Every few years, snowshoe hare numbers in the Canadian Yukon climb to a peak. As hare populations increase, so do those of their predators: lynx and coyotes. Then the hare population plummets and predators start to die off. The cycle is a famous phenomenon among ecologists and has been studied since the 1920s.
In recent years, though, researchers have come to a startling conclusion: Hare numbers fall from their peak not just because predators eat too many of them. There’s another factor, too: Chronic stress from living surrounded by killers causes mother hares to eat less food and bear fewer babies. The trauma of living through repeated predator chases triggers lasting changes in brain chemistry that parallel those seen in the brains of traumatized people. Those changes keep the hares from reproducing at normal levels, even after their predators have died off.
And it’s not just snowshoe hares, as the behavioral ecologists Liana Zanette and Michael Clinchy have shown. Zanette and Clinchy, both at the University of Western Ontario, are a married couple who majored in psychology as undergraduates. Today, they study what they call the ecology of fear, which combines the psychology of trauma with the behavioral ecology of fear in wild animals. They’ve found that fear of predators can cause other wild mammals and songbirds to bear and raise fewer young. The offspring of frightened voles and song sparrows, like those of stressed snowshoe hares, are less likely to survive to adulthood and succeed in reproducing.