Reproductive ecology of human males

Abstract

Recent demonstrations that female reproductive physiology is highly responsive to environmental factors have led to a growing appreciation of the extent of variation in reproductive function and the adaptive nature of much of that variation. Male reproductive function has received much less attention from anthropologists, whether as an aspect of human variation, as a possible contributor to population variation in fertility, or as the product of selective pressures. Yet, an understanding of male reproductive function, along with that of females, is obviously important for a complete picture of human evolutionary ecology. We review our current understanding of male reproductive function, its response to environmental factors, and implications for male fecundity. Studies among Western populations indicate that male reproductive function is sensitive to environmental stresses, including weight loss, dietary composition, exercise, disease, and psychological stress. Indices of reproductive physiology among males in some non-Western populations are consistent with the notion that chronic environmental stresses can lead to suppression of testicular function. The specific causes of this suppression are not known, nor are the implications, if any, for male fecundity and fertility understood. Further research on males is called for, including the effects of hormonal changes on reproductive behavior and characterization of reproductive function in a wider range of populations. Evolutionary models of the selection pressures underlying male reproductive function should be helpful in focusing these investigations. Despite initial skepticism, recent work in the reproductive ecology of females has demonstrated convincingly that environmental circumstances can have a significant impact on reproductive function (see Ellison [1991, 19941 for a recent comprehensive review). In contrast, the effect of ecological factors on male reproductive function has barely been explored. There is substantial evidence from Western populations (reviewed in the following sections) that testicular function can be suppressed by exercise, weight loss, diet, and psychological stress. Furthermore, testicular function in some anthropological populations is suppressed or truncated compared to that of Western populations (Bentley et al., 1993;KL Campbell, 1994;Christiansen, 1991;Ellison et al., 1989). Yet we remain largely ignorant of the impact of environmental factors on reproductive function among males in non-Western populations.The lack of attention to male reproductive ecology stems from a long-standing orientation toward females in demographic studies of fertility, justified by the observation that women are much more likely than men to be the limiting factor in fertility. In addition, two-sex models of fertility or population replacement were 0 1995 Wiley-Liss, Inc.

Predictors of fecundability and conception waits among the Dogon of Mali

“…In this study, however, the potential for such a bias was lessened by the exclusion of women who were prematurely sterile (younger than age 42 but had not given birth in the past 5 years). Although we cannot identify the mechanism behind the effect for husband's age, these results empirically support the suggestion (Campbell and Leslie, 1995) that human reproductive ecologists need to consider the male contribution to reproduction.…”

Predictors of South African male testosterone levels: the THUSA study

“…A second possibility for these null findings is that they are real, and reflective of the fact that relationships between baseline testosterone levels and depression, hostility, and affect tend to be relatively small (e.g., Barrett-Connor et al, 1999;Book and Quinsey, 2005). Moreover, male gonadal function is relatively insensitive to short-term energy balance compared to females (Campbell and Leslie, 1995); consequently, energetic effects on male reproductive function may be more likely to emerge across longer time frames such as puberty and with senescence.…”

“…Differential nutrition, energy expenditure, disease burden, and possibly social circumstances may account for previously-documented population differences in male testosterone levels (Bentley et al, 1993;Bribiescas, 2001;Campbell and Leslie, 1995;Heald et al, 2003;Ross et al, 1995;Sewane-Rusike et al, 2000;Winters et al, 2001), and some of these same variables differ along the urbanization gradient represented in the THUSA study (Vorster et al, 2000).…”

Father absence predicts age at sexual maturity and reproductive timing in British men

“…It is possible that the stress of losing a father may have a proximate effect on adolescent hormonal responses and delay puberty [26]. Regarding reproduction, it may be that fathers are able to influence their son's educational attainment [10] thereby delaying starting a family in favour of investing in non-somatic capital.…”