Rassim Khelifa was standing by a pond in the Swiss Alps collecting insect eggs one day, when he noticed something strange. A dragonfly being hotly pursued by another suddenly took a dive and crashed to the ground, seemingly dead on the spot, before springing back to life and making a grand escape once the coast was clear. The observations that followed confirmed a previously unknown phenomenon – the insect had faked its death to give a wannabe lover the slip.
Khelifa, a biologist at the University of Zurich, had spent the summers of 2014 and 2015 gathering the eggs of dragonflies and damsel flies at ponds in the Swiss Alps, with the aim of running experiments on larval response to temperature. But his chance encounter with these would-be mates lured his curiosity in a different direction – was this kind of behavior widespread?
He then spent two months studying the reproductive behavior of the dragonfly species in question, Aeshna juncea or moorland hawker, across two different ponds in the area. One thing he found was that the females tended to lay their eggs in dense vegetation. This appears to be a protective measure, as females lay eggs on their own and are therefore quite vulnerable to harassment from males during the process ... unless they sneak away into the shrubs for some peace and quiet, that is.
But the aversion to clingy males doesn't stop there. Khelifa says that when the female is leaving the reproductive site is the time at which they are most vulnerable to coercion from the males, and it is here that they enact their most dramatic repudiative measure. Females were always chased by males after departure in his observations, and 31 out of 35 of those witnessed decided to crash to the ground rather than pair up with a mate.
Of those crashes, 71 percent landed the dragonfly in vegetation like bush or dense grass, and 87 percent – 27 out of 31 – were followed by faked deaths. The females do this by laying motionless upside down. This behavior is atypical of a dragonfly, and is apparently so convincing that it allowed 21 of the 27 to trick the male and escape.
Sexual death feigning, as it's called, is incredibly rare but not unsighted in the animal kingdom. It has been documented in nursery web spiders as a way of avoiding cannibalism, two species of robber fly and a European mantis. Khelifa's observations, however, mark the first time the behavior has been seen in odonates, the group of carnivorous insects covering dragonflies and damsel flies.
To see how aware the dragonflies were during their little charade, Khelifa also carried out experiments where he tried to physically catch them by hand while they laid there motionless. He says 27 of 31 females escaped, indicating that they are fully conscious, alert and ready to make a getaway as regular dragonfly would do. Talk about playing hard to get.
Khelifa's research was published in the journal Ecology.