Sexual harassment is prevalent among common hawker dragonflies because males outnumber females at nesting sites and the ones that coerce females are more successful.
Animals do the most amazing things. Read about them in this series by Janaki Lenin.
She squeezed out the last of her eggs in a sheltered thicket of reeds on the banks of a pond and took to the air. A patrolling male dove on her. She could make a dash for it but her chances of outmanoeuvring him were poor. She fell out of the sky, crash-landing into a bush where she lay inert on her back. Seeing her lifeless body, the male abandoned his pursuit and flew away to await the appearance of another female. When the coast was clear, the female common hawker (Aeshna juncea) took off. Of course, another male dragonfly may accost her – but she could always play dead. Besides her suitor, the dragonfly fooled another observer.
Rassim Khelifa, a doctoral student at the University of Zurich, Switzerland, first observed this behaviour at a pond near Arosa in the Swiss Alps in the summer of 2015. When he went closer to the inert common hawker, she flipped over on her feet and flew away.
Many animals, from water bugs and hognose snakes to North American and white-eared opossums, feign death to escape their assailants. Some go to great lengths to put on an act, contorting their bodies, flipping over on their backs and hanging their tongues out of gaping mouths. When you set a hognose snake that is pretending to be dead right side up, it immediately rolls over on its back and resumes its act. And for good reason. Prey movement triggers predatory behaviour. Playing dead is the last resort of desperate prey, even if it means allowing the predator close enough to take a bite. If a quail pretends to be dead, a marauding cat would most likely ignore it and continue stalking one that’s running away. A pygmy grasshopper (Criotettix japonicus) feigning death erects prominent spines that would make swallowing it a painful business for a frog.
While such pretence to escape predators is relatively common, using the same strategy to avoid sexual encounters is rare and restricted to a handful of creatures – mostly insects. Male spiders fear their Goliath-sized predatory mates. They offer silk-wrapped insects as gifts before approaching females. If their mates accept the gifts and start feeding, the males have nothing to worry about. They can go forth and copulate. But sometimes females may ignore the meals, their predatory instincts triggered by even the males’ slow and cautious approach. And like prey animals, the males draw in their legs and pretend to be lifeless.
To female dragonflies, sex-obsessed males are no different than hungry predators. On seeing the female common hawker take off as soon as he came close, Khelifa was surprised and perplexed. He recounted his observation to colleagues, and everyone laughed. “They were both surprised and amused since the story is quite relatable,” he told The Wire.
He investigated whether this was a widespread behaviour in the species. When males chased 35 female common hawkers after they had laid their eggs, 31 crashed into bushes. The four that didn’t take evasive action were forced to mate with the males. Of the ones that plummeted to the ground, 27 lay still on their backs.
But why did those four flap their wings? “There might be behavioural variability among females of the same population – that is, not all females express death feigning,” says Khelifa. “Moreover, it might be related to the fact that after landing, females’ wings get stuck in vegetation and they may flap their wings to escape.” Some males seemed to cotton on to the trick. Of the 27 that played dead, the males saw through the pretence of six.
“It is difficult to say why they detected them because different factors may come into play, such as intrapopulation variation in detection efficiency,” says Khelifa, “or the contrast between female coloration and the background colour where she lands.”
Only 21 – about 60% of the observed females – successfully evaded the males. Since so many female dragonflies tried this trick, Khelifa thinks this is a common behaviour in the species. To check if the female common hawkers were conscious and alert after they crash-landed, he tried to catch them. Of 31 attempts, he caught only four; 27 escaped. He held 50% of the males patrolling two ponds captive for a day. With fewer males to escape from, the females grew bold, laying their eggs in more open vegetation. And they had fewer encounters with males when they soared into the sky.
Is the cost of mating so high that the females have to resort to such extreme measures? “The function of the male penis is not only limited to sperm transfer but also to remove sperm left in the female’s storage organ from previous matings,” says Khelifa. The sophisticated penis structure could damage the females’ reproductive tract while scooping out another male’s sperm. “One copulation is enough to fertilise all eggs. It is not in the females’ interest to carry out multiple copulations per day.”
Sexual harassment is common in the species because, according to Khelifa, males outnumber females at nesting sites and the ones that coerce females are more successful. The females’ tendency to lay eggs solitarily makes them vulnerable to such bullying. A combination of female vulnerability and male-biased sex ratio creates the conditions for the evolution of this behaviour.