In sex life of jumping spiders, size matters

by Marlowe Hood Mon Sep 24, 1:57 PM ET

PARIS (AFP) - From post-coital cannibalism to love at first sight, the sex life of the African jumping spider is full of surprises, according to a new study.

But none is more unexpected than this, say researchers who studied the blood-gorging Evarcha culicivora up close and personal: while virgin females are attracted to meatier mates, a bit of experience sees them switch to smaller partners.

In this and other ways, the jumping spider, native to East Africa, is in a class of its own when it comes to sex, according to a study in the current issue of the scientific journal Ethology.

For starters, both males and females play a roughly equal role in choosing partners, an aberration in the eight-legged world of Arachnida.

That equal opportunity behavior extends to two-way cannibalism as well, with males consuming their loved ones only slightly more often as the reverse.

For spiders that go in for that sort of thing, females gobbling up their sex partners is the norm.

Another finding that surprised a team of researchers led by Simon Pollard, a biologist at the Cantebury Museum in Christchurch, New Zealand, was that E. culicivora -- armed with eye-sight "unrivalled by other animals in their size range" -- picks its partner based on looks and size alone.

And here is where things get strange.

Female adult African jumping spiders vary in size between three and six millimetres, while males range between four and seven, giving rise to the possibility of larger males mating with smaller females and vice versa.

In a series of laboratory experiments, Pollard showed that in three of four possible scenarios, an individual E. culicivora preferred a partner with an extra millimetre or two on its frame.

Virgin females, along with both experienced and inexperienced males, were all more than twice as likely to opt for a meatier mate.

But females that had copulated once before saw things differently: two out of three made a bee-line for the smallest male in sight.

"It is as though females start out prepared to take the risk in choosing larger males," knowing they may be eaten as a post-sex snack, Pollard notes in the study. "And then, once mated, they become less inclined to take the risk again."

But exactly why experienced females prefer to practice "safer sex" the second time around, admits Pollard, "is currently unknown."

To test his hypothesis that E. culicivora makes mate-choice decision based on looks alone, Pollard perched two life-like embalmed specimens of different sizes in come-hither postures inside a large cage, with a single plastic tube leading from an entryway to each one.

The researchers made sure that the spider choosing between the two had a gander at both potential partners before counting the results, which showed that decisions were based on static looks alone.

To ensure that other factors were not coming into play, they repeated the experiment, but this time with live spiders. The results were the same.

The African jumping spider feeds mainly on small lake flied and blood-gorged female mosquitos, which means that vertebrate blood is an important part of their diet.