Scientists in Israel have observed a strange behaviour among brown widow spiders: When given the choice, the males of this species prefer to have sex with older females even though they're less likely to bear offspring. More problematically, these older females are also more inclined to devour their partners after mating, making the males' preference all the more baffling.
New research published this week in the science journal Animal Behaviour chronicles a strange mating behaviour in brown widow spiders that seems to violate the laws of natural selection. Brown widows partake in sexual cannibalism, a practice in which the female turns her mate into a convenient meal after copulating. Sounds bizarre, but scientists say it's a sacrificial act for the male, who, by giving himself up in this nourishing way, improves his odds of reproductive success.
Brown widow spiders in the act of mating.Photo: Iara Sandomirsky
This theory is all well and good for sexually cannibalistic fishing spiders and praying mantises, the males of which make for a hearty, nutritious snack. But in brown widow spiders, sexual dimorphism between males and females is significant; the females are so much bigger than males that scientists say the "nutrition" theory of sexual cannibalism does not apply. The exact reason for this behaviour among brown widows isn't immediately obvious, but the latest observation in the mating practices of this species are clouding the picture even further.
When it comes to choosing a mate, brown widow males have a choice: They can either play it safe and choose immature females, or go the dangerous route of having sex with mature, older females. Immature females, also known as subadults, are a single molting phase away from maturity, but they're capable of mating, storing sperm, and producing eggs. What's more, they don't need a long and protracted courtship display (unlike their older sisters), and they don't partake in sexual cannibalism. Young mature females are fertile, but they're dangerous. Older mature females are significantly less fertile - and they're 50 per cent more likely to eat males after sex compared to young mature females, thus representing the least desireable choice for males. At least on paper.
Indeed, it would make sense for brown widow males to steer clear of older mature females and seek out younger spiders. To test this theory, researchers from Ben-Gurion University of the Negev, with help from the Hebrew University of Jerusalem and The Volcani Center, collected males and females from central and southern Israel. In a laboratory setting, the spiders were arranged such that the male had his choice of which prospective mate to approach: either subadult females, young mature females, or older mature females. The researchers fully expected the males to avoid the older females for three reasons: less time and energy for courtship displays, a greater chance of reproductive success, and self-preservation.
But the observations didn't bear this out. The males, to their apparent detriment, consistently preferred the older females.
"[W]hen given a choice, males preferred mature over subadult females and older over young mature females," write the researchers in their study. "We found no benefit for males in mating with the females of their choice. Older females were significantly less fecund [i.e. capable of producing offspring] than young mature females, and were not more fecund than subadult females."
These observations gave rise to a theory. It's possible, the researchers surmised, that the males were somehow taking advantage of the older female's physiology. Specifically, the scientists wondered if the males were plugging the genital openings of the females by detaching and leaving their pedipalps (the spider version of a penis) inside the females (yes, this is a thing that some spiders do, and also some slugs). It's a dramatic act, but it's a kind of chastity belt that prevents the female from mating again with other males. The researchers reasoned that, if the detachable penis trick happened more often with older females, it would confer an advantage for the males and provide a tidy explanation for why they prefer older females.
During follow-up observations, however, the researchers did not observe this stop-gap measure, so they had to throw this theory out the window.
"Males don't seem to be behaving in their own self-interest and suffer a twofold cost - fewer offspring and no opportunity to mate with another female," the researchers wrote, adding: "Thus, we lack an adaptive explanation for male preference for mature older females."
One possible explanation, however, is that older females are manipulating the males by using larger quantities of pheromones. In other words, the older females may be producing a kind of irresistible perfume that the males cannot resist, and at greater amounts compared to younger spiders. But there's a problem with this theory: It has zero evidence to support it, aside from the strange male behaviour. It's "a hypothesis that remains to be tested," conclude the researchers.
The pheromone theory makes sense, but clearly more work needs to be done in this area. Nature doesn't do anything by accident, and there's likely a very good reason for this odd, seemingly self-destructive behaviour among brown widow males.