Spiders self-castrate in fight for females
PHOTO: A spider from the tropical species Nephilengys malabarensis with only one sperm-filled palp (right).(Supplied: Qi Qi Lee)
Researchers have found male spiders that castrate themselves so their manhood does not weigh them down when they are fighting off competitors.
In a number of spider species, the females are much larger than the males and more aggressive.
The self-castrating spiders belong to one tropical species, Nephilengys malabarensis, in which the female cannibalises the male during copulation.
During mating the males break off one or both of their sperm-containing palps.
The palps, which look somewhat like boxing gloves, have a tube attached which is inserted into the female to enable sperm to be transferred.
When the palps break off they remain attached to the tube and sit on the outside of the female's body, blocking her reproductive tract.
This had led to some scientists to speculate that spiders castrate themselves to create a "mating plug" that continuously transfers sperm to the female after the male is gone - and at the same time stops competing males from inserting their sperm.
But animal behaviour expert Daiqin Li from the National University of Singapore says other males can easily remove the mating plug, which called for other explanations for the emasculating behaviour.
One hypothesis is that castrated males are better fighters because they are not weighed down by their manhood and thus have more energy to fight off competing males.
"The eunuch males are better fighters because they are lighter and they have an increased endurance," he said.
"They try to prevent the other males from mating with the female."
Dr Li said scientists had noticed that the 25 per cent of males that survive the terrifying ordeal of mating are better fighters.
"Relatively the palps are heavy compared to the whole body mass," Dr Li said.
"If they remove one or two palps, they become significantly lighter compared to the intact male and are able to run faster and longer."
Dr Li and his team tested this "gloves off" hypothesis by creating spiders with one or both palps removed.
He says the design of the palps mean they break off easily and his team used forceps to emulate this natural process.
They then weighed the half-eunuch and full-eunuch spiders and compared their weight to intact males.
Finally, they tested the endurance of the three types of spiders by prodding them to run around and measuring the time it took the spiders to get exhausted.
Dr Li and his colleagues found full-eunuchs weighed 9 per cent lighter than intact males, and half-eunuchs weighed 4 per cent lighter.
They also found that full-eunuchs had 80 per cent more endurance than intact males and half-eunuchs had 32 per cent more endurance.
Dr Li says the findings show intact males are more easily exhausted than eunuchs and provides a mechanism to support the "gloves off" hypothesis.
The researchers have reported their findings in Biology Letters.