Sexual cannibalism in spiders

It's not a phrase to be bandied about lightly - and it's actually quite rare in the natural world. It's rarer still in males. Sexual cannibalism is well known in the Black Widow, where the female of the species famously kills and consumes its male partner during copulation.

But a couple of Czech researchers say they've been observing it in male spiders, too, and interestingly it happens before the male and female have a chance to mate.

It all started several years ago, when a research student from Brno's Masaryk University noticed something rather strange in the extensive grounds of Lednice chateau in South Moravia.

Micaria sociabilis - black, brown and sexually cannibalistic

The research student had been sent there by his professor to observe a rare and tiny black and brown spider called Micaria sociabilis, which is confined to the warm grasslands of southern Moravia, Austria and Italy.

Micaria sociabilis had first been catalogued 100 years ago - by a Hungarian zoologist - but it hadn't been captured since. So arachnologists were rather excited at its rediscovery. But what they found next truly astounded them.

"I sent one of my former students there to study their biology. The student had to perform several different experiments - what they consume, how they mate, and how the males get on with the females," recalled Professor Stano Pekar, head of Arachnology at Masaryk University's Department of Botany and Zoology.

"I remember one day he came to me and said - well, the males consume the females. And I said - that's impossible!" Professor Pekar went on.

Females eat males, not the other way round

"This just doesn't happen in spiders. The females should consume the males. So he repeated some of these trials, and he came to the same conclusion," says Pekar, while explaining the chance encounter that - several years later - has led to some fascinating research.

The resulting paper, published in the prestigious scientific journal Behavioral Ecology and Sociobiology, bears the rather racy title "Mate with young, kill old: reverse sexual cannibalism and male mate choice."

It's largely the work of Professor Pekar's PhD student Lenka Sentenska, who's spent several years observing the mating habits of Micaria sociabilis.

Sex. Death. Cannibalism.

"Sexual cannibalism is the situation where typically the female kills and consumes the male. It's said that it comes from sexual conflict. This conflict results from anisogamy - a situation where males are producing a lot of sperm, and females are producing only a restricted number of eggs," Lenka Sentenska explains.

"It means that the females are limited in terms of copulation, while the males are producing a lot of sperm, so they're trying to copulate as many times as possible."

So far, so good. Picky female spiders - most notoriously of the Black Widow variety - kill and eat males during sex. Not only are weaker males weeded out and prevented from reproducing, but male Black Widows that allow the female to attack them actually copulate longer and produce more sperm, thereby increasing the likelihood of her eggs being fertilized.

Young males and old females - a lethal combination

But why do Micaria sociabilis males kill and eat females, instead of mating with them?

It seems to happen mainly in summer, when there are large numbers of younger males and older females in the field.

Some 70 percent of all such interactions end in cannibalism, and it seems the males deliberately choose older females to kill. More than that Stano Pekar and Lenka Sentenska refuse to say - the answer will be published in another, forthcoming study.

Sexual cannibalism in male spiders is extremely rare. It's seen only in two species - Micaria sociabilis and one other. But Professor Pekar points out it could be much more common than we think. Until more research is carried out, we won't know the answer. There are 43,000 species of spider on the planet. Scientists have observed the mating habits of just 700 of them.