Spiders sacrifice genitals to ensure paternity
Friday, 9 March 2007
Sarah Bartlett
Cosmos Online
SYDNEY: Male wasp spiders allow part of their genitals to snap off and lodge in their female partner during sex - impeding other males attempts to mate with her.
The group of German researchers who conducted the study think that the males' behaviour means that their own sperm receives less competition for fertilising their partner’s eggs.
The males have to make their copulation count; female wasp spiders (Argiope bruennichi) usually end their 25-second sexual encounters by attacking and devouring the smaller male, meaning that he generally gets only one chance to pass on his genes.
Researchers had previously shown that 80 per cent of the time, males leave part of their genitals wedged in the females sexual orifice, said Gabriele Uhl of the University of Bonn.
But it had been unclear, she said, whether the spiders, native to central Europe, were simply hurrying to escape, or if their behaviour was a deliberate attempt to sabotage attempts to mate with further males later on.
Male spiders don't have a penis. Instead, they secrete semen into a mass of special silk, and then dip a form of specially adapted leg known as pedipalps into it. These pedipalps are used to transfer semen to the female during sex, and are the appendages that wasp spiders leave broken off in their partners.
By observing many spider matings in the laboratory, Uhl and her colleagues from Bonn and the University of Hamburg found thatwhether a males' genitals snapped off or not had no significant effect on his survival rate. But they found that it did make a significant difference on the success of subsequent matings.
When the pedipalps were left wedged in place in the female’s duct, subsequent matings were an average of just 8 seconds long - less than half the length of matings when the females were unimpeded. Additionally, males copulating with females whose ducts had been plugged were much less successful in breaking off their own genitals.
The team - who reported their findings recently in the U.K. journal Behavioural Ecology - have also shown that other wasp spiders also employ the same mechanism. “We presume that genital mutilation only makes sense if there is hardly any chance of further copulation anyway,” said Uhl - the males sacrifice everything in their one chance of passing on their genes.
"This exciting discovery brings another twist to the evolution of sexual cannibalism," commented behavioural ecologist Marie Herberstein of Macquarie University, in Sydney, Australia, who added that breaking off their pedipalps renders the males sterile.
Broken off pedipalps are not the only structures that male spiders use to interfere with females’ mating encounters. Many are known to use special secretions to create 'mating plugs', a kind of cork they use to block up the female's insemination ducts and prevent further inseminations from rival males.
Herbestein added that sexual cannibalism is common practice in spiders and that in some species, such as the Australia's red-back (Latrodectus hasselti), males actually sacrifice themselves to be eaten during copulation, as female redbacks are much less likely to mate again if they have cannibalised one mate.
Talk about extremes in sexuality and submission.
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