If you’re on a first date and it’s not going so well, what’s your exit strategy? Do you have a friend ready to call you at a predetermined time so you can give a plausible excuse? (“Heeyyy, so I’m having a really great time but my buddy Nate just called and he accidentally flushed his pants down the toilet at a gas station, so um, yeah, I gotta go help him out.”) Or do you just hit the fire alarm and run for it?
One strategy you probably haven’t considered is eating your date. For whatever reason, our society just isn’t into that. But sexual cannibalism is common in a wide variety of invertebrate species, and biologists have plenty of hypotheses to explain this odd behavior.
Closeup of a female redbacked spider (Latrodectus hasselti).
Usually, it’s females that eat males. In one remarkable spider species, the Australian redbacked spider (Latrodectus hasselti), up to 65% of males get eaten by their sexual partners. Remarkably, males even seem to offer themselves to females during mating, performing a sort of “somersault” into the female’s jaws and thereby facilitating their own demise (see a video – the male’s “somersault” begins around 0:20). This might appears to be a poor evolutionary strategy – after all, once you’ve been eaten, your mating days are over. But biologist Maydianne Andrade has shown that this behavior actually makes sense in a macabre sort of way.
For male redbacked spiders, mating opportunities are few and far between, so when a male finds a female he needs to make the most of the opportunity. The longer a male can remain in flagrante delicto, the more offspring he will sire. Males can continue to transfer sperm even while being consumed, so by allowing themselves to get eaten while copulating, males can increase the amount of sperm transferred. Furthermore, females who eat their first mate are less likely to re-mate, further increasing the genetic contribution of the first “suicidal” male. Statistically, even if a male survives his first mating, he is very unlikely to find another female. So by investing 100% in his first and only mating by offering himself to his mate, he actually enhances the transmission of his genes to the next generation.
Much less common is reversed sexual cannibalism, in which females are consumed by males. In most cases of sexual cannibalism, females are larger than males and invest more resources in reproduction. In some species, however, the sex roles are reversed: males invest more heavily in reproduction than females, and it is conceivable that reversed sexual cannibalism could evolve in such species.
In the latest issue of the Biological Journal of the Linnaean Society, biologist Anita Aisenberg and colleagues document just such a situation: reversed sexual cannibalism in Allocosa brasiliensis, a dune-dwelling wolf spider from South America. In A. brasiliensis, males are larger than females and have been observed eating adult females in the wild. Males dig deep burrows in the dunes and line them with silk, and females require these burrows to lay their eggs. After mating, the male abandons his burrow and the female remains inside to lay and guard her eggs. Given this unusual reproductive biology, Aisenberg saw a unique opportunity to test hypotheses about sexual cannibalism in a “sex-role-reversed” species.
A male Allocosa brasiliensis, photographed by Marcelo Casacuberta
If sexual cannibalism is a result of misidentification, then a female’s body size and mating history shouldn’t affect the likelihood of cannibalism. If males consume females when they are very hungry, then males in worse body condition should be more voracious cannibals. Finally, if cannibalism is an adaptation to prevent mating with undesirable females, then males should preferentially consume: 1) females in poor condition (because these females will produce few eggs), and 2) females that have already mated with another male (because the current male’s genetic contribution to the resulting clutch would not be exclusive).
Aisenberg and colleagues brought adult A. brasiliensis into the lab and allowed virgin and previously-mated females to court males with sand burrows. They observed cannibalism in more than a quarter of all courtship attempts, and the females who were attacked had, on average, a lower body weight than females who were not attacked. Males were more likely to mate with heavier females, and were more likely to mate with virgin females than previously-mated females. There was no measurable difference in the rate of cannibalism, however, based on females’ previous mating experience. (And contrary to reports in the echo chamber of the mainstream science media, the authors did not test females of different ages, so we can’t say that male spiders “eat cougars” or any such nonsense!)
The male’s body condition did not affect his propensity to eat a courting female, so sexual cannibalism in this species probably can’t be explained simply by hunger. Instead, the evidence supports the idea that male cannibalism could be an extreme mate choice behavior: if a prospective mate isn’t up to standard, a male should just eat her and be done with it – better than wasting a perfectly good burrow!
Luckily, humans have some subtler ways of dealing with a bad date. Let’s stick with the phone call as an exit strategy.
Work cited:
Andrade, MCB. 1996.
AISENBERG, A., COSTA, F., & GONZÁLEZ, M. (2011). Male sexual cannibalism in a sand-dwelling wolf spider with sex role reversal Biological Journal of the Linnean Society DOI: 10.1111/j.1095-8312.2011.01631.x
