Black widow spiders: Everyone’s heard of them, not only because they’re poisonous but also because of the female’s lurid propensity to widow herself by killing and eating her mate.
Unknown to many, there are also white widow spiders (not to mention brown, red-legged, and others). Even more surprising, perhaps: these sexually cannibalistic arachnids can teach us something about – of all things – monogamy.
Basic biology suggests that males are generally less inclined than females to practice monogamy, simply because males have very little invested in any one reproductive effort (sperm are tiny, cheap to produce and hence, are made in vast numbers), whereas females partake of a different breeding strategy (eggs are large, expensive to produce, and hence, each one is taken more seriously).
Conventional wisdom in evolutionary biology has therefore been that males are limited by the number of females they get to copulate with; females, by the extent to which they get to invest in their offspring. Yet, by adroit use of DNA fingerprinting, biologists are discovering more and more cases in which males limit their sexual attentions, sometimes to just one female, and in which females extend their favors to more than one male. Among the aptly-named love bugs, for example, males may insist on copulating for upwards of 50 hours (during which, of course, they can’t mate with anyone else), while even some “happily married” geese incubate eggs fathered by someone other than the female’s “husband.”
Which brings us to those white widow spiders. Although folk culture may well have exaggerated the murderous inclinations of females – they are known to kill and eat males during and after copulation in the laboratory, but less so in the wild – males do run great risks seeking to inseminate females, who typically are not only much larger and more venomous, but also lethally uninhibited. Those males who succeed, however, even if their lives are drastically shortened in the process, are “fitter” than their more reticent, and celibate, colleagues. Male spiders, as a result, are risk-takers.
But they aren’t gallivanters.
A study of white widow spiders native to Israel, published in the journal Animal Behaviour, found that in this species, a male is likely to hang out in the web of only one chosen female, in the process of which he is sometimes cannibalized, more often merely losing the tip of his copulatory organ inside his inamorata’s genital tract. By contrast, female white widows commonly mate with more than one male; up to six, in fact (these numbers confirmed by simply counting those handy little “copulatory organ tips” that various males inadvertently left behind, in the female’s behind).
From the perspective of a male white widow spider – one can imagine him shaking his head, while regretfully intoning “Can’t live with her, can’t breed without her” - a female in the web is worth any number in the Great Beyond. With females hard to find, even if, once found, they are life-threatening, staying put and taking your chances is a more productive strategy than is wandering around looking for an additional fling. So male white widows are, in a sense, sexually faithful, even monogamous, while the females are “polyandrous,” which is to say, a “harem-mistress” doesn’t hesitate to mate with more than one male. And then, on occasion, to kill them.
Here’s a conundrum, by the way: When a female’s partner dies, we refer to the survivor as a “widow.” What about when a female is associated with many males and one of them dies? Are you a widow if you still have some potential mates sharing your web, who haven’t yet succumbed to your lethal charms? Even more daunting, what are we to make of the fact that the Republic of Kazakhstan has issued a stamp that celebrates the white widow spider? (And does Borat know about this?)
In any event, one lesson taught by the white widow spiders is that monogamy in animals may be dictated by cold-hearted economics, as revealed by, say, cost-benefit considerations – and the outcome is neither obvious nor necessarily the same for both sexes. Questions nonetheless remain. What, for example, is the payoff to a male white widow spider who stays with one female, but as a result, forgoes the prospect of meeting others? If you’re a female, should you eat a potential mate – thereby gaining nutrients – but at the risk of possibly being unfertilized?
And what’s love got to do with it?
David P. Barash, an evolutionary biologist, is professor of psychology at the University of Washington. His most recent book – just published – is Buddhist Biology: ancient Eastern wisdom meets modern Western science (Oxford University Press).