To get a clear understanding of a general process, it often helps to pay special attention to the extremes. This is particularly true of a spectrum phenomenon, of which polygyny is an excellent example (there are varying “degrees of polygyny” just as there are degrees of polyandry). And the species Homo sapiens is both polygynous and polyandrous. Accordingly, let’s take a quick look at an extreme case of polygyny in the animal world, because when we do, we’ll see ourselves - albeit in caricature.
Elephant seals are very, very large. In fact, elephantine. Bulls can reach 16 feet in length and weigh more than 6,000 pounds. Cows are much smaller, about 10 feet long and weighing around 2,000 pounds. This size difference is important, since it arises because of the elephant seal mating system: the species might be the most polygynous of all mammals, with successful males establishing harems of up to 40 females. Since (as in most species) there are equal numbers of males and females, this means that for every highly successful bull seal there are roughly 39 unsuccessful, reproductively excluded bachelors. In the world of elephant seals, every healthy female gets mated, but only a very small proportion of males are comparable evolutionary winners. On average, four percent of the bulls sire 85% of all the offspring.[i] Bulls therefore fight long and hard among themselves for possession of a harem. Success requires large size, a violent temperament, massive canine teeth combined with willingness to employ them, a thick chest shield to provide protection from one’s opponent, and sufficient age and experience.
Female elephant seals wean their babies in late summer and early fall, after spending much of the summer on land, members of a crowded, beachfront harem. It turns out that by the time they are weaned, some young elephant seals are considerably larger than others – as much as twice the size of their fellow weanlings. These over-sized juveniles are known as “super-weaners.” Their greater size conveys a distinct benefit, since after spending a more or less idyllic time on their rocky beaches, nursing from their mothers, at summer’s end and upon being weaned the pups must begin a long sojourn at sea, not returning to land until the following spring. This is, not surprisingly, a stressful time for young elephant seals, and – also not surprisingly – those who were super-weaners are more likely to survive. It isn’t known whether male super-weaners are, in turn, more prone to eventually become harem-masters, but it’s a good bet, since in a highly competitive system, anything likely to provide a “leg up” when it comes to physical condition is likely to bring benefits.
So far, so good, at least for the super-weaners. A question arises, however. Why – given the payoff of being super-sized – aren’t all elephant seals super weaners? It turns out that since elephant seal mothers are limited in how much milk they can produce, there is only one way to become a super-weaner: a pup must obtain milk from two lactating females. How to achieve this? It’s not easy. Females are quite determined to make their milk available only to their offspring, not to someone else’s. This selfishness makes a lot of evolutionary sense, since nursing mothers who were profligate with their precious milk would have left fewer descendants (and thus, fewer copies of their milk-sharing genes) than others who were disinclined to wet-nurse an unrelated pup.
Nonetheless, even though every pup has only one genetic mother, it’s still possible for a pup to get milk from two “mothers.” Elephant seal pups occasionally die while nursing, either from “natural causes” or because they are literally squashed during the titanic battles among oblivious, competing bulls, who have females (not the safety of young pups, who were sired the previous year, possibly by a different male) on their mind. The death of nursing infants provides an opportunity for an enterprising young pup: if he can locate a bereaved mother – quickly enough after her infant has died so that her milk hasn’t dried up - he might induce her to permit him to nurse, in place of the recently deceased infant.
This is an effective strategy, but also a risky one, since most females don’t take kindly to allowing an unrelated baby to suckle. “Sneak sucklers” often get bitten, and may die of their wounds. But successful ones become what are known (in the technical literature, thanks to the detailed research of elephant seal maven Burney Le Boeuf) as “double mother suckers” … and they, in turn, become super-weaners. Here is the kicker: all double mother suckers are male! Chalk it up to the pressure of polygyny, in the case of elephant seals, super-polygyny leading – because of the potential payoff to males of being larger, stronger, and healthier than their competitors - to super-weaners by way of double mother sucking. All of this requires, of course, a willingness to take risks, certainly greater willingness than is shown by female pups, who, as the harem-kept sex rather than the harem-keepers, are pretty much guaranteed the opportunity to breed so long as they survive. For males in a highly polygynous species, mere survival isn’t enough. They must stand out from their peers.
As described in my recent book, a number of human traits can be understood as resulting from our shared human history of moderate polygyny. Human beings aren’t elephant seals. Few – if any – of our fellow Homo sapiens are double mother suckers. Nonetheless, the data are overwhelming that little boys are more risk-taking, on average, than are little girls,[ii] a difference that continues throughout life and is most intense among adolescents and young adults – precisely the age at which reproductive competition was most intense among our ancestors, and to some extent, still is today. Examples of extreme polygyny, such as elephant seals, reveal exaggerations and caricatures of traits found in human beings as well. We are biologically designed to be mildly, not wildly polygynous, but those traits found in such extreme cases as elephant seals, elk and gorillas sheds light on the more modest but nonetheless real and otherwise perplexing reality of what it means to be human.
David P. Barash is an evolutionary biologist and professor of psychology at the University of Washington; his most recent book is Out of Eden: surprising consequences of polygamy (2016, Oxford University Press).
[i] B. J. Le Boeuf and J. Reiter (1988) Lifetime reproductive success in northern elephant seals. In T. G. Clutton-Brock (ed.) Reproductive Success. Chicago: University of Chicago Press
[ii] E. E. Maccoby and C. N. Jacklin. (1974) The Psychology of Sex Differences Stanford, CA: Stanford University Press