杏吧原创

Species:

Habitat: , especially the south-western deserts, pigging out on crickets

If there鈥檚 one trait we can all agree is attractive in a potential mate, it鈥檚 that they are not going to eat you.

Yet there are a few animals that seem to accept being munched as the price of reproduction. The male praying mantis is the poster child for sexual cannibalism, famously having its head bitten off by the female.

Black widow spiders get their name from the females鈥� supposed cannibalistic behaviour, but this has been exaggerated. Among western black widows, in fact, the females go to great lengths to reassure the males that post-coital slaughter is not on the cards.

Don鈥檛 eat me!

Western black widow males court females by stepping onto their webs and tapping out a rhythm with their legs. They then cut some of the lines, cutting off the female鈥檚 avenues of escape, before throwing a 鈥渂ridal veil鈥� of silk over her body and mating with her. The webs , secreted with the silk, that signals their owners鈥� sex and which is .

The females hardly ever go in for sexual cannibalism: climax with the male being eaten. The only black widow species that is the southern black widow, 鈥� though the related Australian redback spider also does it, with the male , apparently on purpose.

The trouble with is that they might decide not to try mating with you 鈥� not withstanding that some species mate as they鈥檙e being eaten. Males will do better if they seek out females that are less likely to kill and eat them, and these more obliging females will get more mates.

According to and colleagues at Arizona State University West in Glendale, that is exactly what鈥檚 happening. His team have been conducting a long-term study of western black widows in Arizona鈥檚 state capital, Phoenix. The spiders are flourishing in the city, as the gardens and parks support a large population of prey insects like crickets.

In fact there is so much prey that the spiders often kill an insect, then eat only part of it. On the face of it this is foolish, because catching prey costs effort, and the victim might well fight back.

Gluttony

Johnson tried to work out why the spiders do it: perhaps it was because they had evolved a voracious attitude to survive in deserts where food was scarce, an attitude they were now stuck with? He kept 13 black widows in separate boxes in his lab, and starved them for 2, 7 or 14 days. Then he put four into each spider鈥檚 home, and watched what happened.

Spiders that had been starved for longer attacked the crickets sooner and killed more of them. They also made sure not to waste anything. So it seems they could adapt as prey became more or less scarce: but if that鈥檚 the case, why waste their food?

In an as-yet unpublished study, Johnson found that males were more likely to court females if they were sitting on a web and had been recently fed. That was sensible on the males鈥� part, as well-fed females were less likely to attack them: a pattern .

He suggests that female black widows kill more prey than they strictly need to, and display the half-eaten carcasses in their webs, to let males know that they aren鈥檛 hungry. Gluttonously wasting food, it seems, is a promise of safe sex.

Journal reference:

Read previous Zoologger columns: The butterfly that sleeps its way to safety, How to get elected in a termite democracy, Away in a vermin-infested manger, Child clones shape-shift to escape hunters Weaponised eggs turn predators鈥� stomachs, The hardest bat in the world, Houdini fly inflates head to break walls, A primate with eyes bigger than its brains, The solar-powered electric hornet, The miniature cuckold fish, Lemmings swap suicide for infanticide.