Males of a tropical species of orb-web spider castrate themselves,either partly or fully,after mating. It might seem a self-destructive habit,but in removing their genitals,a new study reports,the spiders are actually improving their reproductive success.
By becoming a half eunuch or full eunuch,the spider reduces its weight by 4 to 9 per cent. It then stands guard by the female it has mated with. Its lighter weight allows it to better fend off other males.
When invadersintact malescome in,eunuchs fight and do much better than the intact males, said Daiqin Li,a behavioural ecologist at the National University of Singapore and one of the studys authors.
By keeping away other males and preventing females from mating again,the eunuchs can reduce sperm competition and maximise their own reproductive success. The researchers found that half eunuchs had a 32 per cent increase in their endurance levels and full eunuchs an 80 per cent increase.
All this is contingent on male spiders even surviving to this stage,however. About 75 percent of the time,females of the species Nephilengys malabarensis cannibalise males after copulating. Females are aggressive,and much bigger and heavier than males,Li said. Males can mate a maximum of two times in their lifetime,depending on whether they remove one or both of their sperm-delivery appendages.
The study appeared in a recent issue of the journal Biology Letters.