ivory towers

Is it healthy for male spiders to be eaten by their mates? A short paper, "Sexual cannibalism in an orb-weaving spider Argiope aemula" (Animal Behaviour, vol 49 1995, 1119-21) by Takeshi Sasaki and Osamu Iwahashi, explores the possible benefits. The researchers reach the conclusion that: "...sexual cannibalism is a male mating strategy that may represent a form of terminal parental investment".

The courtship routine is predictable: male arrives at female's web, courts, copulates, then tries to jump away. Slow or unlucky males, however, are eaten. If not, they try again. Previous research had not revealed whether males tried to escape after the second bout. This study, involving the observation of 87 female and 55 male spiders in the wild and during "staged matings in the laboratory", suggests that they do not.

In the field, there were 11 observed matings. In seven of these the male was eaten after his first observed insertion. (There was no way of knowing whether this was indeed the first mating.) Of the four males who survived for a second try, two were eaten, one escaped and one, though not eaten, was found to be dead anyway.

In the laboratory, 98 males were introduced to females and 52 tried to copulate. None was eaten before mating, but 20 were after the first bout. Among the wounded was one spider that lost five legs, another that lost four legs and two that died the next day.

Of the 28 bouts that went into a second round, all but one ended with the male being eaten. The one exception left his mate apparently unharmed but was dead by the next day.

Subsequent experiments, involving the removal of males by forceps during the second mating, indicated that most of them died during mating.

The behaviour suggests "male complicity" in sexual cannibalism, increasing the chance of reproductive success by enhancing the female's diet. "Further study of the mechanism of male death during the second insertion is required."