Hypothesis for the Evolution of Human Breasts and Buttocks

Human females are unique in the concentration of fat deposits in breasts and buttocks. Although breasts develop in some primates during first pregnancy, human females are further distinguished by breast development during puberty, usually several years before pregnancy. Humans have been interested in breasts and buttocks, in one way or another, for a long time, as illustrated by the so-called Venus figurines of Upper Paleolithic Europe. More recently attempts have been made to explain the evolution of these unusual anatomical features. Here I present an integrated hypothesis, proposing that breasts and buttocks evolved to signal the female's nutritional state to males, in the context of facilitating female choice of mate. High male parental investment (Trivers 1972) is critical to this point of view. I reach the conclusion that humans stand out against a general background of sexual selection wherein many animals exhibit male ornaments to attract females. In at least some cases the connection between the ornaments and reproductive success is dubious (Dawkins 1976, p. 170). In humans, regardless of whatever male traits may have evolved to attract females, females possess ornaments whose signal meaning is directly related to factors bearing on reproductive success. Before proceeding it is useful to review some existing explanations. Bernard Campbell (1974, p. 304), for example, believes the traits to be sexual attractants: if they did not increase the chances of a particular female's finding a mate, they did increase the chances of her mating with a genetically better endowed male. " R. V. Short (1976) also focuses on intersexual attraction, and presents a more complex argument: As brain size increased during human evolution so did infant dependency, thus posing increasing constraints on the mother's freedom of movement and necessitating male parental investment in her offspring. Breasts function to attract a male and promote a pair-bond with him because "it makes evolutionary sense for the woman to become sexually attractive to the man at an early age, so that she can establish a pair-bond with the future father of her child before she becomes fertile" (p. 8). Finally, Morris (1967) believes that breasts mimic fleshy buttocks, thereby providing a clear face-to-face signal in upright bipeds, but he does not explain the evolution of buttocks in the first place. It is not clear from these and similar explanations how natural selection might have produced males with an interest in breasts and buttocks. Just how could such a preference contribute to male reproductive success? We can proceed by the following steps: (1) Female choice of mate is always favored so long as there is variation in male qualities affecting the female's offspring. There appear to be two different kinds of qualities. The first is the male's genetic endowment because a female's offspring receive, at least in mammals, one-half their genes from the father. (Doubts about the "good genes" criterion in female choice are discussed by Williams [1975] and Maynard Smith [1978].) The second sort of quality concerns parental investment (PI) in offspring by the male. Parental investment is