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Lecture 13

Sexual Dimorphism and sexual selection

Sexual dimorphism largely results from sexual selection (see handout). It is easy to understand how natural selection can favor longer legs and sharper fangs in animals. It is difficult to understand why selection would favor the male peacock's tail.

Examples of sexual dimorphism (slides): Elephant seals (males much larger than females); baboon canines (male canine twice as large as female's); cottontop tamarin (females slightly larger than males); male lizard throat sack; polyandrous phalaropes (females have brighter plumage than males); uakari and mandrill (males in both species brightly colored); bower bird (male builds bower to attract female); human haircuts and ornamentation.

Darwin proposed that sexually dimorphic characteristics are related to a critical hostile force: mate shortages. Sexual selection is a form of natural selection. Rather than favoring traits that improve an individual's chance of survival or getting food, sexual selection favors traits that improve an individual's chances of locating, attracting, and fighting for access to mates. The intensity of sexual selection depends on the kind of mating system.

Several factors are related to sexual dimorphism. These are:

(1) Mating system: Polygynous species are more sexually dimorphic than monogamous species. However, few species are highly polygynous and male-male competition is difficult to investigate in nocturnal primates.

(2) Larger species are more sexually dimorphic than smaller species. Luetenegger has investigated this allometric relation but its causes are unclear.

(3) Terrestrial species tend to be more sexually dimorphic than arboreal species. Perhaps male-male competition in terrestrial species is more dependent on body size. A notable exception to this pattern is the orang. Orangs are arboreal and polygynous. Males are much larger than females. A male's territory encompasses several females. (Orangs have long birth intervals, five to six years.) Males defend their territories from other adult males. The dominant male tolerates subadult males in his territory. Occasionally, the "beta" males (subadults) coerce females to copulate—even young males are much larger than females. These force copulations do not typically occur when the female is in estrus.

Sexual dimorphism is greater in uni-male societies than in multi-male societies. Perhaps male-male competition is equally intense but males in the two social systems compete in different ways. In a uni-male society, male body and canine size may be important in acquiring and guarding a harem. In a multi-male society, coalitions of cooperating males may be important in defeating dominant males and acquiring matings. In a multi-male society, perhaps, intelligence rather than size is more important.

Mating systems and environment

Why do anubis and hamadryas baboons have different mating systems? Recall that they live in different environments. Environment has a strong influence on mating system.

Polygynous Systems

Resource-defense polygyny. If critical resources (i.e., fruit trees, water holes) are unequally distributed in an environment, males may be able to monopolize and defend them. Males that defend these resources will attract mates. Males without resources will not have a mate. Males with sufficient resources may attract additional mates. A female may join a second female and a territorial male if that male has many more resources than territorial males without mates.

Example: Red-wing blackbirds. During the mating seasons, male blackbirds establish territories. Females chose males by the size and quality of their territories. A female may choose to join a male who already has a mate if that male has a significantly larger territory than other males. The polygyny threshold refers to the size of territory a male must have to make it worthwhile for a female to share his territory with a second female. If his territory is sufficiently larger than the polygyny threshold, two females will share it.

If resources are distributed unequally, females, especially primate females who need resources to rear their young, will be distributed unequally (clumped). Access to females is a critical factor in male reproductive success so males will also be unequally distributed.

Mate-control polygyny. Males coerce females into polygynous relations. Dominant males can herd females into harems (e.g., hamadryas baboons). Or males exclude males from their harems (e.g., gorillas and orangs). Females become "defensible."

Monogamous Systems

Monogamy may be a consequence of high parental care an infant requires. But it also occurs in environments where resources are evenly distributed (females are spread out).

Dispersal patterns and philopathy

Philopathy means "love of place." Female philopathy occurs in almost all primate species. Females stay in their natal group (group they were born into), males move out. In some species, young males are kicked out of the group. But in most species, young males are not forced out but migrate voluntarily.

The evolutionary function of this dispersal is to prevent inbreeding. Inbreeding increases the chances that offspring will be born with genetic disorders. By moving away from their group, males and females avoid inbreeding. But why do males disperse and not females? Or, put another way, why do females stay together?

Wrangham hypothesized that females in most primate species need many resources to support their offspring. Primate offspring tend to be altricial rather than precocial. They are born helpless and are dependent on their mothers for several years. Wrangham suggest that females, rather than males, should be most concerned about acquiring and maintaining resources. A solitary female, without her kin, is less able to defend a critical resource such as a fruit tree than a band of related females. Females should be expected to remain together and cooperate in defending a tree or territory from other groups.

How do we test this hypothesis? Not all primate species are female philopathic. In chimps, humans, red colobus monkeys, hamadryas baboons, and spider monkeys, females disperse and males stay together. In chimps and humans, coalitions of males do not defend resources, they defend females against males of other groups. Gibbons are neither female philopathic nor male philopathic. In this monogamous species there is no advantage of either sex of offspring to stay with their parents. Both male and female offspring move away and avoid encroaching on their parents' territory. To test Wrangham's hypothesis we can compare species with and without female philopathy. More on Thursday.

Gorilla