Male Health and Reproductive Maturation

The transition from childhood to reproductive maturity in males involves the redirection of energetic and behavioral investment from general somatic growth and development to physiological functions related more directly to reproductive effort. This includes the growth of sexually dimorphic tissues such as bone and skeletal muscle, spermatogenesis, and an increased motivation toward mate-seeking behavior. This transition is also associated in all human societies—traditional and modern with a spike in mortality in young males that is correlated with risky behaviors (Fig. 6.1).

Fig. 6.1 Sex differences in mortality in Ache foragers of Paraguay (Hill and Hurtado 1996)

Risk-taking behavior during adolescence and young adulthood is an important public health concern worldwide. Many young male deaths occur as a consequence of accidents, homicides, suicides, substance abuse, war, and other risky or violent behaviors (Daly and Wilson 1983a). We categorize these causes of mortality as “extrinsic mortality” in contrast to intrinsic causes of mortality (those that lead to death via senescence or disease) (Partridge and Barton 1993; Carnes et al. 2006). Males, especially young adult males, are much more likely to engage in these activities compared to females than (Byrnes et al. 1999). For example, Wang and colleagues (2009) found that male college students in the USA are more likely to participate in risky behavior of all types, from chasing a bear out of a campsite to having unprotected sex. Males also commit more violent crimes, which can be seen in the higher rates of men killing unrelated men in Chicago, Detroit, England, and Canada (Daly and Wilson 1983b). These examples provide a snapshot of the ways in which males in industrialized societies engage in risky behavior; comparable scenarios can be found in all human cultures. In fact, this discrepancy between male and female risk taking and subsequent mortality follows the general mammalian pattern. In order to understand the ultimate cause of this discrepancy, it is important to consider sexual selection theory in general.

Greater male extrinsic mortality compared to females is central to understanding sex-specific reproductive strategies (Bonduriansky et al. 2008; Trivers 1972). Females tend to engage in significant metabolic investment in reproduction compared to males. The production of energetically expensive, internally fertilized eggs, gestation, lactation, offspring care, and provisioning compared to the relatively low metabolic costs associated with spermatogenesis underlie the contrasting energetic investments between men and women. Female fitness is most often limited by access to resources to support the energetic costs of reproduction. Males, on the other hand, are largely unencumbered by the direct energetic requirements of reproduction related to gamete production, gestation, and lactation. Male fitness is then not limited by resources, but by access to females and energetic investment in somatic tissue that is commonly deployed toward reproductive effort, such as sexually dimorphic muscle tissue (Bribiescas 1996).

This difference in reproductive investment has important implications for the variance of reproductive success between males and females. A male can potentially impregnate many females at the same time and conceive many offspring. Females, however, have much less variance in their fitness because they have a finite number of resources and time in which to reproduce. A commonly cited figure for most offspring sired by a single human male is 888, although historical records report approximately 1157 (Oberzaucher and Grammer 2014; Busnot 1712). In contrast, the current record holder for the most offspring born to a female without reproductive technology is 69 offspring (Clay 1989). Between-male variance in fitness is also greater. In those societies where the level of effective polygyny and male fitness variation is high, risk taking tends to be elevated (Kruger and Nesse 2006). This idea also holds when considering economic inequality, as access to resources and status confer fitness advantages to males.

Differences in potential evolutionary payoffs are predicted to lead to differences in male and female mating strategies and behavior (Bonduriansky et al. 2008; Vinogradov 1998; Bateman 1948). When mating opportunities are limited, males compete for mating opportunities with other males because the differential in reproductive payoffs from engaging in this competition can potentially be great. Consequently, the payoffs vary more in men compared to women in polygynous societies, although humans exhibit a greater range of variation in mating strategies and behaviors compared to other primates (Brown et al. 2009). Also see Kokko and Jennions (2008). The timing of this risk taking during the life history is also important. It pays to take the greatest risks at the beginning of one’s reproductive career, when the mating “market” is more open because females have not yet chosen longterm mates. The adolescent period can thus be viewed as an “inflection point in developmental trajectories of status, resource control, mating success, and other fitness-relevant outcomes” (Ellis et al. 2012, p. 601). It is consistent with this view, then, that male adolescents and young adults experience the highest rates of mortality. The male/female mortality ratio peaks at 3.01 in the 20-24-year age class in US males in 2000, and if one only considers extrinsic mortality (deaths from causes unrelated to illness), that ratio increases to 4.03 (Kruger and Nesse 2006).

