She's A Man-Eater, And That's OK With Male Orb-Weaving Spiders

Jun 1, 2016
Originally published on June 1, 2016 6:42 pm

Male orb-weaving spiders get devoured by the females they mate with, but a newly published study shows that at least the poor guys get to choose the lovely lady who will cannibalize them.

Usually in nature, it's the females who survey the males and make their selection. But when biologist Eric Yip was working at Ben-Gurion University of the Negev in Israel, he and some colleagues wondered if that held true for Cyrtophora citricola, a kind of orb-weaving spider native to the Mediterranean.

Two things made them think these males might be choosy. First, they have just one shot to mate and they have to make it count. That's because researcher Na'ama Berner-Aharon recently found that this species has a very high rate of sexual cannibalism — 100 percent of males that successfully mate get eaten by the female.

Second, this species lives in colonies of hundreds or even thousands of spiders. That means males can potentially encounter multiple females, unlike more solitary spiders that have to wander around and possibly get killed before they find a single mating partner.

To see if these male spiders were picky and, if so, what they looked for in a female, the researchers did a series of careful experiments.

It turns out that, indeed, the males won't mate with just anyone. "We had a lot of males that just sat in the females' webs for days and days and days and never copulated," says Yip, now at Pennsylvania State University. "She was an adult female, ready to go, a virgin — and the male just had no interest."

The males would wait for a gal with certain key traits. "The ideal female is a very well-fed, recently molted female to adulthood," Yip says. "So she's got a lot of resources for eggs, she's a virgin so he may not have any competition for those eggs with other male sperm, and she's young, so she hasn't expended any of these resources that she's gathered throughout her juvenile life."

The males also preferred female spiders that had been raised on a feeding regimen that gave them lots of extra food. Yip says that's probably because well-fed females produce more eggs — not because the males were hoping she wouldn't be hungry enough to eat them.

"Females on the high and low feeding regimes were equally likely to cannibalize their males," he explains. "Therefore, preference for well-fed females cannot be explained by a male sense of self-preservation."

Older males spiders, however, were less choosy, presumably because they were running out of time to do the deed.

One thing the researchers noticed was that females often would begin plucking their webs and initiating courtship before the males did. "And that was really interesting because it's not very common for that to happen," Yip says. "Maybe the females are actually having to compete or advertise to the males to get mating opportunities."

The researchers describe their work in the journal PLOS One.

"I thought this paper was really interesting because there are only a few people who have studied cannibalism and choice in the context of a social spider," says Maydianne Andrade an evolutionary ecologist at the University of Toronto. "The male kind of has a smorgasbord of females to choose from, potentially."

She says because cannibalism is so high in this species, "it's a really high-stakes decision for that male, and has all these options to choose from."

But it's still unclear, in these orb-weaving spiders, how complicit the males are in their own demise. Females of many spider species will attack males right after copulation. "The female needs sperm, but after copulation, she has no use for the male," Yip says. "There's no reason not to eat him, really. He's a nice calorie-rich meal for her."

While some male spiders try to get away, the males of other species seem to actively court death. Australian redback males, for example, actually twist their bodies around and somersault into the mouth of their mate.

This is because males that sacrifice themselves seem to produce more offspring. They can feed a female so she is more successful in producing young, and perhaps copulate for a longer period of time and transfer more sperm.

"If these males have been selected to do this over time, I would actually predict that they enjoy it," Andrade says. "They're just completely different from us. None of our assumptions apply."

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Among insects and spiders, it's not uncommon for a female to gobble up her mate. That's how the black widow spider got her name. It's a grim fate for the guys, but a new study suggests that some male spiders at least get to choose the lady who will devour them. NPR's Nell Greenfieldboyce has more.

NELL GREENFIELDBOYCE, BYLINE: A female spider needs sperm to fertilize her eggs.

ERIC YIP: But after copulation, she has no use for the male. There's no reason not to eat him really. He's a nice, calorie-rich meal for her.

GREENFIELDBOYCE: Eric Yip is a biologist who currently works at Penn State University. Recently, he was in Israel studying an orb-weaving spider that lives in big colonies.

YIP: And these can be very large groups of spiders containing hundreds or even thousands of spiders.

GREENFIELDBOYCE: Yip and some colleagues wondered if so many spiders together might lead to something unusual. Typically in nature, it's the females who are picky when it comes to selecting a mate.

But here, the males were surrounded by females, and as soon as they mated with one, they would become dinner. So the scientists did some experiments to find out if this state of affairs made the males the choosy ones. And it turns out that, yeah, the males did a lot of rejecting.

YIP: We had a lot of males that just sat in the females' webs for days and days and days and never copulated.

GREENFIELDBOYCE: They were waiting for the one. It looks like their ideal female spider is a young, fat virgin. Yip says that's because she's likely to be fertile and not full of any other spiders' sperm.

YIP: The best thing they can do is to pick the very best female that they can find. I mean, you could sort of see it as making the best of a bad situation.

GREENFIELDBOYCE: The researchers described their findings at a scientific journal called PLOS One.

MAYDIANNE ANDRADE: I thought this paper was really interesting because there are only a few people who studied cannibalism and choice in the context of a social spider.

GREENFIELDBOYCE: Maydianne Andrade is an evolutionary ecologist at the University of Toronto Scarborough. She studied red-back spiders. Males of this species have no real choice. They're solitary and struggle to find even a single female to mate with. They're so desperate that during mating they do a somersault to throw themselves into the female's open jaws.

ANDRADE: It's fascinating because it's so different from what we usually think of in mating or in general about the evolution of traits. Like, don't people - don't individuals evolve to try to survive?

GREENFIELDBOYCE: But self-sacrifice can work to a male's advantage. Becoming food could help his mate produce more of his babies.

ANDRADE: If these males have been selected to do this over time, I would actually predict that they enjoy it.

GREENFIELDBOYCE: If, of course, spiders are capable of contemplating sex and death and the meaning of it all, which they probably aren't. Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.