Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Video: Asexual stick insect fights off randy males

Species: Extatosoma tiaratum

Habitat: foliage-rich areas in Australia

WHO needs men? Female Australian spiny leaf stick insects (Extatosoma tiaratum) have figured out how to reproduce without males, and they fight off potential suitors ferociously.

In fact, they’re so into their newfound all-female lifestyle, they’ve even evolved an anti-aphrodisiac to keep the males away as they go about their business in the leafy undergrowth of eastern Australia.

If that doesn’t work, they have a few self-defence moves up their sleeves. Females curve their heavy, spiny abdomen tightly over their back, making it impossible for the male to hold on. They then kick the daylights out of the males with their hindlegs.


Making the switch

The leaf insect is facultatively parthenogenetic, which means females can shift between sexual and asexual reproduction. If a female hasn’t mated with a male before laying eggs, she produces eggs parthenogenetically – embryos develop from unfertilised eggs, and females give birth to more females. If a male manages to find a female and mate with her, the eggs are fertilised, resulting in both male and female offspring.

Animals are generally sexual; parthenogenesis is relatively rare. Very few animals – certain salamanders, for example – have become completely parthenogenetic. Parthenogenesis is thought to have evolved from sexual ancestors during times when low population densities have resulted in a scarcity of males.

If this were true in E. tiaratum, the female would be trying her best to find a mate. But this is definitely not the case – in fact, females are turned off by male odours and do whatever they can to keep males at bay.

Unwelcome advances

Just after the final moult, virgin females secrete a pungent chemical – an “anti-aphrodisiac” – to repel males. If a male still finds her and tries to mate, she fights tooth and nail to keep him away.

“Taken together, these results suggest that it’s probably unlikely that parthenogenesis evolved in this species as an adaptation to mate scarcity or low density,” says Nathan Burke from the University of New South Wales.

So why did parthenogenesis evolve in E. tiaratum? The costs of sex include attracting and finding mates, potentially harmful mating, the chance of contracting sexually transmitted diseases and the cost of breaking apart well-adapted genotypes, says Burke. “The benefit of parthenogenetic reproduction is that females can completely avoid the costs of sex by producing offspring clonally.”

Why sexual reproduction persists in such a species is an evolutionary puzzle, because you’d think that costly sex would become extinct when in competition with less costly parthenogenesis, he adds.

Fighting back

The answer, in the case of E. tiaratum, seems to lie with the males. It seems they have adaptations that help them overcome rejection. Males find females very attractive, and go all out to mate with them. They can fly and seek out females, while females have stunted wings and cannot escape easily. Males also have clasping organs at the tip of their abdomen to try to hold the female abdomen in place.

Is the species on course for an all-female future, then? “It will all depend on who comes up trumps in the tug-of-war between the sexes,” says Burke. “Any adaptation in females that reduces the likelihood of attracting mates is usually answered with counter-adaptations in males which allows males to easily overcome female barriers to mating. For this reason, I think males are here to stay.”

Journal reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2014.12.017