However as anthropologists, we are often interested in the applicability of modern circumstances to the conditions that were more common during human evolution. In essence, our current environment of sedentism and high caloric availability is likely very different from our evolutionary past. It is therefore common for anthropologists to look at hunter-gatherer populations for an alternative perspective. This research strategy is not comprehensive as forager populations vary considerably in their ecologies and environments. However, it does provide a useful socioecological model to compare with modem industrial societies. Among the Ache of eastern Paraguay, the male/female mortality ratio is 1.8 for this same age class (Hill and Hurtado 1996). The difference in the mortality ratios between the Ache, a traditional society, and industrial societies, such as the USA, is not surprising since the discrepancy between male and female mortality rates has been increasing in developed nations for the last century. Kruger and Nesse attribute this to an overall decrease in infectious disease in industrialized societies, leaving a larger portion of mortality risks attributable to risky behavior undertaken primarily by men (Kruger and Nesse 2006). Male-skewed mortality during adolescence has been viewed as a public health problem, so it would be useful to deploy a life history perspective to address this challenge (Bell et al. 2013).

Recently, several authors have drawn attention to this discrepancy in mortality between the sexes. Ellis and colleagues (2012) make a persuasive case for the adoption by policymakers, scientists, and lawmakers of what they term the “evolutionary model” of adolescent risk taking, as opposed to the “developmental psychopathology model.” In the developmental psychopathology model, risk taking is considered maladaptive because it threatens a person’s “health, development, and safety,” while the evolutionary model focuses on the potential fitness benefits of such risky behavior (Ellis et al. 2012). Kruger has also highlighted the need for public health professionals to embrace life history theory when confronted with risk-taking behaviors and strategies. He emphasizes the need for public health officials and researchers to be aware of life history strategies in humans that manifest themselves in earlier reproduction and higher homicide rates, for example (Kruger 2011).

Moving beyond a basic understanding of the evolutionary forces behind risk taking and other competitive and violent behaviors that lead to higher male mortality, how can life history theory inform interventions aimed at curbing male mortality? If risky behavior has potential fitness benefits, this could pose a challenge for those attempting to develop interventions aimed at curbing male mortality. By understanding the motives of males who undertake risks, namely, increases in status, one can then meet those goals in less lethal ways. Ellis and colleagues cite an example wherein schoolchildren earn points for the group by refraining from exhibiting “problem behaviors” (Ellis et al. 2012).

Kruger, on the other hand, emphasizes the reduction in status differentials between males by using egalitarian societies as a possible model (Kruger 2011). Regardless of the methods proposed, it is exciting that researchers are exploring interventions that consider the evolutionary rationale of such behavior. Hopefully, future health policy will incorporate an understanding of sexual selection and its effects on male risk taking when implementing measures to curb male mortality.

While peak lifetime testosterone levels coincide with risky behavior and the spike in male mortality in the late teen years and early twenties, the actual relationship between circulating testosterone and young male risk taking is less straightforward. While testosterone is highest during the second decade of life in many economically developed populations, foragers and males under greater energetic stress exhibit much smaller or no age-related differences in testosterone levels (Ellison et al. 2002; Uchida et al. 2006). Therefore, it is likely that pubertal testos?terone increases are likely to be more contributory to risky behavior and mortality regardless of absolute adult-level testosterone. Additional increases in testosterone in more energetically rich populations probably reflect priming of luteinizing hormone (LH) and gonadotropin-releasing hormone (GnRH) receptors and greater somatic investment in lean body mass (Smith et al. 1975; Spratt and Crowley 1988).

Overall, adolescence is an important life stage in men, one in which mortality rates are significantly greater than in females. Comparative investigations strongly suggest that this period of high male mortality during reproductive maturation is a conserved trait common to many mammals, including primates (Pereira and Fairbanks 2002). Tolerance for risk and a hampered ability to accurately assess hazards contribute to health threats in young men (Cohn et al. 1995). Looking beyond testosterone, serotonin levels are associated with greater mortality in young male primates (Macaca mulatto), possibly hindering risk assessment and increasing risk tolerance (Higley et al. 1996).

We hypothesize that the development of interventions to decrease young male mortality is challenging given the potential fitness benefits that could promote risky behavior, as well as the possibility of transgenerational influences that may contribute to population differences in morbidity and mortality (Jasienska 2009). Nonetheless, changes in perceptions of environmental risk and socioeconomic factors hold some promise for promoting positive change. For example, van Anders et al. (2012) reported that testosterone was positively associated with safer sex attitudes, especially those most closely tied to STI (sexually transmitted infection) risk avoidance. Among some African American communities, young male mortality remains stubbornly high compared to that in the general population. An examination of 66 US counties that exhibit African American mortality rates in line with those in the general population report, not surprisingly, that income, poverty, and education are delineating factors between counties with high and low African American mortality (Levine et al. 2013). These factors in turn strongly contribute to perceptions of future survivorship and well-being.