These Male Spiders Attack Females and Tie Them Up Before Sex to Avoid Being Cannibalized

Male spiders of the species Thanatus fabricii bite and tie up their female partners with silk before mating to avoid being eaten, researchers have found.

A team of scientists from the Czech Republic observed the spiders, which are native to Israel, in a lab using a video camera, uncovering the unusual behavior.

Male spiders across different species typically court females in order to convince them to mate, but in some species, they actually coerce the opposite sex into engaging in this behavior.

Scientists think one of the reasons the males use this coercion is to avoid being cannibalized by the females—a gruesome ritual that is not uncommon during spider mating.

Male coercion during mating is an extremely rare behavior in spiders given the physical superiority of females, but the Czech researchers found that T. fabricii males never mated unless the female was bitten and immobilized, according to a study published in the journal Animal Behaviour.

This suggests that this strategy on the part of the males is necessary for mating to occur in this species.

For their study, the authors collected male and female Thanatus fabricii from a site in Israel and placed them together in a lab setting to observe the species' mating behavior.

The scientists found that the males would first bite the females on the legs. The females would then draw up their legs close to the body and become completely inactive and immobile.

"Spiders sometimes spend hours luring females to court them, but these guys just go and bite," Lenka Sentenská, an author of the study with Masaryk University in the Czech Republic, told New Scientist.

The male would then mount the female and quickly bind her legs and body up with his silk before inseminating her.

Once mating was finished, the females would often lie motionless for some time after the males had left, before freeing themselves by tearing up the silk threads they were covered in.

After mating, the scientists observed that the females were less mobile and less successful in catching their ant prey, suggesting that they had been injured in some way.

The researchers suggest that the males use this coercive strategy to avoid being eaten by the females, which are slightly larger, as well as to overcome their resistance to mating.

"Female immobilization, which is typical of forced copulation, may be particularly advantageous if males are at risk of being attacked and cannibalized by their mates during courtship," the authors wrote in the study.

Sexual cannibalism is common in spiders and other invertebrates in cases where the females are larger.

"In spiders, females generally dominate and control sexual interactions and so physically restraining a female would be highly advantageous for a male. Although their size typically does not allow them to coerce females to mate by physical force, spiders possess weapons such as silk which males can use to immobilize females," the authors said.

The strategy of the T. fabricii males was not sufficient to keep them safe in all instances, however. In 11 percent of mating cases, the females actually attacked and consumed the male before copulation.

In addition, the researchers say that despite the apparently forceful nature of the mating behavior, it remains unclear whether the females are actually completely immobilized by the males or if this inactivity is simply a signal that the females are receptive to the males' approach.

"Females that fall into the state of immobility will allow the male to mate, while unreceptive females will continue struggling and thus will reject the male," the authors wrote.