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Introduction to Biological Anthropology: Notes 14
Mating: Primate females and males
 Copyright Bruce Owen 2011
− We want to understand the reasons behind
− the lifestyles of our non-human primate relatives
− their physical traits, both the ways they are similar to ours and how they are different
− their behavior, both how it is similar to ours and how it is different
− that is, we want to understand the evolutionary pressures that led various primates to become
the way they are
− if we develop tools to understand how natural selection has shaped non-human primates, then
− we can use those tools to understand the evolution of our own ancestors
− and infer things about our ancestors that are not directly preserved in the fossil record
− like aspects of their behavior
− their cognitive (thinking) abilities
− the size and organization of the groups they lived in
− Reminder of a key concept: reproductive success
− what drives evolution is variation in reproductive success
− reproductive success: the number of offspring that an individual has that are fertile and
survive to reproductive age
− reproductive success is the net result of an individual’s success at:
− surviving
− being healthy enough to be able to mate and have offspring
− successfully competing for mates
− caring for offspring so they make it to reproductive age
− parental care is important in some species, and less so in others
− plus anything else that affects the number of surviving offspring
− so to explain traits (physical and behavioral) of non-human primates…
− we must understand how they affect reproductive success
− by improving (or reducing)
− the individual’s chances of survival
− overall health
− success in mating
− survival of offspring, etc.
− mating and parenting behavior must have a big effect on reproductive success
− so mating and parenting behavior should be particularly easy to explain in terms of costs
and benefits to reproductive success
− Evolution can affect only heritable traits
− that is, physical traits and heritable behaviors or general tendencies
− NOT specific, learned behaviors
− since learned behavior is not inherited by the offspring, explanations based on
reproductive success don’t apply to behaviors that are learned
− so we use evolutionary reasoning to explain only
Intro to Biological Anthro F 2011 / Owen: Mating: Primates females and males p. 2
− general, heritable, inborn tendencies
− including the ability to learn
− but not the specific things that are learned by individuals
− But first, a word about explaining behavior
− Many species have behaviors typical of most individuals of the species
− dogs act like dogs; cats act like cats
− some animals live in large groups, other species are solitary
− some defend a defined territory, others don’t
− and many, many others
− so these behaviors must be to some degree heritable, that is, at least partially determined by
genes
− so we should be able to understand the evolution of behaviors in the same way as we
understand the evolution of physical characteristics
− usually by looking at the relative cost and benefit of a given behavior
− “cost” means how something reduces an individual’s reproductive success
− “benefit” means how something increases an individual’s reproductive success
− just as we looked at the costs and benefits of beak depth among finches
− if the benefits of a behavior outweigh the costs, the net effect is an increase in the
individual’s reproductive success
− if there is a genetic component to the behavior, then it will become more common with
each generation
− so we can “explain” behaviors by figuring out how they create a net increase to the
reproductive success of the individuals who do them
− Explanations in terms of “strategies”
− strategy: a behavior that is viewed (or “explained”) in terms of its results.
− example: howler monkeys eat small amounts of many different leaves, rather than gorging
on any one kind
− effect: ensures that they get all their different needed nutrients, and reduces exposure to
any particular plant toxin
− this is a “strategy” of eating wide variety of foods
− as if the howlers were doing it for the purpose of getting varied nutrients and avoiding
toxins
− This does NOT mean that animals make conscious choices based on reproductive success
calculations!
− it means only that these behaviors have this effect
− if the behavior is common, presumably it must have improved reproductive success
− it does not matter what physiological or psychological mechanism causes the behavior
− continuing with the howler monkey’s “strategy”…
− maybe they have small stomachs, so they get uncomfortable if they eat very much in one
sitting, so they tend to move and encounter some other kind of food
− maybe their taste receptors respond less after they have been stimulated for a while by a
given food, so any given food provides a positive sensation only for a short time, so they
move on to something else
Intro to Biological Anthro F 2011 / Owen: Mating: Primates females and males p. 3
− maybe howlers have a tendency to be skittish and nervous, so they rarely stay in one
place long enough to eat much of a given kind of food
− the exact mechanism could probably be figured out by careful observation or laboratory
study,
− but for the purposes of evolutionary explanations, it does not matter
− all that matters for the purposes of explanation using evolutionary theory is that the
“strategy” affects reproductive success
− as in “it makes them healthier, so they have more offspring”
− ideally, we would actually measure how much the strategy improves individuals’
reproductive success
− but this is hard to do in practice
− so we often assume that if a behavior improves survival or health, it probably improves
reproductive success
− Male and female reproductive strategies differ
− Female strategies are fairly consistent across different species
− females have to support the offspring during pregnancy and lactation
− so females have to invest a lot in each infant
− Male strategies vary a lot from one species to another
− males can reproduce by simply mating and leaving, investing nothing more at all in the
offspring
− or they can invest some or a lot of energy in the offspring
− defending the female and offspring’s access to food or water
− defending the offspring from predators or infanticidal males
− carrying the offspring from one feeding or sleeping place to the next, etc.
