The sex difference in body composition, with females having greater body fat accumulation than men, seems to transcend culture and time (101). Moreover, such a marked sex dimorphism in body fat distribution is rather unique to the human species (1). The evolutionary mechanisms underlying these dimorphisms and their teleological significance will be briefly discussed in this section.
Hoyenga put forward a theory stating that through sexually dimorphic natural selection, females were placed under more pressure to survive in times of short food supplies than men (101). According to this theory, females would have evolved ways to conserve calories and withstand starvation, leading them to accumulate more fat in times of abundant food supplies. Though very interesting, this theory could, however, be better extended by focusing on mechanisms of sexual selection, which involve reproductive success, rather than mechanisms of natural selection, which involve likelihood of survival. Darwin's original natural selection theory could not explain extravagant traits that actually decrease survival ability (e.g., peacock's tail), and this led him to reason that in a sexually reproducing species, any heritable trait that could help compete for sexual mates would tend to spread through the species, even if they somewhat compromise survival (102). According to this concept, mate choice and competition for mates could have shaped organic form to maximize reproductive success. According to Miller (102), some traits would have evolved as aesthetic displays and sometimes also as indicators of fertility. The pattern of body composition and fat distribution may be one of these sexual selection-driven traits.
In this regard, reproductive roles possibly represent the origin of the body composition and fat distribution dimorphism in humans. Trivers (103) defined the concept of parental investment as "any investment by the parent in an individual offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring." This concept includes metabolic investment in generating gametes, gestation, and feeding and guarding the offspring (103). According to this theory, women would have evolved to maximize reproductive success through parental investment. The presence of larger body fat reserves in women is consistent with these reproductive roles, which are very demanding from the energetic standpoint. Peripheral subcutaneous adipose cells are also fairly stable in response to physiological lipolytic stimuli (31), which is consistent with these depots being able to provide energy supplies in the long term (Fig. 5).
According to Trivers (103), the sex whose typical parental investment is highest becomes the limiting resource, and individuals of the sex investing less will compete among themselves to breed with
members of the sex investing more. Quite consistently, male gamete production is not an energy demanding, nor limiting process. Rather, typical male reproductive roles involve searching, fighting, and competing for mates (102-104). Hence, men would have evolved to maximize reproductive success through better motor performance. This is consistent with a higher body fat-free mass/body fat mass ratio in men (105). The presence of central, visceral fat depots that are rapidly mobilized due to high responsiveness to catecholamine stimulation is also consistent with these reproductive roles (Fig. 5). In addition to providing readily available energy resources, the predominance of central fat depots would have only a modest effect on the center of gravity (1).
These traits apparently coevolved in males and females both as courtship traits (e.g., breasts and buttocks in females, upper-body mass in males) and indicators of nutritional/reproductive status, in a context where males compete for females, who in turn select mates among the males that they attract (102).
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