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Dot-tailed whiteface dragonflies begin their life as aquatic larvae. Undergoing metamorphosis into an adult like the one above is particularly stressful, and larvae forced to live next to predators were less likely to make the transition successfully. (Shannon J. McCauley/California Polytechnic State University)

Stress can kill on a dramatic scale, according to a new study of immature dragonflies forced to live next to scary predators.

In a newly published study, dragonfly larvae living within sight and smell of a predators such as fish had 2.5 to 4.3 times higher mortality than dragonfly larvae living in a predator-free environment — even though the dragonfly larvae were protected by a cage and could never be eaten by the predators.

As part of the experiment, the predator neighbour consumed two members of their dragonfly species, three times a week, in their presence.

Nevertheless, the high death toll is "a pretty extreme response," said Shannon McCauley, a biologist at the California Polytechnic State University who led the research and conducted part of it while she was a postdoctoral researcher at the University of Toronto.

She added that it was particularly surprising because dragonfly larvae are typically so hardy and robust — they can withstand dirty water, crowded conditions and even being out of the water they normally live in for short periods of time — qualities that make them good subjects to study in the lab.

Because of that, McCauley suspects that stress can have significant impacts on mortality in other organisms.

"It would really surprise me if they were that different from other animals."

McCauley said lots of lab experiments have shown that stress has a negative impact on animals' immune system, and she thinks this may be what is causing the higher mortality in the dragonflies.

"We don't think they're like 'Aaaahhhh, it's a predator!' and then have little heart attacks. [It's that] the common things that they normally deal with, infections and parasites — those things become lethal when they're stressed."

She suggested that a significant link between stress and mortality hasn't really been documented before in other animals because researchers were mostly focused on other, non-lethal effects. They may not have run experiments for long enough or used extreme enough conditions.

She did her own experiment in two stages.

In the first, the dragonfly larvae were in smaller tanks and therefore were in closer proximity to the predators at all times. That led to very high mortality.

In the second experiment, conducted at the University of Toronto, the dragonfly larvae were placed in bigger tanks more similar to a natural environment and allowed to mature to adulthood. In that case, the larvae suffered only 10 per cent higher mortality in the presence of a predator.

However, the dragonfly larvae that lived with predators were five times more likely to die while metamorphosing into an adult — an event that results in additional stress.

The results were published in the journal Ecology.