Health and Fatherhood
If male mortality is high, evolutionary theory suggests that males should invest more in mating opportunities with multiple females, since this would provide the greatest increase in fitness in the face of shortened life spans (Trivers 1972). The evolution of significant paternal investment, on the other hand, should coincide with an increase in male survivorship. Moreover, paternal investment should be associated with an increase in offspring survivorship, although fathers can also place burdens on families (Hewlett 1992; Flinn and England 2003). But can fatherhood itself serve as a means of decreasing male morbidity and mortality?
Garfield et al. (2006) contend that fatherhood is indeed an important factor for understanding patterns of male morbidity and mortality. Among other observations, they suggest that fatherhood may improve men’s health, since the responsibility of
Fig. 6.2 A number of daughters are associated with longer paternal life spans among a rural Polish community. Sons had no effect. Numbers of both sons and daughters are associated with shorter postmenopausal maternal life span (Jasienska et al. 2006) raising a child creates a greater awareness of the need to be mindful of a parent’s own health and well-being. Fatherhood is also associated with lower health risks in some cultures. American men over the age of 50 with two or more children have a significantly lower risk of cardiovascular death compared to married men with no children (Eisenberg et al. 2011). A demographic study of rural Polish fathers reported that number of daughters significantly increased paternal survivorship, while number of sons had no effect (Fig. 6.2) (Jasienska et al. 2006). These researchers suggest that the patriarchal structure of rural Poland encourages daughters to stay home, contributing to household tasks, hygiene, and supporting their fathers’ overall well-being, while sons are more likely to disperse from their homes.
In iteroparous species with altricial offspring (species that reproduce repeatedly and whose young mature more slowly), it is often beneficial for mothers to survive long enough to care for their offspring to mature. For males, this is often not the case. Internal fertilization results in uncertainty of paternity, which is predicted to compromise the potential for male paternal investment. Therefore, instead of investing in paternal care and long lives (like females), males benefit by investing in male- male competition for access to fertile females. While mating with multiple males also may augment female fitness in a number of ways, greater offspring output is not one of them (Hrdy 2000). This pattern holds for most mammals; however, humans exhibit a unique divergence from the common mammalian pattern. Humans
Fig. 6.3 Range of human male offspring investment (Bribiescas et al. 2012)
have diverged from the typical mammalian male mating schema, in which fathers contribute little to offspring care; we are among the few primates, and only great ape species, that engages in significant care of both young and mates (Gray and Anderson 2010; Bribiescas et al. 2012; Gettler 2010). Moreover, human males have evolved the broadest range of paternal investment patterns of any primate, including extensive paternal care and provisioning (Fig. 6.3).
The extensive (and presumably adaptive) range of behavioral reproductive strategies in men has required the evolution of the neuroendocrine plasticity that is conducive to paternal offspring investment and pair-bonding. To date, numerous studies have reported that testosterone levels are lower in fathers (Gray and Anderson 2010; Kuzawa et al. 2009; Gray 2003; Gray et al. 2002, 2007, 2006; Burnham et al. 2003), supporting the idea that there is adaptive phenotypic plasticity in these levels. In a longitudinal study, Gettler et al. (2011) noted a decline in testosterone in Filipino men after they became fathers, although they also report that young men who had attenuated declines in testosterone with fatherhood engaged in more sexual intercourse with their partners, compared to men who had significantly greater declines with fatherhood (Gettler et al. 2013). These data are consistent with the idea that environmental and social modulation of neuroendocrine function are reflected in the plasticity of male reproductive strategies.
The ability of organisms to respond adaptively to morbidity and mortality challenges is a major predictor of population survival, and a major tenet of life history theory is that natural selection in the form of mortality pressure will shape patterns of investment by males in offspring and mates. Hence a critical prediction of evolutionary theory is that male neuroendocrine function should respond adaptively to environmental challenges that correlate with mortality and morbidity risk. While the paternal neuroendocrine profile is complex and involves many hormones, such as oxytocin, vasopressin, prolactin, and testosterone, the latter is particularly impor?tant, since it is closely associated with variation in immunocompetence, morbidity, mortality, and risk tolerance.
Changes in testosterone levels in response to fatherhood may be an adaptive response to facilitate care and bonding behavior and decrease mate-seeking behavior. However, other possibilities include lowering testosterone to augment paternal immunocompetence (see Chap. 7). Testosterone lowers immune function, so men with high testosterone levels could run the risk of endangering their immunologically naive offspring by transmitting contagious diseases (Muehlenbein and Bribiescas 2005). Moreover, decreases in testosterone may also facilitate the deposition of adipose tissue to aid in paternal survivorship during periods of energetic stress (Lakshman et al. 2010; Bribiescas 2001).
Several investigations have explored the relationship between health-seeking behavior and fatherhood, particularly in reference to the possibility of couvade or “sympathy pregnancy” symptoms presented by expectant fathers in association with the pregnancies of their partners (i.e., weight gain, nausea). Interestingly, American men with expectant wives visited health-care providers less during the 9 months of pregnancy compared to a control group of men without expectant wives. The decrease in health-seeking behavior during pregnancy did not support the prediction of couvade behavior; however, this may be due to the increased attention to the pregnant partner and her health needs (Fig. 6.4) (Quill et al. 1984).
We hypothesize that the evolution of paternal behavior coincided with decreases in morbidity and mortality in men, possibly in association with the emergence of the genus Homo (Bribiescas et al. 2012). Moreover, we suggest that the broad range of male offspring investment strategies now seen in human cultures was affected by males’ ability to accurately assess and manage mortality risks. An intriguing example can be seen among indigenous Amazonian human groups, in which paternity is visibly parceled out and negotiated between men, resulting in the identification of
Fig. 6.4 Expectant fathers’ health-care seeking immediately before and after the pregnancies of their partners. Dark bars are matched controls; open bars are expectant/ subsequent fathers (Quill et al. 1984)
primary, secondary, and sometimes tertiary fathers of a single offspring. This is likely a way to decrease social tensions, to increase ties with kin, and to manage conflict and male-male competition (Beckerman and Valentine, 2002; Walker et al.
2010). It is also likely that changes in hormone levels, particularly testosterone, in association with fatherhood, serve as an adaptive response to improve paternal survivorship and morbidity through the attenuation of the behavioral, immunological, and metabolic liabilities associated with testosterone (Gray and Anderson 2010).