Last time, we talked about acute stress: the animal’s physiological and behavioral responses to a brief stressor. This time we’ll talk about what happens when an animal is stressed for weeks, months, or even years: chronic stress.
Animals who are chronically stressed tend to be lowkey stressed all the time, to freak out after a stressor, and to take a long time to return back to baseline. Again, the experience of chronic stress is something that you’re probably familiar with, unless you live a charmed life. Among humans, poverty, chronic illness, social isolation or rejection, grief, depression, and abuse can all cause chronic stress. When you were in one of those situations, you probably noticed that you were always on edge, that you blew up or started crying when something minor went wrong, and that it absolutely ruined your day.
Chronic stress is bad for your health. In humans, chronic stress is linked to insomnia, tension headaches, heart disease, and even getting colds more often. In nonhuman animals, it is linked to immune system suppression, muscle loss, lower growth, inhibition of the reproductive system, and even neuronal cell death.1
Some researchers—most famously Robert Sapolsky—argue that wild animals don’t experience chronic stress under even vaguely normal conditions. We can expect the stress system to evolve to prepare the animal for the sort of stressors they normally face. Chronic stress is bad for your health, so it would be selected against. Modern humans would be chronically stressed if we were constantly worried that someone might eat us, but extending it to nonhuman animals is an inappropriate anthropomorphism. Nonhuman animals have evolved to accept it as part of life.
An interesting case is snowshoe hares, who are very possibly one of the worst-designed species. Instead of reaching equilibrium like a normal species, snowshoe hares have a characteristic nine- to eleven-year boom-and-bust population cycle.23 As you might expect, in the decline period when the hares are overpopulated and constantly being eaten by predators, they are pretty damn chronically stressed.
If you’re Sapolsky, you argue something like this: “Because there are more snowshoe hares during the peak and decline than during the low and the increase, most snowshoe hares spend much of their lives being chronically stressed. But because snowshoe hares reproduce less during the peak and the decline, hares who successfully reproduce are far less likely to be chronically stressed. So chronic stress isn’t particularly selected against, and being a snowshoe hare is a bad time.”
But Boonstra (2013) argues (and I basically agree) that chronic stress can be adaptive.4
It’s important to be careful here. Animals of any species can experience chronic stress when they’re faced with repeated stressors they didn’t evolve to deal with: across vertebrate taxa, animals tend to be stressed by human disturbances like tourism and noise. In addition, some species evolved to experience chronic stress in particular situations that they evolved to deal with. For example, snowshoe hares probably evolved to be chronically stressed during the decline, because nearly all snowshoe hares alive during the decline die; constant terror is the hare’s only chance at survival.5 Like, let’s be real here, if you’re a snowshoe hare in the decline and you live long enough to die of stress-induced heart disease, you’ve won.
Boonstra argues that chronic stress about predators is adaptive if two conditions are met. First, predator risk has to be persistently high sometimes and persistently low other times: for snowshoe hares, it’s low during the increase and high during the decline. Deer experience only acute stress from predation, because there’s no real year-to-year variation in how likely a deer is to be eaten. Second, the animal shouldn’t be too short-lived. If you’re a snowshoe hare, there’s a chance that you’ll survive the decline and be able to have an absurd number of offspring during the low. If you’re a vole, another cyclic species which happens to live much less than a year, there’s no point. You’re not going to survive the decline regardless, so you might as well ignore the risks, have as many kids as possible, and hope some of them make it.
These two criteria can be fruitfully applied to other sources of chronic stress. In particular, even Sapolsky admits that outcast animals in social species are basically always chronically stressed. Both of Boonstra’s criteria apply. Some animals have a persistently higher risk than other animals of getting beat up if they try to go for desirable food or mates—and social species generally live long enough that it’s worth it to bide your time and try to increase your status until you can get the mates without the assault.
Unfortunately, measuring chronic stress is really really hard. You can’t assume that a measure that works in one species will work in a closely related species. An animal’s level of stress hormones is affected by sex, age, reproductive condition, time of day, and season of the year, so all of these must be accounted for in the analysis. Individuals have different physiological responses to stress, so an individual might look non-stressed when really they are, or vice versa. Any particular measure of chronic stress is subject to a lot of random nonsense like “this animal eats different food, which changed the chemistry of their feces, so their results can’t be compared to the others’ results.” And there is no consensus endocrine profile for chronic stress in wild animals: there are general trends, but it’s impossible to predict whether any particular marker will increase, decrease, or show no change at all in a particular stressful situation for a particular species.
Further, I am intrigued by the argument that, like acute stress, chronic stress isn’t necessarily bad. Animals are sometimes chronically stressed during reproduction, and birds with large broods or who are putting in more parental effort are more likely to be chronically stressed. Of course, maybe it’s unpleasant to be a bird with a lot of offspring. But you can certainly imagine a sort of avian All Joy And No Fun. Maybe having a lot of offspring puts you on edge, but in a good way. It’s stressful, but in a rewarding way that you’d choose to experience, much like parenting often is for humans. There’s been very little research along these lines, but I think it’s worth looking into.
A thing which I love to expect I may be experiencing.
Here’s a good explanation of how it works.
Yes, yes, a lot of other species have boom-and-bust cycles. Still bad design.
In the sense of increasing fitness, not in the sense that you’re having a nice time.
Nature is such a charming place.
Thank you for this explanation! Do you have any sense of how far off we are from applying foundational research in this field?