Male/female Strategies
Female
Females choose mates based on prior affiliative patterns (Stanford, 1998). Female chimpanzees also induce a state of stress in males in order to expose individuals that may have multiple diseases (Freeland, 1976). Also to a female’s advantage, births are aseasonal and ovulation occurs at the end of swelling, allowing them to exploit their sexuality for a longer period of time and select mates accordingly (Stanford, 1998). Females guard offspring so strictly to reduce contact while infants have low immunological competence (Freeland, 1976), but this could also be a tactic to reduce infanticide. Females avoid physical attacks (Wilson, et. al., 2001), but will protect offspring in agonistic encounters (Richard & Schulman, 1982). Female chimpanzees avoid competition and agonistic encounters, and are essentially only concerned with finding food and protecting their offspring.
Male
Male groups, which may increase fitness, have been thought to be “macro-coalitions,” usually with kin, used in order to gain political power (Te Boekhorst & Hogeweg, 1994). Male chimps defend females rather than specific geographic boundaries (Wilson, et. al., 2001). They defend females in order to have mating privileges, but could also defend them because if she mates with foreign males there are risks of disease contamination (Freeland, 1976). Male defense of the territory is crucial. The male dominance hierarchy is based on mating success, but higher ranking individuals are not required to defend more than lower ranking ones. However, dominant males advertise their presence more than subordinates. In response to an intruder, females will remain stationary or retreat, but males become aggressive and will approach if current party size is larger than three. In this case, they travel in single file toward the intruder in a random order (Wilson, et. al., 2001). Male chimpanzees participate in activities in order to increase mating opportunities.
Computer model
Te Boekhorst & Hogeweg (1994) developed a computer model based solely on the search for food in females and the search for sex in males. The model shows the same fission-fusion pattern as in the wild. Males are most likely to increase selected matings, while females are more likely to increase food intake and therefore improve reproductive success. Mutual control of females in estrous causes aggregations, a form of the “dear enemy effect.” Most interestingly, this model perfectly replicates chimpanzee social organization, yet uses no higher-level decision making, a trait possibly over-emphasized in chimpanzee existence.
Image adapted from: Te Boekhorst, I.J.A. & Hogeweg, P. 1994. Self-Structuring in Artificial “CHIMPS” Offers New Hypotheses for Male Grouping in Chimpanzees. Behaviour, 130 (3-4), 229-252.