different group compositions 8  An observation Primate females always provide lots of care for their young Primate males do so in very few species WHY? 9  Why females invest more Why is it female primates that invest more, and not males? – In fish and bird species the males sometimes provide more/most parental care Mammalian reproductive system constrains female strategies – Must carry offspring to term – Must nurse them until they can forage independently – Infant’s survival (hence mother’s fitness) in mammals depends on these factors, more than in other animals 10  Why females invest more Primates: extended pregnancy (large brain), altriciality Extra time and energy per infant Relatively small number of surviving infants produced over lifetime Each infant = greater portion of lifetime fitness 11  Why males invest less Males typically less involved in care of offspring – Male care often less important for survival of offspring – Time, energy and resources are limited – Paternity uncertain – Often better strategy to use their resources to access additional females 12  When males invest less Males will be less involved: – 1) when attracting additional mates is relatively easy (low cost) – 2) when fitness of offspring raised by one parent is high (benefit of additional care is minimal) When benefits of investment outweigh costs of seeking new matings, males will provide more care (tamarins, owl monkeys – females can’t provide enough care?) 13  Reproductive Strategies Sexes invest unequally in offspring – Female investment typically higher Causes of variation in RS are likely to be different: – Females: securing resources (food) – Males: securing mates 14 Female Reproductive Strategies Female RS depends on ability to obtain enough resources (esp. food) for her and her offspring Abundance leads to high birth rates, shorter interbirth intervals (e.g., provisioning by humans) Resource crashes lead to low rates, long intervals (e.g., cyclones) (provisioned years in open squares, 1964-1971) 15 Female Reproductive Strategies Access to food is important, so females often compete for this access Dominance hierarchies often form – High-ranking females gain access to more/better quality food 16 Female Reproductive Strategies Rank is positively correlated with RS 1. Macaques: rank 2. Hanuman langurs: and group size rank and age influence RS influence RS 17 Female Reproductive Strategies Rank is positively correlated with RS High rank Mid rank Low rank 3. Chimpanzees: Mother’s rank influences survival to weaning in offspring (hence RS) 18 Female Reproductive Strategies Socializing can positively influence RS May insulate some females from cost of low rank – Benefits from association with high ranking females? J B Silk et al. Science 2003;302:1231-1234 19  Reproductive Strategies Females Females produce few offspring, invest heavily in each, and can raise them (largely) without males Limiting factor that causes competition and influences the RS of females = access to food Males Males less constrained by food, seldom invest in offspring, hence can potentially mate with many females Limiting factor that causes competition and influences the RS of males = access to mates 20  Male Reproductive Strategies All about access to mates Traits that increase success in competition for mates evolve through a special form of selection: Sexual Selection 21  Sexual Selection Originally proposed by Darwin Special category of natural selection Accounts for features with no obvious survival function (secondary sexual characteristics) Helps explain traits that seem maladaptive in terms of natural selection (e.g. costly) 22  Natural vs. Sexual Selection Natural Selection Sexual Selection Favours phenotypes related to Favours phenotypes that survival/reproduction, e.g. increase success in competition predator avoidance, resource for mates acquisition... Affects one sex more strongly, Affects both sexes the one whose access to mates is limited 23  Components of Sexual Selection Intrasexual Selection – Competition within the sex(es) – Usually stronger among males – Common in primates Intersexual Selection – Mate choice – Usually stronger in females – Common in birds, ?primates 24 Intersexual Selection: What do females want? Females have high parental investment Also reproduce slowly Low reproductive potential Should mate with “quality” mates – Do not want to waste rare reproductive opportunities on unhealthy males or those that might produce less competitive offspring 25  Intersexual Selection Females can choose males that: – Will increase their fitness Defend resources, infanticide protection – Show good “genetic quality” Will produce more fit offspring Measured by condition, and costly traits Text suggests that intersexual selection not important in primates (female choice limited) But may occur via matings with multiple males and possibly cryptic female choice (female sperm