Female Spiders Attack Mates After Sex – But Why?

Many spider and insect species are known to cannibalize their mates after intercourse – an act scientists have characterized as uncontrollable aggression. But a new study of female raft spiders suggests the behavior may simply be a female's way of testing her male suitor and weeding out the weak ones. 

Researchers from the University of Melbourne conducted two trials to test sexual cannibalism. In the first trial, 11 out of 16 females attacked males during or after copulation, but only four of these attacks were fatal. In the following trial, six out of eight females attacked their mates, but only two attacks were fatal. Furthermore, females that were aggressive in one trial were not necessarily aggressive in the next. This contradicts the popular idea that females are unable to "tone down" their aggressive mindset after hunting for prey, according to a news release.

"Females are sexually cannibalistic because they are testing the males, rather than just being inherently aggressive," Dr. Mark Elgar, one of the study researchers, explained in the release.

In addition to the females' inconsistent tendencies, researchers found that they were more likely to attack smaller males, which could be because they are easier to capture.

"We conducted extensive experiments and found no correlation between female foraging aggression and that displayed towards males," Dr. Elgar continued in the university's release. "It actually appears there are many other potential factors involved, including the size of the males in comparison to the female, the female's age and whether or not she is a virgin."

Raft spiders (Dolomedes fimbriatus) have large oval brown bodies, often lined with a white stripe, and long sturdy legs. This species can be found across Europe near bogs and wet grasslands. During courtship, males attract females by making surface waves on the water by jerking their abdomen and waving their legs.

Their study was recently published in the journal Animal Behavior.   

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