Sometimes Male Spiders Eat Their Mates, Too

Many female arthropods - black widows, praying mantises - eat their male mates, but sometimes the reverse is true

A female orb-weaver chowing down on a male. Photo: Kumon

The female black widow spider, as her name suggests, infamously devours her would-be suitors as they attempt to mate with her. These spiders are not the only sexual cannibals in the arthropod kingdom, either. Female praying mantises cannibalize their mates, sometimes decapitating and eating them while they are still mounted. Female orb-weaving spiders eat the smaller, more timid males and mate with the larger, more aggressive ones.

In an interesting case of role reversal, however, researchers just discovered that some male spiders also eat their mates. Micaria sociabilis, a small brown spider that lives in Europe, is more likely to eat his female mate than be eaten by her. The researchers see this phenomenon as evidence of male mate choice.

The study, which was only carried out with spiders in the lab rather than those observed in the wild, involved pairing male and female M. sociabilis of different sizes, ages and mating status to see what would happen. All of the spiders were fed ahead of time to discourage cannibalism due to hunger.

Reverse cannibalism, it turned out, depended heavily on the month in which the spiders met. Males tended to eat females most often in July. In the summer, males tended to be larger and also more cannibalistic, so the researchers speculate that male mate-eating aggression may be correlated with size. Cannibalistic males would eat their potential mates both before and after copulation.

Cannibalism occurred most frequently when large, young males from the summer batch met older females from the spring generation. So the behavior could also be based upon female age. Female body size did not turn out to be a significant factor in whether or not the female gets eaten, and neither did virginity.

Whatever the underlying reasons, in the case of M. sociabilis, males clearly call the shots on who they prefer to mate with and who will just serve as another convenient snack.

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