storage) 26  Intrasexual selection in males Most basic form of male-male competition is to fight and drive other males away Winners have advantage in mating opportunities = higher RS Intrasexual selection favours traits that enable males to be effective fighters 27  Intrasexual selection in males Increased male body size and canine size are common targets of intrasexual selection Often leads to sexual dimorphism (SD) 28  Intrasexual selection in males SD should be most pronounced in groups where males compete most = single-male/multi-female 29  Reproductive Strategies: Single- male groups A male tries to establish residence in an unrelated group of females, and then restricts access to other males Other males constantly try to take over e.g. baboons, geladas, Hanuman langurs 30 Reproductive Strategies: Multi- male groups In most primates, females have estrus In multi-male/multi female groups, estrous females can mate with several males in group Less direct competition over access to females Here, intrasexual selection favours increased sperm production – “sperm competition” 31 Repro Strategies: Multi-male groups In multi-male/multi-female groups, dominance hierarchies may also form among males – Determine access to estrous females Dominance hierarchy reflects competitive ability (threats, etc.) Younger males have higher ranks – better physical condition? RS related to age Rank and RS in baboons 32  Reproductive Strategies: Pair- bonded males In pair-bonded species (e.g. gibbons), males don’t compete for access to females RS depends more on finding mates, defending territory, and rearing surviving offspring Mate guarding is a common rep. strategy Grooming in white-handed gibbons 33  Reproductive Strategies: Infanticide Adaptive strategy for males After group takeovers, males kill dependent offspring Females return to estrus allowing infanticidal male to reproduce more quickly (before they are ousted) Infanticide is costly for females – So expect counter-strategies such as false estrus, female coalitions, spontaneous abortions, male “friendships”, etc. Observations have been made in a large number of primate species, in the wild and in provisioned populations. 34 Infanticide Counterstrategies Concealed ovulation Regularly sexually receptive – with multiple males Terminate pregnancy after takeover (spontaneous abortion) Social supports/friendships to try to avoid infanticide 35 12. Human Mating Systems Anthropology 201 Fall 2024  Objectives Take what we learned about primate mating strategies and apply it to humans Understand basic challenges in studying human mating patterns from a biological perspective Understand human mating preferences Know the different mating systems found in humans and examples of these Explore human perspectives on paternal investment, infanticide and adoption Reading: Chapter 15 2  Human Mating Patterns Challenging to Study Strongly culturally mediated – Culture is a better predictor but we CAN learn from evolutionary theory – Likely to be changing rapidly (globalization, availability of birth control, etc.) People LIE when asked – Under/over-report sexual activity/partners – Paternity Human sexuality and romantic relationships are complicated! – Focus here is on mating patterns (i.e. child-bearing) 3 Male and Female Fertility Differs with Age 4  Males Paternal investment can be highly variable Expect differences in behaviour based on paternal investment Should prefer female mates that have higher potential fertility – Correlates with female AGE – Prefer indicators of high fertility that can be judged – physical appearance Sexual fidelity should be very important 5  Females Maternal investment in offspring is always high Should be very selective in choosing mates Select mates who can provide resources for them and their offspring Minimal concern about age – show preferences for older males – more resources available Fidelity is important, but less so than for males Conservative in initiating sexual relationships 6  Both Sexes Value physical attraction and love Value stability, pleasantness, sociability and overall compatibility Why? 7 Some Preferences are Culturally Mediated Traits; Others More Universal 8 Preference for number of sexual partners (per month) 9  Human Mating Systems Humans form unions to mate and rear offspring: “Marriage” – usually incorporate exclusive sexual access (but practice often departs from principle) – investment by male and female in offspring – defines social status of offspring Functions: – minimize male-male competition (as does concealed ovulation?) – protect females from mating aggression – maximize paternity certainty (not absolutely though!) Variable in : – number of mates and children – how marriages are regulated (official?, arranged, termination) – relationships between individuals 10 – where they live Are Humans Monogamous? 