Dragonflies can literally be scared to death of fish. Who knew? In a study published in November in the journal Ecology, researchers found that dragonfly larvae reared in the presence of fish were four times more likely to die before reaching adulthood, compared to larvae raised in an environment without predators. Similarly, 2.5 times more dragonflies croaked when raised in the same tank as an invertebrate predator. The larvae were kept in cages in full view of the predators, although the cages kept the predators from entering, and each one contained a small cup where the larvae could hide.

The study also found dragonfly nymphs raised in tanks with a fish were 10 percent more likely to die while metamorphosing into their winged adult form that we know so well. Apparently growing up is not only stressful for humans, and being constantly reminded of one’s mortality doesn’t help. (But of course, I’m anthropomorphizing their metamorphosizing.)

The researchers also measured body size in surviving larvae and adult dragonflies (a species called the dot-tailed whiteface, or Leucorrhinia intacta). They found no difference between the different groups, which came as a bit of a surprise since previous studies on other animals have shown that various types of stress can affect development and growth rate, affecting adult body size.

So why did higher numbers of dragonflies die when raised alongside predators? The researchers don’t know but have several guesses. For one, the stressed-out larvae may have an impaired response to disease or pathogens. It’s well-established that stress can hamper immune response in a wide variety of animals (including humans), and the fact that a small percentage of the predator-free larvae died hints at a low level of disease in the population, gathered from lakes in Michigan. The researchers also suggest stress may lead baby dragonflies to use and store energy less effectively. But their best guess is that stress causes a variety of negative responses that can combine synergistically. This has been shown in other animals; for example, tadpoles exposed to predatory newts are more susceptible to herbicide poisoning.

All this seems somewhat intuitive, but few studies have examined concrete effects of stress in invertebrates. The findings could help pave the way for understanding stress in other animals, including the one that complains about stress the most: us humans.

Reference: Shannon J. McCauley, Locke Rowe, Marie-Josée Fortin. The deadly effects of “nonlethal” predators. Ecology, 2011; 92 (11): 2043 DOI: 10.1890/11-0455.1

Image: Gary Yankech / Flickr