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Spider self-castrates to counter cannibalism during sex

02 February 2012

Despite breaking off its reproductive organ (red box), this male orb-web spider did not escape being eaten by the female partner

Photo: Joelyn Oh
An NUS study has discovered that some male spiders sever their genitals during sex as an adaptive counter-measure to being eaten by their partners.

Led by Assoc Prof Li Daiqin at the Department of Biological Sciences, the researchers examined the behaviour of Nephilengys malabarensis, a species of orb-web spider which practises cannibalism. About 75 per cent of the much bigger females eat up the males during copulation.

The team observed the mating of 25 pairs of virgin spiders. All the males suffered genital injuries - the sperm-delivery palps of 22 were completely snapped off while the rest were partially damaged.

On dissecting the females, the group found that the palps had delivered only a-third of their sperm after the females stopped copulation. However, the sperm continued pumping through the apparatus lodged in their reproductive organs.

The results demonstrated that the "remote copulation" served to increase the chances of fertilisation success, thus perpetuating the male's genes. The researchers postulated that the behaviour evolved in response to female-controlled short copulation, and ultimately the female's frequent cannibalism.

This study was published in the journal Biology Letters on 1 February.

Previous work conducted by the team found that the detached palp can prevent other males from mating with the female, thus ensuring the paternity of the self-castrating spider. The eunuch also becomes more aggressive and guards the female from competitors while the sperm is being transferred.