Stress Everlasting: The Cautionary Tale of Stressed-Out Bunnies

Whether at work or at home, we all experience the effects of stress, rarely heeding its red flags. The anxiety, the loss of appetite, the insomnia—most of us dismiss stress as little more than a psychological irritant, an ephemeral feeling that can knock us down; from which we can always bounce back.

But stress packs a heavier punch than we might think. Contrary to how we experience it, the physical effects of chronic stress go beyond the short term, and perhaps even beyond the long term. Because the latest research in stress physiology indicates that some effects of stress may not only last a lifetime, they could leak into your children and even their children—permanently.

While many stress physiologists examine stress in the lab, Dr. Rudy Boonstra, from the Centre for the Neurobiology of Stress at the University of Toronto, looks at stress in snowshoe hares in the forests of the Yukon. At first glance, snowshoe hares might seem like an odd choice for a stress research subject, but a closer look at their unusual life cycle indicates that these bunnies could be the poster-children for paranoia. These guys are seriously stressed out—pretty much all the time.

The Original Stress Bunny: Snowshoe Hares

“In the war between predators and prey, the snowshoe hares have lost,” Boonstra says flatly. “Animals under these conditions are making the best of a bad situation.”

Boonstra’s research opens a one-of-a-kind window into a mysterious mammalian life cycle that has puzzled biologists for years. Snowshoe hares go through a characteristic 10-year cycle, in which they happily pump out babies for a couple of years until the population peaks, and then die off for another four to five years until numbers start to creep up again.

Boonstra and his colleagues have spent years crossing off possible causes for the widespread crashes in snowshoe hare populations. So far, predation is the leading cause for why they crash, but the jury is still out for why populations can’t recuperate years down the line.  Chronic stress is now a tantalizing hypothesis to explain their inability to bounce back.

“We don’t know what causes it, but I suspect it’s that they’re suffering from the ‘ghosts of predators past,’” Boonstra says, explaining further: “Imagine that you are a mother, and you have a child, and that child for whatever reason…dies. You will not be the same individual the next year as you were before…In other words, the chronic stress of having to deal with that situation may cause irreversible changes in the way your hippocampus is organized, which then shapes you for the future.”

Boonstra points to the research of neurophysiologists like Bruce McEwen and Robert Sapolsky, who have studied how chronic stress can actually shrink the hippocampus, the brain’s centre for memory, learning, and the regulation of stress itself. Because of its unique receptivity to stress hormones, the hippocampus can end up battered by long-term hormone exposure, causing reversible neuronal changes over the course of weeks, and irreversible neuron loss over the course of months. And since the hippocampus controls how we respond to stress, these changes can compromise our ability to deal with stress in the future.  What Boonstra’s actually implying is that snowshoe hares’ brains could be so physically traumatized from the stress of predation that they just can’t calm down enough to raise healthy families later on.

Stress: The Gift That Keeps on Giving

But it doesn’t stop there—researchers are now looking into the inter-generational effects of stress in animal (and human) populations. In fact, the snowshoe hare cycle might be an extreme example of how stress can negatively impact future generations, by permanently modifying their expression of stress-associated genes.

Recent research by McGill psychobiologist Michael Meaney demonstrates that this kind of thing does in fact happen. In a surprisingly simple experiment, Meaney found that stressed-out mother rats who neglect their pups end up programming their babies to be stressed-out as adults. How? It turns out that Mom’s grooming and licking releases serotonin in pups’ brains, which triggers the expression and de-methylation of stress response-inhibiting genes. In other words, Mom’s nurturing not only turns on her babies’ calming genes, it programs her babies to be calmer permanently. Take away Mommy’s love, and you take away Baby’s ability to adapt to stress in the future.

Now researchers are wondering if the transfer of these kinds of permanent, epigenetic brain changes could account for poorer fitness in subsequent generations of snowshoe hares. “That’s one hypothesis, and until you actually test it, you can’t know for sure,” Boonstra admits. “But so far, it’s the most plausible one.”

The cycle of snowshoe hares, long cited as a classic example of predator-prey interactions in ecology textbooks, could soon come forth as an illuminating icon of what chronic stress is capable of in the natural world. And from it, scientists are realizing that stress is truly a force to be reckoned with, one that can affect us more than we’d like to admit.