Male Spiders May Break Off Genitals To Boost Paternity
February 1, 2012

Male Spiders May Break Off Genitals To Boost Paternity

Perhaps it is not a thought that crosses the minds of the average male, but spiders have more to consider than most other creatures when it comes to choosing a mate. Biologists have long known that some male spiders sacrifice their genitals in the act of reproduction and that doing so leaves them sterile, but no one could adequately explain why this was so.

An answer, however, may now be at hand following a series of meticulous experiments by Daiqin Li, a behavioral ecologist at the National University of Singapore, reports Ian Sample, science correspondent for the Guardian.

Li and his fellow researchers studied a species of orb-web spider called Nephilengys malabarensis. The males are much smaller than the females and more often than not are eaten by their partners during copulation.

The male´s sexual organ, known as a palp, is broken off inside the female - behavior known as the “eunuch phenomenon”. Often the female breaks off the palp, but males engage in the behavior too. Male spiders are usually born with two palps.

The broken palp, left behind in the female is thought to function as a “plug” to stop other males from later mating with the same female, it also makes sense in light of the female propensity for cannibalizing males after sex.

The researchers found that sperm transfer from the broken palp continued after sexual intercourse had been terminated and that the longer the palp was left in the female before its removal, the more sperm were transferred, writes Jennifer Walsh for MSNBC.

About 30 percent of sperm were transferred to the females before the palp break off, and about 70 percent of sperm were still left in the broken palp,” Li said. “In our experiments, it took about 20 minutes to transfer about 85 percent of [the] sperm.”

Li thinks the same sperm-transfer bonus may apply in other species, as well, “this may apply to other spider species and other animals in which males castrate their genitals, for example, in fire ants, ground beetles, scorpions and cephalopods,” he tells BBC News.

The research, detailed in the publication Biology Letters, claims that there may be other benefits of the castration such as an enhanced aggressiveness of the de-sexed spiders, which can lead to gains in choice web spaces for the spiders.


Image Caption: Eunuch male spider with no palps. Photo credit: Lee Qi Qi (C)


On the Net: