Some female stick insects just don’t need males around, ever. They clone themselves, alter their pheromones to stay inconspicuous to unwanted suitors, and when males try to copulate with them, they literally kick them off. Oh, and virgins also produce anti-aphrodisiacs. The reason is that sex is costly, according to new findings published in Animal Behaviour.
Parthenogenesis is an asexual way of reproducing, where female offspring develop from eggs that weren’t fertilized by males. It’s pretty rare in the wild compared to sex, and parthenogenetic animals did come from sexual forebears. So why did some animals make that transition, and how does it persist in nature? After all, sexual reproduction offers benefits like genetic diversity. Well, sometimes females turn to parthenogenesis when males are rare. But now, according to a team led by Nathan Burke from University of New South Wales, the sex-to-parthenogenesis transition might also happen when sexual interactions are costly. And these costs include time wasted looking for mates, venereal diseases, increased predation risk, and a shorter lifespan, the authors explain in The Conversation. "The benefit of parthenogenetic reproduction is that females can completely avoid the costs of sex by producing offspring clonally,” Burke tells New Scientist.
To test the mate scarcity hypothesis against the sexual conflict hypothesis, the team conducted a series of experiments with spiny leaf stick insects (Extatosoma tiaratum). These are facultatively parthenogenetic, which means they reproduce asexually, but aren't restricted to that form of reproduction only. The females, they found, appear to neutralize the costs of sex by using “counter-evolved resistance traits.” Pre-reproductive virgin females produced an anti-aphrodisiac that repelled males, while parthenogenic females altered their pheromones so that they would stay inconspicuous to males. Females who were paired with males avoided sex by curling their abdomens to make it impossible for males to grab hold, and when males did attempt to copulate, the females vigorously kicked at them with their hindlegs.
Parthenogenic females who switched to a sexual reproductive mode suffered higher mortality and a decline in egg production—compared to exclusively parthenogenetic or exclusively sexual females—suggesting how females in these experimental circumstances at least benefit by avoiding mating. "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," Burke tells New Scientist. Sexual conflict mediated by female resistance, they write, could contribute to the evolution and persistence of facultative parthenogenesis.
But that doesn’t mean there will be no more males in the future. "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,” he adds. “I think males are here to stay."