San Man from Namibia
Here’s a moment to shine a spotlight on the old guys. This one’s for you. For several converging lines of evidence point to the importance of older male humans fathering offspring from an evolutionary standpoint. To show why, let’s start with great apes and with age-related female reproductive function. And let’s talk about human hunter-gatherers as well as chimpanzees in the wild; since chimpanzees and bonobos (though little is known of bonobos compared to chimpanzees) are our closest living relatives, we can learn much from this contrast, including with respect to making inferences about our evolutionary ancestors.
For females, the rate of chimpanzee oocyte depletion is remarkably similar to human oocyte depletion. Both chimpanzees and humans lose eggs at similar rates, in other words, to eventually face lower fecundity from the late 30s and menopause around 50 (if chimpanzees stay alive that long, which happens rarely in the wild but a little more often in captivity). Think about this a bit—it’s quite striking, and speaks to the importance of phylogenetic constraints. Human females, whether in hunter-gatherer societies or among large cities today, typically live much longer than chimpanzees under the best of coddled captive circumstances, but the age-related reproductive declines show remarkable similarities. The stark female fertility difference is in interbirth interval, or time between births; human foragers have births spaced much more closely, enabling higher lifetime fertility. The intensity of extractive female foraging techniques (think digging up tubers, and cooking them) as well as resources from fathers and grandmothers provide human females with an energetic bounty enabling more rapid return to fecundity, shorter interbirth intervals, and eventually higher fertility.
Now we return to a focus on fathers. A couple of studies of wild chimpanzee male fertility, relying on DNA-based paternity testing, have been conducted in recent years, including at Gombe National Park in Tanzania. These studies show that most of the offspring are fathered by chimpanzee males in their early 20s. There are a few offspring fathered by males in their 30s, but that’s not very common. The multi-male, multi-female mating system of chimpanzees means that lots of males have sex with a given fertile female, with preferential mating access to a dominant male during times she is most fertile. The nature of male mating competition and shorter lifespans mean that fewer older males are around or able to have mating opportunities with fertile females. And for males who do father offspring, they typically do not exhibit any paternal care.
Among human hunter-gatherers such as the !Kung and Ache, and forager-horticulturalists such as the Tsimane of Bolivia, the pattern of age-specific male fertility extends to older ages than in chimpanzees. Many offspring are fathered by men in their 20s through 40s, but a fraction among men in their 50s and 60s too. There are several major reasons for this. One is that more males are alive at these later ages, and thus capable of continued fathering. Other reasons draw upon social behavior: human hunter-gatherers form long-term reproductive partnerships (i.e., marriage) and have fathers who provide a significant amount of paternal care. Men tend to be a few years older than wives and are more likely to remarry, contributing to older ages of male fathering. Human forager male-male relationships are egalitarian, with attempts by an upstart to politically dominant others usually shot down; that’s a distinct contrast with chimpanzee male social hierarchies, where being an alpha male can have its reproductive benefits along with a high degree of reproductive skew (or variation in reproductive outcomes). Human forager social status also tends to privilege hunting prowess and being a good shaman; those are capacities that older, more experienced males in their 40s and 50s can achieve with success. Featuring economic, social and political prowess—rather than sheer muscle-power—helps make older males valuable contributors to society and as mates and fathers. Since human foragers typically reproduce within long-term relationships, this also has the effect of dampening male reproductive skew; in that way, egalitarian male social behavior has a mirror in less variable reproductive success too.
To summarize this discussion, a contrast between chimpanzee and human age-related reproductive function highlights similarities in females, but shows dramatic differences in males. The fact that older male hunter-gatherers have offspring is linked with socioeconomic capacities, and the obvious but central point that, unlike our closest living relatives, humans form long-term reproductive partnerships within which kids are typically conceived and raised. This, combined with wider patterns of male ape and monkey social behavior, point toward dramatic and important shifts in our recent evolutionary ancestors giving rise to derived patterns of long-term partnerships and paternal care. With respect to discussions about aging, these patterns also show why it can pay for males to stay alive at later ages than, say, chimpanzees; males gain direct fitness benefits from doing so. It has even been suggested that older male reproduction could be the driver of slowed rates of human senescence (yielding longer lives); that view is a contrast with versions of the so-called grandmother hypothesis that argue for female indirect fitness benefits as drivers of extended human lifespans.
In the end, give a hand to the !Kung, Ache, and other forager men in their 40s and 50s who have fathered children. They offer a window into the importance of older male human fathers from an evolutionary perspective.
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