It's hard to be a showshoe hare. The northern animals are in a constant race for survival with their predators, always cycling between population booms and busts. In hard years, hares are understandably stressed. And that stress can leave its signature not just on those animals, but on several future generations. When life is good, populations of showshoe hares (Lepus americanus) can roughly double every year. But the hare's predators—lynx, foxes, coyotes—also increase in numbers as their food multiplies. Then the hare population crashes: nearly every animal becomes a meal. Predators also lose numbers as their food disappears. This creates a constant cycle of about 8 to 10 years, with predator populations lagging a year or two behind hares. This repeating story has one mysterious chapter, though. After hares and their hunters have both crashed, the hares don't hop back right away. Their numbers stay low for another 2 to 5 years, even though a new generation of hares is born every year. The varying length of this slump is an "enigma," writes Pennsylvania State University ecologist Michael Sheriff. Based on earlier research, Sheriff had a hunch that the explanation might be epigenetic. In other words, new generations of hares might inherit something from their stressed-out parents that keeps them from reproducing well. (Epigenetic means that even though the genes themselves haven't changed, parents can pass down an altered way of expressing those genes.) To search for the answer, Sheriff and his coauthors scoured data from 6 decades' worth of snowshoe hare population data. The numbers were collected between 1961 and 2013 in Alberta and Yukon Territory. This covered 6 boom-and-bust cycles for the hares. For each cycle, the researchers calculated the rate of decline during the bust phase. How quickly were predators munching through the hare population? Then they counted the number of years that passed before hares started reproducing at their normal rate. They saw a clear relationship between the two factors: the sharper the decline had been, the longer the population slump lasted. The authors ruled out possible explanations that don't involve stress. The length of the population slump doesn't seem tied to a lack of food, for example, or to predators that are still hanging around. They also ruled out the idea that hares are so rare during their population slump that they can't find each other to reproduce. As the slump ends, the hares' reproductive rate increases even though their population density hasn't changed. This left the epigenetic explanation. "The variation in the length of the low phase could be due to inherent, long-lasting changes within the animal caused by factors acting during the decline," the authors write. The more severely hares are hunted, the more future generations are damaged by the stress. The reasons for this are likely "very complex," Sheriff says. But some of his earlier studies have shed light on how it might work. While predators are obliterating the hare population, mother hares have increased stress hormones. These chronically stressed moms give birth to fewer young, and the babies they do have are smaller than usual. These offspring, too, have higher stress hormones, and their stress response systems are more reactive. Sheriff says this disrupts the hormones that regulate reproduction, and the hares' sexual behaviors might even be affected too. This is the first time, Sheriff says, that scientists have found a connection between the severity of a stressful event and the length of time for which that stress can be inherited. We've seen before how stress can affect future generations, even in humans. But the research in hares shows that the degree of that stress makes a difference to how many generations are harmed. Conservationists might have to keep in mind that even after a stressor is gone—a fire, a drought, meddling humans—its memory can linger. An animal may not know what happened to its great-grandparents, but their story is recorded in its body.
Image: by Denali National Park and Preserve (via Flickr)
Sheriff, M., McMahon, E., Krebs, C., & Boonstra, R. (2015). Predator-induced maternal stress and population demography in snowshoe hares: the more severe the risk, the longer the generational effect Journal of Zoology DOI: 10.1111/jzo.12249