Imagine you're home alone on a stormy night. This is usually fine, but you hear ominous footsteps outside, the window latch jiggles – your heart-rate quickens and your skin gets sweaty. Even though you know the windows are locked, you run to your car and speed off to safety (phew!). Just as you acted in this risky situation, a wild animal who senses a predator lurking nearby might also flee for the comfort of safety. But what happens if they can't easily escape? New research by Frédéric Dulude-de Broin and colleagues at the University of Laval, The Arctic University of Norway, and the Toronto Zoo wanted to address just this: what happens when animals live under constant threat of a predator attack?
Dulude-de Broin and colleagues focused their study on a species with little opportunity to run away from predators: mountain goats. Aptly named, mountain goats are often confined to mountain tops where their only option to evade predators, the likes of grizzly bears, cougars and wolves, is to scramble onto inaccessible cliffs and precarious ledges. This research group has been tracking a mountain goat population on Caw Ridge, Canada for 23 years. They have routinely trapped goats and marked them with colourful ear tags or collars so the researchers could identify the animals on daily visual scans with binoculars and spotting scopes during the spring and summer. Realising that they also observed predators while scrutinizing the goats, the team hypothesized that the goats would be more stressed in years when a large number of predators shared their slopes, just like you felt when responding to that potential home intruder. In this case, the physiological stress response can be a good thing, because it improves the likelihood of surviving a threat by increasing glucocorticoid hormones to free up stored energy and allow animals to mount the classic ‘fight-or-flight’ response. However, the stress response can also cause problems for animals in the long term, because it diverts energy away from routine body maintenance and reproduction.
Knowing this, Dulude-de Broin's team explored whether the goats were more stressed in years when there were high numbers of predators and whether chronic stress would be detrimental to mountain goat reproduction, potentially contributing to population decline. Reviewing their annual surveys, the team paid close attention to the females that had kids to measure how fertility varied each year. They also collected faeces from the goats that they trapped to measure the glucocorticoids in their systems in order to find out how stressed the animals were.
In years when the team observed large numbers of predators on Caw Ridge, the mountain goats were more stressed: for every 50 more predators spotted, the average glucocorticoid concentration increased by 53%. But what was the cost to the goats of living in a semi-permanent state of alarm? Well, normally, 50% of the female goat population produced a kid, but that percentage plummeted to 20% in the year following a period when a high number of predators prowled the peaks. The authors concluded that chronically high stress suppresses female fertility, likely delaying it to a less stressful season.
While mountain goats aren't at particular risk in Canada, these findings provide exciting insights into why the Caw Ridge herd has declined by 80% over the last decade. This team is the first to convincingly show that predator presence can suppress fertility in a wild, large-bodied mammal. Altogether, their work underscores that even just the risk of being eaten should not be underestimated when studying how predators shape prey populations.
