Cannibalistic spiders may just be choosy guys

Male spiders that treat some females as meals instead of mates may be expressing their preference for younger partners.

“Normally male spiders are viewed as active, let’s say, nondiscriminatory machines that are trying to copulate with every female they encounter,” says Lenka Sentenská of Masaryk University in Brno, Czech Republic. But that’s not the case for the small spider Micaria sociabilis, she and colleague Stanislav Pekár report in an upcoming Behavioral Ecology and Sociobiology. “We found males are choosy, and they can be choosy in an extreme way,” she says.

In more than 160 laboratory encounters between the spider sexes of all ages, about one-third ended with the male killing the female, Sentenská reports. She never saw a female kill a male, the iconic way that spider sex goes cannibal.

Sentenská estimated the spiders’ ages based on what time of year she caught them and her knowledge of their life cycle. Among spiders hatched in spring, females are the last to die. In the next generation, born in summer, males mature first. So by July, females were almost certainly a generation older than males, she says.

Meetings between these older females and younger males ended with a dead female more than 60 percent of the time, Sentenská says. At other times of year, when males and females were more likely to be from the same generation, she found less cannibalism. She hopes to investigate whether availability of other prey has an effect on cannibalism.

A male Micaria sociabilis encounters a female but instead of mating, kills her. July, when this was filmed, seems an especially perilous time for spider meetings.
Courtesy of Lenka Sentenská