How Can the Rules of Cannibalistic Sex Change?

Jumping spider females behave differently

By on September 24th, 2007 18:06 GMT
There are some people who act like crazy in their search for sex. In many spiders, mating is a risky game, as the (usually) much bigger female will kill and eat the male during the mating process. In jumping spiders, sexes are quite similar in size, still the larger partner will eat the smaller.

A new research made on the East African Evarcha culicivora, a jumping spider feeding on blood-filled mosquitoes, will not be hampered by the cannibalistic sex drive of larger partners, preferring them for mating. Virgin females found the larger males to be most attractive when losing their virginity; after that, they shifted to the safer smaller male.

In the case of E. culicivora, the size of both genders varies differently from the spider norm, leading to the evolution of a different mate choice behavior. Large males are more cannibalistic towards smaller females than larger females towards smaller males.

Consequently, virgin females prefer larger males as mates, in spite of the increased risk of cannibalism, but once the females have mated, they change their attitude and prefer smaller males.

In contrast, males make the same choice regardless of whether they are virgins or not and prefer larger females as mates overall. The team made of New Zealand and Kenyan researchers focused on determining if females' choice was based purely on size selection and not courtship behavior. "Like all Salticids (jumping spiders), E. culicivora has exceptional eyesight that is unrivaled by other animals of a similar size. Because E. culicivora can see so well, we could test what size mates virgin and non virgin males and females preferred by showing them different sized, dead conspecifics arranged in life-like postures.", explained lead researcher Dr Pollard.

"These motionless individuals were coated with a plastic adhesive and allowed us to test for decisions based only on the size of the potential mate and not its behavior. To check whether the decisions made with static models related to actual mating, we also repeated the experiment with the choice being live mates."

The female of E. culicivora was smaller, so weaker, so it only mated once with a larger male; whole male E. culicivora made less discrimination when choosing a partner, but female's motivation for this behavior remains a puzzle. This is "the first experimental evidence of pronounced mutual mate choice in a Salticid and also the first experimental evidence of a Salticid making size-choice discriminations in the context of mating.", said Pollard.

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