Reversal of Ancient Black Widow Myth: Male Spiders Eat Old Females Rather Than Mate with Them

The choice of sexual partners among male spiders is commonly chosen by attractiveness of the female. Yet, it is also commonly known that female partners may typically eat their partners. However, a new study suggests that there is not as much evidence about how males may sometimes dictate the choice of partner. In the Micaria sociabilis species in particular, reverse cannibalism may be a type of male mate choice.

Researchers examined male and female Micaria sociabilis spiders over a two-year period and studied their behavior by mixing males and females of the species at different time points. All the spiders were well fed to discount cannibalism that may have been from hunger. Authors observed what happened when they paired young adult male and female spiders together, either from the same generation or from another generation. The pairings with females of different size, age and mating status, the researchers hoped to be able to identify whether the reserved form of sexual cannibalism was an adaptive mechanism for male mate choice.

Their study found that cannibalism took place early after the first contact and before any mating took place. The researchers also observed that reverse cannibalism differed significantly, depending on what month it was -- most of the incidents were in July. Males from the summer generation tended to be bigger than males from the spring generation and they were more cannibalistic. This would suggest that male aggression may be related to male size.

The authors noted that the highest frequency of reverse cannibalism occurred when these larger, young males from the summer generation met old females from the previous spring generation. This suggests they may have based their choice on female age. Female body size, even though considered to be a sign of quality, did not affect rates of cannibalism. The authors also noted no difference in male cannibalization of females who had previously mated or virgin females. This evidence demonstrates that in some species and some cases, the males make a very clear choice about who they will mate with.

"Our study provides an insight into an unusual mating system, which differs significantly from the general model," the authors wrote. "Even males may choose their potential partners and apparently, in some cases, they can present their choice as extremely as females do by cannibalizing unpreferred mates."