11  Polygyny Men have multiple wives Varies between men – hunting ability, wealth (land, other resources), political rank all correlated with RS – Wealth and power enable some men to attract more mates and to provide them with resources – in post-agricultural societies, potential for extreme polygyny Recall this means some men have no wife 12  Even with polygyny: – actual rates of polygyny vary widely – often tolerated, but not the norm – men have fewer wives than they aspire to Even with monogamy: – extramarital sex is not infrequent biological father is not the mother’s husband non-paternity highly variable - ~10%? – serial monogamy and frequent divorce 13  Infidelity Major cause of marriage termination across cultures Rates vary greatly between cultures, as does social acceptance Does appear to be more common in males than females Female infidelity ‘matters more’ from a biological standpoint – More restrictions placed on women – Social punishments more severe for women Very challenging to study! 14  Financial Considerations Bridewealth/ Bride Price – Paid TO the family of the bride by the groom/his family – Compensation to the bride’s family for loss of her labour – More common in polygynous societies because man may marry other wives and divert resources to other offspring Dowry – Paid BY the family of the bride to the groom/his family – Rare but more common in highly stratified societies and in monogamous societies – Strategy to marry females to higher status males whose status will be passed on to the offspring 15 Example of Polygyny: Kipsigis Kenya Females marry in late teens Males marry in their 20s Many marriages are arranged Bridewealth is paid in livestock and cash Competition for females is strong (normally receive several offers) 16 Kipsigis Females 17 Polygynous Females – Competition among wives for their mate’s resources – Prefer males who can offer the largest amount of land per wife, regardless of number of wives AND – Males whose co-wives have produced the fewest children – Those women who balance these two considerations have the highest RS. 18 Kipsigis Males 19  Polyandry One woman married to several men Rare (0.5% of human societies) Men in polyandrous marriages have lower RS Fraternal polyandry is most common form in humans – woman marries brothers – reduces the impact on fitness because children he is helping to raise are his brother’s children Share some genetic material 20  Example of Polyandry: Nyinba Nepal Fraternal polyandry where brothers set up a communal household with one woman Mixed industries – agriculture, herding, long- distance trade Paternity is tracked and considered important 21  Paternity Uncertainty Women KNOW who their children are Men don’t necessarily know Degree of paternity uncertainty varies by individuals, cultures, and circumstances Paternity uncertainty is predicted to influence paternal investment and investment of paternal family 22  Paternal Investment Male investment in offspring will vary – are they his biological children? – is their mother his current mate? Balancing current and potential future offspring (old version of textbook) 23 Paternal Investment: New Mexico, US 24 Paternal Investment: South Africa 25 Grandparent Care 26  Human Infanticide Different from non-human primates because can be carried out by the mother Infanticide happens when: – Child is unlikely to survive (birth defects, illness) – Parents cannot care for child (multiple births, lack of resources, mother dies in childbirth) related to social/mating systems rarely give up 1st born children – Child not sired by the husband (similar to primates) Recall limited options in many societies… 27  Human Adoption Offspring raised by someone other than biological parents Seen in other animals, but extremely rare – More common in non-mammals than mammals? Variable rates in humans, but very common in some societies Normally adopted by kin – grandparents, aunts, etc. Idea of adopting non-kin considered odd in many societies 28  Adoption and Relatedness 29 0.25 is the same amount of relatedness as grandparents, aunts/uncles  Adoption When parents are alive, give up children: – reluctantly – because of lack of resources – prefer wealthy relatives with no other children – maintain access to children and can reclaim them Asymmetries exist between resources given to biological and adopted children Why do people adopt non-related children? 