Mating systems- the way animals find mates and care for offspring, plays a crucial role in understanding primate societies
“Understanding the diverse reproductive strategies of nonhuman primates illuminates human evolution because we share many elements of our reproductive physiology with other species of primates.”
The Language of Adaptive Explanations
Strategy refers to behavioral mechanisms that lead to particular courses of behavior in particular functional contexts, such as foraging or reproduction
The terms cost and benefit refer to the effect of particular behavioral strategies on reproductive success
Beneficial: increase genetic fitness of individuals
They can easily use their resources to acquire many additional matings
Caring for their offspring would not appreciably increase the offsprings fitness
Ideas of investing and non-investing male parents deal with the idea that caring for offspring takes time and effort away from competing for access to females, getting food, etc.
Unequal parent investment (one cares for offspring more htan other) favored whne one or both of these is true:
Acquiring additional mates is relatively easy, so considerable gains are achieved by allocating additional effort to attracting mates
The fitness of offspring raised by only one parent is high, so the payoff for additional parental investment is relatively low.
The mammalian reproductive system commits primate females to investing in their offspring; females lactate and males do not
Female primates invest heavily in each of their offspring because:
Pregnancy and lactation are time-consuming; pregnancy takes so long b/c brain tissue development a slow process
It takes so much time and energy, so females can only have a few offspring in their lifetimes
A female’s reproductive success depends largely on her ability to obtain enough resources to support herself and her offspring
Changes in resources can affect females’ ability to reproduce or support their offspring
Sources of variation in female reproductive performance: high-ranking females tend to reproduce more successfully than low-ranking females (have high priority access to valued foods)
Reproductive trade-offs: females must make a trade-off between the number of offspring they produce and the quality of care that they provide
There is sometimes conflict between mothers and infants over the amount and extent of maternal investment.
Parent-offspring conflict: a fundamental asymmetry in the genetic interests of mothers and their offspring
Sexual selection leads to adaptations that allow males to compete more effectively with other males for access to females
Sexual selection often much stronger than ordinary natural selection
Two types of sexual selection:
Intrasexual selection resulting from competition among males; in species in which females cannot choose their mates, access to females will be determined by competition among males
Intersexual selection resulting from female choice; selection will favor traits that make males more attractive to females.
Competition among males for access to females favors large body size, large canine teeth and other weapons that enhance male competitive ability.
The fact that sexual dimorphism is greater in primate species forming one-male, multimale groups than in monogamous species indicates that intrasexual selection is likely the cause of sexual dimorphism in primates
In muiltimale, multifemales groups, where females mate with several males during a given estrous period, sexual selection favors increased sperm production.
Intersexual Selection
Favors 3 kinds of traits in males:
Traits that increase the fitness of their mates
Traits that indicate good genes and thus increase offspring fitness
Monogamous pair-bonding is generally associated with high elvels of paternal investment. In species with this, males do not compete directly over access to females.
In cooperatively breeding species, males invest heavily in offspring, but the reproductive benefits to males are not clear.
Male-male competition in non-monogamous groups
In non-monogamous groups, the reproductive success of males depends on their ability to gain access to groups of unrelated females and to obtain matings with receptive females.
In species that normally form one-male groups, males compete actively to establish residence in groups of females
Residence in one-male groups does not always ensure exclusive access to females
For males in multimale groups, conflict arises over group membership and access to receptive females
There is growing evidence of substantial variation in the reproductive success of males over the course of their lifetimes.
Infanticide
Sexually selected male reproductive strategy
Associated with changes in male residence or status
Males kill infants whose deaths hasten their mother’s resumption of cycling
Males kill other males infants and not their own
Infanticidal males achieve reproductive benefits
Can be a substantial source of mortality
Females have evolved some responses (support of other males, confusing males about paternity of infants)
Altruistic: costly for actor, beneficial to recipient
Mutualistic: beneficial to all
Spiteful: costly for all
Altruism: A Conundrum
Altruistic behavior cannot evolve by ordinary natural selection because it decreases the fitness of the individual performing the behavior.
Primates perform altruistic behaviors in nature
Altruistic behaviors cannot be favored by selection just because they are beneficial to the group as a whole
Kin Selection
Natural selection can favor altruistic behavior if altruistic individuals are more likely to interact with each other than chance alone would dictate
When individuals interact selectively with relatives, altruistic acts are more likely to benefit the genes of those for whom the act is costly
Hamilton’s Rule: Hamilton’s theory of kin predicts that altruistic behaviors will be favored by selection if the costs of performing the behavior are less than the benefits discounted by the coefficient of relatedness between actor and recipient. According to Hamilton’s rule, an act will be favored by selection if:
rb > c
r = average coefficient of relatedness between actors and recipients
b = sum of fitness benefits to all individuals affected by behavior
c = fitness cost to the individual performing the behavior
Two insights from H’s rule: altruism is limited to kin; closer kinship facilitates more costly altruistic actions
Evidence of kin selection in primates
Primates may use contextual cues to recognize maternal relatives (phenotype matching)
Phenotype matching may play some role in paternal recognition
