Male spiders who give their all for sex

PUBLISHED : Sunday, 05 February, 2012, 12:00am
UPDATED : Sunday, 05 February, 2012, 12:00am


What would you give up to have sex? For the males of the orb-web spider Nephilengys malabarensis, it's a stark choice - either your life or your sex organ.

The females of the species are incredibly aggressive, and they kill and eat their mates. So, to avoid becoming dinner, the males snap off their own genitals - either all or in parts - to make an escape.

But new research by Li Daiqin of the National University of Singapore, published in the current issue of Biology Letters, found something equally astonishing: the genitals - a pair of structures called palps attached to the sides of the head - continue to pump sperm into the female after being severed.

Since only one palp is needed for mating, the lucky male that escapes has a second chance to do it again.

By giving up their weighty genitals, the partial eunuchs become more agile and therefore, better ighters to ward off potential sexual competitors.

'Why males castrate their genitals was then unclear,' Dr Li told the South China Morning Post.

'This 'eunuch behaviour' seemed maladaptive. We tested two hypotheses: the mating plug hypothesis [the broken male palp parts may effectively plug the female genitals against other males] and the good-fighter hypothesis [males may become agile to be more aggressive, or motivated to fight against other males as they are light and have nothing to lose].'

The explanation actually lies, Li discovered, in what he called 'the remote-copulation hypothesis'.

'The lodged male sperm-transferring organ in the female genital allows continuous sperm transfer even after the male has been detached,' he explained.

Because the females are usually in a hurry to break off copulation, the detached organ also allows more time to transfer the sperm.

According to Li's calculations, the palp delivers only about a third of the sperm by the time the female pushes the male away; the detached organ pumps more and at a faster rate.

The new research, according to Li, also has implications for understanding sexual evolution, such as how cannibalism and eunuch behaviour evolve together.

Despite being an authority on spiders, Li said he wasn't interested in them when he first started research.

'I was actually forced to study spiders just after graduating from university. 'Nevertheless, I quickly got interested in spiders - I was amazed by their biology. Now l love spiders and their behaviour.'