Male fishing spider might offer himself as snack to females to help offspring

Dark fishing spiders (female on the left, male on the right) get their name from the fact that they often live near water and hunt for insects and small fish by walking on the water(Credit: Karina I. Helm)

The urge to reproduce in the animal kingdom can often be stronger than the urge to survive. Case in point – the male dark fishing spider. After the arachnid mates with his female companion, which is roughly double his size and can weigh up to 14 times more than the diminutive male, he dies and the female eats him. New research out of the the University of Nebraska-Lincoln postulates that the macabre ritual might have to do with ensuring healthy and abundant offspring.

Unlike other spider species, such as the famous black widows, the female fishing spiders aren't responsible for killing the males. The death of the males is due to the fact that after they deposit sperm into the female using an appendage known as a pedipalp, a bulb inside it inflates. In most other spider species, after mating is over, that bulb deflates. In the male dark fishing spider however, the bulb stays swollen, which traps some of the spider's circulatory fluid. This causes the male to curl up into a death-like state in which his heart stops beating after a few hours.

Researcher Steven Schwartz of the University of Nebraska-Lincoln first discovered this odd arrangement in the dark fishing spider species in 2013. In the new research, Schwartz and his colleagues wanted to see if any benefit was arising from the sacrificial death of the male and the subsequent devouring of her mate by the female.

Sure enough, they found that females who ate their mates gave birth to about double the number of baby spiders than those who went without the meal. Also, spiders with the cannibalistic moms grew roughly 20 percent bigger and lived about 50 percent longer than those whose mothers abstained from the post-coital snack.

At this point, the researchers aren't quite sure why females' munching on the males conveys such a benefit to the offspring. To ensure it was definitely the male body that was boosting the brood, Schwartz and his team fed some females crickets that were just about the same size as the male spiders after sex, but they didn't affect the offspring at all.

"There might be a nutrient, or maybe a cocktail of nutrients, that is somehow concentrated in the males' bodies," says Schwartz. "We don't know what that is, but there is something going on there."

The work of the team has been published in the journal Current Biology.

The following not-creepy-at-all video shows the mating between two dark fishing spiders followed by the male's death.

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