Role Reversal: Younger Black Widow Males Cannibalize Older Females

Alan McStravick for – Your Universe Online

The idea that the female of a species would, after completing coitus, approach, kill and eat her mate is a sentiment that instills, if not fear, unease in many the male reader. This scenario, in fact, is exactly how the Black Widow spider got its name. However, according to researchers out of Masaryk University in the Czech Republic, the popular knowledge behind this spider might not necessarily be so.

Study authors Lenka Sentenska and Stano Pekar found through the course of their research the male spiders of the Micaria sociabilis species are more likely to eat the females than be eaten.

The study, conducted over a two-year period, involved the collection of both male and female M. sociabilis spiders. The researchers noted the animals´ behaviors by mixing the males and females at different time points. This intermingling of the spiders allowed the observers to witness what happened when they paired young adult male spiders with single female spiders either from the same generation or from another generation. The researcher team wanted to learn, by pairing males with females of different size, age and mating status, if they were able to identify a form of reverse sexual cannibalism and whether or not it was an adaptive mechanism adopted for male mate choice.

The results show cannibalism took place early after the initial contact between the male and the female. Important to note, this cannibalism occurred before any mating took place. In addition to the females’ age and size relative to the male, the team also learned reverse cannibalism differed significantly based on the month it was occurring. Males from the summer generation, it was noted, tended to be bigger than males born in the spring. They also exhibited more cannibalistic tendencies. The team inferred male aggression in M. sociabilis may be related to size.

The highest frequency of reverse cannibalism occurred when the larger, summer males would be paired with older females from the spring generation. It is then understood the age of the female may be the deciding factor on whether or not the male opts to cannibalize her. Also interesting to the research team was there was no difference in male cannibalization of females who had previously mated or were virgins. This, they say, demonstrates how, in some species and in some cases, the male makes 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 stated. “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.”

Results of their research were recently published in the journal Behavioral Ecology and Sociobiology.