Human and Primate Life History and Reproductive Strategies
In general, primates are relatively large, long-lived mammals with big brains and complex social behavior. Humans are longer lived than our closest primate relatives, the chimpanzees and gorillas, and other mammals of similar body size. Modern humans in industrialized countries can now live up to 30 years or more past the peak reproductive years. Humans also have a long reproductive life span characterized by large investment in each offspring, with females generally investing more than males. As in the great apes, reproduction is costly energetically and pregnancy and childbirth relatively risky. Females must make trade-offs between the number of offspring produced and the quality of care provided. For example, women with high parity often have poorer health and shorter life spans.
Investment in an unsuccessful reproductive episode is also costly in terms of lost alternative reproductive opportunities. The seasonal timing of reproduction can be critical in terms of the availability of mates or food, as can the timing in terms of the life course: very young and very old women, for example, are not as successful on average at conceiving or bringing a pregnancy to term. Males, on the other hand, may incur their biggest challenges in establishing mating relationships and obtaining access to fertile, receptive females.
Primate reproductive patterns are expected to vary with environmental conditions and population variables (including population density). Reproductive variation can take the form of adaptive behavioral, developmental, or physiological responses to environmental cues (phenotypic plasticity)—all limited by the genetic constraints of the population. When environmental variation exceeds the ability of animals to respond adaptively within phenotypic norms, natural selection (in the form of mortality or reproductive failure) will act to produce changes in gene frequencies and, with these, new phenotypic norms.