Mating System

Hierarchical Promiscuity

Although promiscuous, most mating occur within the context of consortships, suggesting some degree of pair bonding (Stanford, 1996). Small (1988) claims that chimpanzees go against traditional promiscuous or polygynous theory because males do not show direct parental care. Also, close male relationships prevent infanticide, thus there is no need to confuse paternity. Females incur costs for non-conceptive matings by interacting with potentially aggressive males and limiting the amount of energy they can expend in foraging. Females invest more per gamete, but it is important to remember that sperm is not unlimited. Therefore, female chimps keep a steady supply of semen in their reproductive tracts. Also, repeated matings or non-fertile matings reduce sperm supplies for other sexually receptive females, making it beneficial for females to have sex frequently. Additionally, females are not necessarily uninterested in sex when not in estrous, the males are simply unattracted.

Kin Selection

Kin selection may have influenced the evolution of chimpanzee social structure because degrees of relatedness among males, who are philopatric, and females, who disperse, are significantly different (Morin, et. al., 1994). Genetic relatedness is not the only factor in determining party associations, but the age and sex of the other chimpanzee is also considered. However, other things being equal, investment should be concentrated on the most closely related relative (Richard & Schulman, 1982). Morin, et. al., (1994) further suggest that social behavior in chimpanzees may be partially explained by kin selection theory. Gombe, the famous chimpanzee study site, shows deficiency in heterozygosity, as expected if there is inbreeding or high male-male relatedness. It has been determined that males at Gombe are on the level of half-siblings. The high within-community variation is attributable to gene flow from female migration, and historical gene flow has been significant. Genetic erosion, such as genetic drift and inbreeding, may increase significantly, yet habitat loss and human-borne diseases pose a more serious threat.

Infanticide

Infanticide can serve a number of purposes. It can provide food, reduce local competition, or concentrate investment on other offspring. It can also allow sexual selection by killing the offspring of rivals and can induce estrous in females (Richard & Schulman, 1982). In chimpanzees, infanticide is likely used to increase future mating opportunities for males (Wilson, et. al., 2001). Males are unlikely to kill young within the community, but have been known to attack infants near the borders. However, occurring within the group is female infanticide, and this also reduces future competition (Case, 2004).