− Summary of female mating strategy
− a female can only have a limited number of offspring in her lifetime
− because pregnancy and nursing take a long time for each one
− so effectively all females can easily get a mate when necessary
− since a female only needs to get pregnant a few times in her life
− and it is to the benefit of any male around to be the father
− so among females, there is not much variation in success at mating
− so there is not much selection pressure for females to attract mates in general
− females of some species may be picky about choosing a mate (depends on species and
possibly other factors)
− if the offspring’s success varies a lot depending on the father, then selection will favor
female traits that help get the best mates
− this apparently only happens in certain species… more on this later
− what most affects female reproductive success is how much food she gets
− allowing her to provide plenty of energy to the fetus
− and then plenty of milk for the infant
− keeping her healthy and strong to carry the infant, defend it, etc.
− so natural selection on females has most strongly favored behavior that increases her
access to food
Intro to Biological Anthro F 2011 / Owen: Mating: Primates females and males p. 4
− Summary of male mating strategy
− males can have almost unlimited numbers of offspring
− since males are not forced to invest a lot in their offspring
− so male reproductive success can vary over a much wider range than females’
reproductive success
− in a typical species, females might range from zero to five offspring
− in the same species, males might range from zero to fifty offspring
− so the payoff in reproductive success for frequent mating can be enormous
− The more a male mates, the more offspring he has
− investing in any one offspring (carrying it, defending it, shooing others away from its
food, etc.) increases the male’s r.s. only slightly, by slightly increasing the odds of
survival for that offspring
− but mating again might add an entire additional offspring to the male’s r.s.
− so in mammals such as primates, natural selection on males has usually most strongly
favored behavior and physical traits that increase mating
− Female reproductive strategy basics:
− female mammals are obliged to invest a lot in each offspring
− they have to pay the energetic costs of:
− pregnancy
− lactation (nursing)
− both require the female to find and consume more food than she would otherwise
− she has to travel more to do so
− while carrying the dependent suckling infant
− because of all this, exposing herself to greater risk of malnourishment or predation
− because of the duration of gestation and lactation, females can only have a limited number
of offspring in their lifetime
− unlike males, who can have very large numbers of offspring
− so each offspring is a big part of the female’s total reproductive success
− example:
− say a typical female can have five offspring
− then if just one dies, her reproductive success is reduced substantially (20%)
− biology requires a female to “put all of her eggs in just a few baskets”
− vs. a male, who could theoretically have hundreds of offspring
− so the survival of one more or less doesn’t make much difference
− so among females, selection should favor traits that improve the chances of each offspring
surviving to reproductive age
− a female’s reproductive success can vary a lot depending on her ability to get enough food
(and possibly other resources, like safe sleeping places, etc.) for herself and her infant
− females must be reasonably well nourished to be fertile (to be able to get pregnant)
− they must get sufficient food to have a successful pregnancy and to produce enough milk
− they must be sufficiently well nourished to watch, carry, and defend the infant
− field observations provide evidence that that finding enough food really is a limiting factor
in female reproductive success
Intro to Biological Anthro F 2011 / Owen: Mating: Primates females and males p. 5
− in places where wild primates have been fed by people, populations shoot up
− due to increased production of offspring (not just immigration of animals from other
areas)
− because better-fed females mature faster, live longer, and produce more offspring
separated by shorter intervals
− so food was the limiting factor in female r.s.
− in one case, among baboons in Amboseli National park in Kenya, when environmental
degradation reduced food supplies…
− female birth rates and infant survival rates declined
− so food was, again, the limiting factor in female r.s.
− So, natural selection must have favored female behavior that maximizes access to food
− so there is often strong competition for resources (especially food, but also water, sleeping
spots, etc.) between females of the same group
− Female competition
− it often happens that two females will both want the same resource
− especially food, but also a good spot for drinking water, a resting place, etc.
