A hungry fish can kill prey with a quick bite. That is, of course, if its prey hasn't already died of fright. Take tasty dragonflies. The mere presence of predators—even caged ones—is enough to scare dragonflies to death, according to a study in the journal Ecology. [Shannon J. McCauley, Locke Rowe, and Marie-Josée Fortin, The deadly effects of ‘nonlethal’ predators]
Researchers collected wild dragonfly larvae, and placed them in tanks with fish or insect predators. The larvae could see and smell their hunters—but were kept safe by underwater cages. After two months, the researchers took a head count—and found that dragonfly larvae sharing quarters with their killers were two to four times as likely to die off, compared to counterparts living in predator-free waters. And they had slimmer chances of surviving metamorphosis, too.
The authors suggest a couple reasons why. First, prey tend to make fewer forays for snacks when predators are lurking around, so they may not be as nutritionally fit. And previous studies have shown that the presence of predators ups stress levels in prey, weakening their immune systems and making them more vulnerable to disease—and death.
As if being eaten wasn't enough to worry about, looks can kill too.