30 13. Evolution of Social Behaviour Anthropology 201 Fall 2024  Objectives Define altruism Understand why ‘group selection’ doesn’t exist – Why it can’t explain the evolution of altruism Understand kin selection – Hamilton’s Rule – Kin biases in behaviour Understand the conditions for the evolution of reciprocal altruism Reading: Chapter 7 2  The story so far… We have explained evolution of morphology and behaviour in terms of individual reproductive success – e.g. primate reproductive strategies But primates live in groups, interact socially, and often perform altruistic behaviours Altruism: “selfless concern for the welfare of others”, i.e. an act that benefits others at one’s own cost 3  Example: Grooming Virtually all social primates groom other group members Beneficial for recipient à hygiene, relaxation Costly for giver à time consumed away from other tasks like foraging, courting mates 4  Altruism Other examples: – Alarm calls – Coalitions in conflicts – Food sharing – Alloparenting How do we explain “selfless” acts in terms of fitness? – They decrease fitness of the giver by definition – Should be eliminated by natural selection 5 If natural selection favours individually advantageous traits, how can it explain the evolution of altruism? 6  Social Interactions Most primates are highly social Sociality produces high degree of contact among individuals with varied relationships Tempting to think of mutualistic behaviours as benefiting the group as a whole – But ‘group selection’ generally not supported (natural selection acts on individuals) Need theoretical framework for understanding how individuals act in their own (adaptive) interest in a social context – Fields of Sociobiology, Behavioural Ecology, and for specific research areas, Evolutionary Psychology 7  Social Interactions Often analyzed in terms of costs & benefits to the actor and/or recipient Ultimate goal would be to apply costs & benefits to fitness, but this is difficult for each individual behavioural interaction Can classify interactions as: – Selfish - benefits the actor and costly to recipient – Altruistic (opposite) - benefits recipient and costly to the actor – Mutualistic - beneficial to both – Spiteful (opposite) - costly to both 8  Group Selection Once popular mechanism to explain the evolution of selflessness and altruism Applies Darwin’s postulates to groups, not individuals – Competition among groups – Groups vary in ways that affect their survival – Some of this variation is heritable Those behaviours that benefit the group as a whole should increase in frequency? 9  Example: Alarm Calling If one monkey gives an alarm call, other group members benefit If every individual gave the call, all members would be better off than if no call given Since every individual benefits, alarm calling should be favoured by NS? 10  But selection acts on individuals That reasoning confuses effect on group with effect on individual It doesn’t matter to selection that other members benefit, all that matters is the effect on the caller 11  Example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white) Compare fitness of the two Alarm call benefits all recipients to the same extent (+) Relative fitness of callers and non-callers is the same – so relative frequency of callers would not change by NS In fact, caller bears a cost (-), so non-callers have higher fitness 12  Example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white) Non-callers are favoured Relative fitness of callers and non- callers is still the same – all slightly reduced (-) Here however, non-caller does not bear a cost May even have an advantage, i.e. non-callers still have higher fitness NS should favour non-callers even though the group as a whole benefits 13  An impasse? Natural selection favours individually beneficial traits Group selection cannot explain evolution of altruistic behaviours Q: How does altruism evolve? 14  An answer: Kin Selection 1964, W.D.Hamilton Our alarm call example assumes altruists and non-altruists interact with equal frequency Hamilton’s insight: what if some factor causes altruists to associate selectively with other altruists? – e.g., what if the group’s members are related? – Relatedness may facilitate evolution of altruism – What you do can benefit your kin at your expense 15  Let’s tweak our example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white). Groups consist of full siblings Frequencies of the calling/non- calling genes don’t change, but distribution does In the caller’s group, 5 of 8 siblings have the calling gene (4/8 because of inheritance*, + 1 of the 4 remaining (= population frequency) * For any pair of siblings, 50% chance they share a given allele 16  Let’s tweak our example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white). Groups consist of full siblings 5/8 > ¼ = frequency of gene for calling is higher in the group than in population at large If call is given, more callers benefit than non-callers Caller raises the average fitness of callers relative to non-callers in that group compared with population at large 17  Let’s tweak our example Hypothetical primate species in which calling has a genetic basis, ¼ are “callers” (pink), ¾ “non-callers” (white). Groups consist of full siblings For a non-caller, 7/8 siblings share that gene Only 1 caller in group, less than in population at large Not calling lowers the fitness of non-callers relative to callers in this group compared with population at large 18  Kin Selection When individuals interact selectively with relatives, callers (altruists) more likely to benefit than non-callers (non-altruists) Benefits of calling will favour genes for calling BUT! Calling costly, will be favoured only if benefits sufficiently greater than costs When is this trade-off satisfied? 