Insects are scared to death of fish

Oct 27, 2011
This is a dot-tailed whiteface dragonfly. Biologists at the University found that the mere presence of a predator causes enough stress to kill a dragonfly, even when the predator cannot actually get at its prey to eat it. Credit: Shannon J. McCauley

The mere presence of a predator causes enough stress to kill a dragonfly, even when the predator cannot actually get at its prey to eat it, say biologists at the University of Toronto.

"How prey respond to the of being eaten is an important topic in ecology, and we've learned a great deal about how these responses affect predator and prey interactions," says Professor Locke Rowe, chair of the Department of Ecology and (EEB) and co-principal investigator of a study conducted at U of T's Koffler Scientific Reserve.

"As we learn more about how respond to – whether it's the presence of predators or stresses from other natural or human-caused disruptions – we increasingly find that stress brings a greater risk of death, presumably from things such as infections that normally wouldn't kill them," says Rowe.

Shannon McCauley, a post-doctoral fellow, and EEB professors Marie-Josée Fortin and Rowe raised juvenile dragonfly larvae (Leucorrhinia intacta) in aquariums or tanks along with their predators. The two groups were separated so that while the dragonflies could see and smell their predators, the predators could not actually eat them.

"What we found was unexpected - more of the dragonflies died when predators shared their habitat," says Rowe. Larvae exposed to predatory fish or aquatic insects had survival rates 2.5 to 4.3 times less than those not exposed.

In a second experiment, 11 per cent of larvae exposed to fish died as they attempted to metamorphose into their adult stage, compared to only two per cent of those growing in a fish-free environment. "We allowed the juvenile dragonflies to go through metamorphosis to become adult dragonflies, and found those that had grown up around predators were more likely to fail to complete metamorphosis successfully, more often dying in the process," says Rowe.

The scientists suggest that their findings could apply to all organisms facing any amount of stress, and that the experiment could be used as a model for future studies on the lethal effects of stress.

The research is described in a paper titled "The deadly effects of 'nonlethal' ", published in Ecology and highlighted in Nature this week. It was supported by grants to Fortin and Rowe from the Canada Research Chairs program and the Natural Sciences and Engineering Research Council of Canada, and a post-doctoral fellowship awarded to McCauley.

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DavidMcC
1 / 5 (1) Oct 28, 2011
How do they know it's psychological stress and not just plain old poisoning from the smell? They don't seem to worry about insectivorous birds, do they?
EDIT: In other words, the "stress" need not be psychological. In fact, if it was, wouldn't they try to fly away from it? Even if they did, it could be that they just can't stand the smell of fish! :)
TrevS
not rated yet Oct 30, 2011
Is there an evolutionary benefit to this?
DavidMcC
1 / 5 (1) Oct 31, 2011
Is there an evolutionary benefit to this?


If the "stress" on the dragonfly is toxic stress rather than pstychological stress, then it might well be an evolutionary benefit to the fish, because the poisoning helps it to hunt. If, on the other hand, it is just a side-effect of the dragon fly's "fear" of the fish, then it would be the down side of the fly's "evolutionary counter-measure" to predation - the fear that is a precursor to flight (flight by flight, that is!).
DavidMcC
not rated yet Nov 16, 2011
Discussing this on a different site, I have realised that there were several issues with this article. I may well get another "1" for this, but I suspect that what is really happening here is that the exposed dragonfly nymph is being denied the chance of feeding by the presence of the fish, because, normally, the nymph would stop hunting in order to stay still to avoid being spotted by the fish. However, in this case, the fish never goes away. Thus, the insect isn't dying of fright (which has never been shown to happen in insects).

DavidMcC
not rated yet Nov 16, 2011
Maybe the journalist who composed the title of this article should also get a "1".