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Page last updated at 09:23 GMT, Tuesday, 12 April 2011 10:23 UK
Cannibal wolf spiders are ladykillers
A male Allocosa brasiliensis eats a female (c) Laura Watson
To mate or to eat? A male spider makes an extreme choice

Male wolf spiders cannibalise older females, scientists in Uruguay have discovered.

In several species, female spiders are known to eat males, but this is the first time biologists have seen the roles reversed in the wild.

The male spiders were observed mating with virgins and eating older, less reproductively successful females.

Researchers suggest that harsh habitats force males to prey on females for food.

Their findings were published in the Biological Journal of the Linnean Society.

The species in question, Allocosa brasiliensis, is a nocturnal wolf spider found in South America's sand dunes along riverbanks and the Atlantic Ocean coast.

This is not only the first report for spiders, but also this is extremely rare for the animal kingdom
Dr Anita Aisenberg
Clemente Estable Institute of Biological Research

The researchers were studying the species because its status is considered an indicator of the health of coastal habitats.

After observing a male spider eating a female in the wild, Dr Anita Aisenberg and her team from the Clemente Estable Institute of Biological Research, Montevideo, set out to find an explanation for the behaviour.

"In spiders in general, females are larger than males and they are the selective sex, while males are small rovers that go out and look for potential sexual partners," explained Dr Aisenberg.

In A. brasiliensis though, researchers found that the males were unusually dominant.

Role reversal

Although sexual cannibalism has been widely documented in spiders, the researchers were surprised to see male A. brasiliensis eating females.

"This is not only the first report for spiders, but also this is extremely rare for the animal kingdom," Dr Aisenberg told the BBC.

Researchers observed the male wolf spiders waiting in their burrows for visiting females looking for a mate.

SEXUAL CANNIBALS
Allocosa brasiliensis at sunset (c) Marcelo Casacuberta
Sexual cannibalism is common in many spiders and scorpions
Widow spiders are so-called because some females have a tendency to eat males after intercourse
Female mantises have been observed biting the heads off mates during copulation

"Males mated more frequently with virgins with high body condition, as a way of ensuring a future successful progeny," said Dr Aisenberg.

"In this species the first egg sac is the most successful in [terms of the] number of eggs, so with virgins they ensure this first brood."

"And females with better body condition will provide more eggs and, consequently, more sons and daughters."

Older females, and those with lower body condition, were in for a nasty surprise as males took advantage of them for other needs.

In what the researchers described as an "extreme sexual choice", the male spiders selected whether to mate with females or eat them; a choice that appeared to be based on their mating potential.

According to Dr Aisenberg, the environment in which the spiders live may have moulded their unusual sexual behaviour.

"The habitat of A. brasiliensis can be considered harsh with extreme temperatures, strong winds, scarce refuges and very unpredictable in prey abundance," she said.

"[Males] cannibalise females of low reproductive value and take advantage of them as prey, and mate only with the females that ensure them high reproductive success."



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