− this is contest competition: competition in which one individual gets something, and
the other loses it
− a “zero-sum game”
− versus scramble competition: competition in which each individual goes out and
independently gets as much as it can
− as in collecting seeds from a field of grasses
− how successful one is does not affect the success of any other
− yet some might end up being more successful
− because they have better eyesight, work longer, or whatever
− if one individual consistently wins over the other in contest competition, then it is the
more dominant of the two
− the other is the more submissive of the two (in that pair)
− in some species, dominance may be independently negotiated between pairs
− in this case, there is no pattern of which individual is likely to be more dominant
− in other species, certain females may be dominant relative to many others
− if, so, there is a dominance hierarchy
− in the most clear-cut cases, there is a single pecking order from least to most
dominant
− this is a transitive dominance hierarchy
− if A beats B, and B beats C, then A will beat C; A>B>C.
− in less “transitive” systems, this might not always hold true
− species vary in how strong and transitive their dominance hierarchies are, or even if they
have them at all
− species vary in how long-lasting and permanent their dominance hierarchies are
− in some species, these relationships are very long-lasting
− in others, they are readjusted more or less frequently as individuals age, get sick, etc.
− the most clearly defined, stable, transitive dominance hierarchies among females occur in
primates in which there is the most within-group contest competition for food
Intro to Biological Anthro F 2011 / Owen: Mating: Primates females and males p. 6
− effects of higher dominance rank for females:
− higher-ranking females get greater access to preferred foods
− (obviously, since this is how dominance rank is defined)
− in some species, higher-ranking females have been shown to have greater reproductive
success
− as measured by things like age at first birth (younger is better, from an evolutionary
point of view)
− interbirth interval (shorter is better)
− births per year (more is better)
− infant survivorship (more survivors mean better success)
− there are some species in which the dominant individuals do NOT have higher
reproductive success, but these are rare cases
− for now, we will ignore these exceptions
− Question: if more dominant females have higher reproductive success, why do any
females ever behave submissively?
− because losing a dominance fight could be costly
− possible injury
− energy wasted
− so selection should favor females that
− strive to be dominant over others (to win in contest competition)
− but are able to recognize when they probably won’t win, and back down submissively
with minimal cost
− you can see how selection could favor greater intelligence in order to make these
judgments accurately
− to more successfully play the “politics” of female dominance hierarchies
− this was probably one factor in increasing primate intelligence
− Female strategies involve a trade-off between the amount of time the female can invest in
each infant and how many infants she can have
− there must be an optimum somewhere between two extremes:
− investing all her effort in just one offspring
− having the maximum number of infants by abandoning each at birth and immediately
conceiving another
− female primates really are observed to regulate their investment in offspring
− initially, they care for the infant a lot
− stay in full-time contact with it, carry it around, etc.
− as the infant matures, the mother starts to cut down her investment
− she is less cooperative when the infant wants to suckle
− does not always pick the infant up when it is time to move, or is less cooperative with
carrying it
− this happens as the infant is getting bigger and heavier
− so the cost of producing more milk for a bigger infant is rising
− and the cost of carrying it is rising
− at the same time, the infant is more able to care for itself
Intro to Biological Anthro F 2011 / Owen: Mating: Primates females and males p. 7
− so the benefit of caring for it is getting less
− the mother has to wean the infant (tapers off nursing it) in order to have another
− because lactation inhibits ovulation
− although I wouldn’t count on it if I were you…
− this produces an interesting conflict of interest between mothers and infants
− the mother would maximize her r.s. by
− giving just enough investment to each infant to maximize her total reproductive
success
− so she should eventually limit her investment in one infant so she can have another
− the mother will want to wean the infant and cut back on carrying it around, watching
it, etc.
− the infant would maximize its r.s. by
− getting the maximum possible investment out of the mother
− even though this is at the expense of previous and potential future offspring of the
mother: the infant’s own siblings
− the infant would do better if the mother kept feeding and caring for it indefinitely
− so selection will favor infants who use any possible method to get the mother to
provide more care
− crying, whining, clinging, manipulating, acting more helpless than it really is, etc.
− including at the expense of other siblings
− so there is an evolutionary reason for both
− mother-child conflict
− and sibling rivalry, competing for the mother’s attention and support
− This applies to humans, but be cautious
− we definitely have some heritable, inborn behavioral tendencies
− for example, most individuals seek sex
− most females care for their children
− and many other human tendencies
− these behaviors are
− so universal that they must be at least partially genetic
− and so obviously related to reproductive success that they must be strongly affected by
natural selection
− but our inborn tendencies are shaped by a lot of learned behavior and very complicated
individual thinking
− there is a raging debate about how far to take evolutionary explanations of behavior in
humans… just keep that in mind