Male brown widow spiders seek to mate with older, less-fertile females who are 50 percent more likely to eat them after sex, according to Israeli researchers in a new study published in the journal Animal Behaviour. The researchers are still puzzling as to why.
Researchers at Ben-Gurion University of the Negev, The Hebrew University of Jerusalem and The Volcani Center collected male and female brown widow spiders from central and southern Israel and then positioned the females in such a way as to give the males the choice of which ones to approach — the immature, sub-adult female spiders or the mature ones. Immature females are able to mate and produce viable offspring, and in contrast to mature females, they do not cannibalize their mates after copulation.
The researchers then observed the interactions.
“We originally thought the males would prefer the sub-adult females, as they are more fertile and far less likely to cannibalize them, but we were surprised to discover that was not the case,” the researchers said in a statement.
Indeed, the male spiders, when given a choice, preferred mature over sub-adult females and older over young mature females.
“We found no benefit for males in mating with the females of their choice,” the researchers said in the study. “Older females were significantly less fecund than young mature females, and were not more fecund than subadult females.”
The researchers sought to determine if there were any possible advantages for the males in mating with the cannibalistic, mature females, e.g., longer copulations or the ability to plug the female genital duct with their male copulatory organs, to stop her from mating with other males. They also tried to find possible specific disadvantages in mating with the younger spiders, such as a higher risk of remating.
“None of these explanations was supported. Thus, we lack an adaptive explanation for male preference for mature older females,” said the researchers.
One possible explanation is that older females are manipulating the males by using strong signals to attract them, the researchers said. This is, however, a hypothesis that still needs to be tested.
“We suggest that older females produce more pheromone to attract males and that males are thus misled into mating with older, more aggressive and less fecund females,” the researchers said.
The study is part of the M.Sc. thesis of Shevy Waner of The Hebrew University of Jerusalem. She was advised by Prof. Uzi Motro from the Department of Ecology, Evolution and Behavior and the Federmann Center for the Study of Rationality at HUJI, and Prof. Yael Lubin from the Marco and Louise Mitrani Department of Desert Ecology at Ben-Gurion University, and Prof. Ally Harari of the Department of Entomology at The Volcani Center.