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Female Stick Insects Go To Extremes to Avoid Sex

Some women just can’t stand men; they would rather be alone than being in the company of men. The same goes for some female stick insects: they do their best to keep the males away. They don’t even need the men for reproduction; they counter this predicament by cloning themselves. They even go the extra mile: they modify their pheromones so that they do not attract males. If any unwanted male does approach one of them, it is quickly repelled. As though these mechanisms were not already extreme, the virgins among them manufacture anti-aphrodisiacs.

Extatosoma tiaratum female

Female stick insect

Why would the female stick insects go to lengths to keep their males away? A new study has brought more insight into the topic.

The findings reveal that sex is costly for the insects. As a consequence, the females take to an asexual mode of reproduction known as parthenogenesis whereby the offspring hatch from eggs not fertilised by males.

Parthenogenesis is considered to be rare in the wild. Those animals who have adhered to this method did come from sexual parents though.

Females are thought to go for parthenogenesis when males are rare. The new study presents another possible reason: the transition from sex from to the asexual method might have been caused because of the sexual interactions being costly to the animals. The difficulties entail (the costs) the time-consuming nature of spotting mates, the risk of contracting venereal diseases, and the chances of coming across predators. Furthermore, sex is also associated with a shorter lifespan.

One of the authors summarised the reasons as follows:

The benefit of parthenogenetic reproduction is that females can completely avoid the costs of sex by producing offspring clonally”.

The team worked on the spiny leaf stick insects, the Extatosoma tiaratum. The latter reproduce by parthenogenesis but are not restricted to that means only.

stick insect

Extatosoma tiaratum (female)Photo credits: Stephan M. Höhne

The females studied by the researchers seem to counter the costs of sex by making use of “counter-evolved resistance traits”.

Virgin females of the species would repel potential mates by producing an anti-aphrodisiac. Those of reproductive age would use modified pheromones as an invisibility cloak to males: the males would not be attracted since they would not recognise the pheromones.

It is no exaggeration to say that these females go to absolute lengths to avoid sex. For instance, those who were paired with males would curl their abdomens so that the males were unable to grab them. Those males who tried to copulate in spite of the ‘acrobatics’ of the females would be severely kicked off by way of the latter’s hindlegs.

Adding to the bulk of information, the authors found that those parthenogenetic females who would have sex would suffer from higher mortality. Furthermore, they would experience a decrease in their egg production, relative to the exclusively parthenogenetic and even to the exclusively sexual females. This suggests that avoiding sex happens to be beneficial for the females.

The scientists therefore concluded that parthenogenesis does not seem to be an adaptation to the lack of mates. Rather, they argued that sexual conflict coupled with the interference of resistance from the females might have encouraged facultative parthenogenesis. Does this imply that the males are to disappear? As a matter of fact, as the scientists have argued, the males are not expected to remain passive to the females’ adaptations.

“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,” said one of the authors in a statement.

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