Female Dragonflies Play Dead To Avoid Amorous Males

Pretending to be dead to avoid predators is a rare behavior, but a new study reports that female dragonflies have adapted this behavior to avoid unwanted sexual attention from males of the species

When ecologist Rassim Khelifa, a graduate student at the University of Zurich, went in to the field to collect eggs from several dragonfly species that he studies in the lab, he did not expect that he would see a previously unrecorded behavior.

"On 5 July 2015, while I was waiting at a pond near Arosa, at about 2000 m[eters] elevation, I witnessed a dragonfly dive to the ground while being pursued by another dragonfly," Mr. Khelifa writes in his paper (ref) that was recently published in the journal, Ecology.

"As I approached the two insects, I realized that [...] the individual that crashed was a female, and that she was lying motionless and upside down on the ground."

"Upside down is an atypical posture for a dragonfly," Mr. Khelifa noted dryly.

"The male hovered above the female for a couple seconds and then left. I expected that the female could be unconscious or even dead after her crash landing, but she surprised me by flying away quickly as I approached," continued Mr. Khelifa, who had never previously seen this behavior in his 10 years of studying dragonflies.

Crashing headlong to Earth

Mr. Khelifa had witnessed a female moorland hawker dragonfly, Aeshna juncea, suddenly plunging to Earth and then playing dead to avoid mating with a male of the species.

At 2.9 inches (74 millimeters) long, the moorland hawker is a large sexually dichromatic dragonfly. The male of the species is colorful: its black abdomen is decorated with paired spots of blue and yellow on each abdominal segment, and it has bold stripes on the upper side of its thorax. In contrast, females have a brown abdomen with yellow spots. This common dragonfly is widespread throughout the Palearctic ecozone, which stretches from Ireland to Japan, and it also occurs in northern North America.

Because their larvae are aquatic, female moorland hawkers must lay their eggs in ponds, and this is where amorous males await -- and harass -- them. Mating once fertilizes all of the female's eggs, but a second encounter can harm the female by drowning her or by damaging her reproductive tract.

Did the female just trick that male? Mr. Khelifa wondered. Did she fake her own death to avoid male harassment? How common is this behavior in this species?

To get some answers, Mr. Khelifa designed several experiments to clarify the reproductive behaviors of moorland hawkers. First, he divided their reproductive behavior into four major sequential events (figure 2):

1. female arrival at reproductive site: mature males congregate close to the pond waiting for females to arrive
2. copulation: when the female arrives at the pond, the male intercepts her in midair and they form a copulatory wheel. After copulation, the male detaches and flies away to seek another mate
3. oviposition: at this point, the female is alone and unguarded -- unlike most dragonfly species where males engage in mate-guarding behavior -- so she retreats to dense vegetation alongside the pond where she seeks a suitable site to lay her eggs
4. female departs from reproductive site

Mr. Khelifa assessed female approach and departure from the reproductive site, which is when they are most vulnerable to male attacks. Of the 35 females that he followed, he found that 31 crashed to Earth when a male dragonfly chased them. Those that did not crash were all intercepted by a male. He also found that, of the 31 females that crashed, 22 (71%) crashed into dense vegetation rather than onto open ground. Additionally, Mr. Khelifa then observed the females playing dead in 27 out of 31 crashes (87%). Of these "dead" females, 21 (77.7%) successfully deceived the male.

After he knew what to look for, Mr. Khelifa frequently observed this behavior at two separate sites, and concluded that "playing dead" is a common behavior for this particular species. He noted that remaining motionless was critical to avoiding the male because dragonflies are strongly drawn to motion.

He also had to know if these crashing females had knocked themselves out. To test whether "dead" females are conscious or unconscious, Mr. Khelifa attempted to catch them by hand -- which is nearly impossible for a person to do if the dragonflies are fully aware of their surroundings. Of his 31 capture attempts, 27 dragonflies escaped. Mr. Khelifa concluded that the "dead" females are fully aware of their surroundings and thus, are quite capable of avoiding marauding males, human researchers and predators.

Field work is critical to discovering and understanding nature

Like all good research, this study raises more questions than it answers; most importantly, how did "playing dead" to avoid sexually aggressive males first evolve? Many dragonfly species are known to "play dead" to escape a predator, so it's possible that female moorland hawkers simply expanded this anti-predatory behavior as a response to marauding males. The moorland hawker reproductive system is different from that of many other dragonfly species because it does not include mate guarding, and, since there is a strong bias for males to group around ponds where females must lay their eggs, the females were pressured to somehow adapt to lessen their vulnerability to male harassment. But the overall rarity of this behavior in nature makes it difficult to study.

This study underscores the value of field studies to document and to better understand nature. Even common and widespread species, such as moorland hawker dragonflies, have surprises that we can appreciate -- if we only take the time to actually see them. Looking to the Animal Kingdom may also shed light on our own deeper truths -- or, as JK Rowling said: "We've all been there."

Source:

Rassim Khelifa (2017). Faking death to avoid male coercion: extreme sexual conflict resolution in a dragonfly, Ecology, published online ahead of print on 24 April 2017. doi:10.1002/ecy.1781

Female Dragonflies Play Dead To Avoid Amorous Males | @GrrlScientist