19  Hamilton’s Rule An altruistic act will be favoured by selection when the following inequality is satisfied: rb > c Where b = sum of fitness benefits to all recipients, c = cost to giver, r = coefficient of relatedness between them 20  Coefficient of relatedness (r) Measures genetic relationship between interacting individuals, or the average probability that they share an allele from a common ancestor Probability that A & B share an allele at a given locus, vs. B & C? Probability that A and B get the same allele from their shared mother = 0.5 * 0.5 = 0.25 Probability that B and C get the A and B are half-sibs, B and C full sibs same allele from their shared mother OR FATHER = 0.25 + 0.25 = 0.5 21 Coefficient of relatedness Relationship r Parent and offspring 0.5 Full siblings 0.5 Half siblings 0.25 First cousins (from full sibs) 0.125 Unrelated individuals 0 r decreases as you become more distantly related to kin 22  Kin Selection Hamilton’s rule (rb > c) has two important predictions: – 1) altruism should be directed towards kin (because r = 0 for unrelated individuals) – 2) closer genetic relatedness allows for more costly altruism e.g. if r = 0.5, then b must be at least (2 x c) to satisfy Hamilton’s rule If r = 0.125, then b must be at least (8 x c) 23  Kin Recognition For kin selection to work, primates must be able to recognize kin – Phenotypic matching = smell or likeness to self – Contextual cues = familiarity, proximity, observe patterns of associations 24  Kin Recognition Easier to recognize maternal kin: using contextual clues, can identify your siblings as those who spend time with mom too Paternal kin: harder to identify, but age may provide clues in polygynous species à age- matched peers likely fathered by same male 25  Kin biases in altruistic behaviours Examples of altruism in primates that provide support for kin selection theory 1. grooming Costly: time-consuming, decreased vigilance Beneficial: hygiene, reinforce bonds 26 1. Grooming more common among kin Rhesus macaques on Cayo Santiago (Puerto Rico) Females groom kin more than non-kin (Hamilton’s 1st prediction) Females groom closer kin more than more distant kin (Hamilton’s 2nd prediction 27  2. Coalitions Primate disputes often between two individuals Sometimes, another individual may come to the support of one of them = coalition – Beneficial to individual who receives aid (may win the dispute or avoid injury) – Costly to the ally (time, energy, may get injured) 28 2. Support directed towards kin 29  2. Male coalitions Males do cooperate, even though they also compete for mates Coalitions last longer and are more intense when males are related (e.g. brothers or half-brothers) Examples: – Coalitions to take over groups – Mutual defense of territory Cooperative breeding (polyandry) is an extreme example where some males suffer big declines in RS – But offset by supporting offspring of close male relatives30  Parent–Offspring Conflict Kin selection helps explain why there is conflict between parents and offspring – Offspring shares genes with siblings, but only 0.5 or 0.25 Fitness of future offspring comes at expense of current offspring Mother wants to invest in future offspring Current offspring want mother to continue to invest in them! 31  What about interactions with non-kin? Reciprocal Altruism = altruism can evolve even among non-kin if the behaviour is balanced between partners over time – Take turns giving and receiving Requirements: – 1. Must have opportunities to interact often – 2. Must be able to keep track of support given & received – 3. Must provide support only to those who help (satisfies the selective association condition for altruism to evolve) Reciprocity, not kinship, drives this type of altruism “Tit for tat” – same or different currency can be used in exchange (e.g. grooming for meat sharing) 32 Example: Coalition recruitment in vervets Tape-recorded recruitment calls in pairs with or without prior grooming Rates similar for kin Non-kin more likely to respond if recently groomed by caller Note difference in exchange currency! 33 Example: Food sharing in chimpanzees Chimps will sometimes share food Most successful at getting food if you recently groomed the possessor 34  Take-home Message Altruism can evolve through natural selection (not group selection), so long as altruists are more likely to associate with other altruists (whether kin or unrelated) = nonrandom social interactions Will increase Inclusive Fitness Relative genetic contribution of the individual plus the contribution of close relatives 35