Humans increasingly stress over climate change. Sometimes the stress is acute, for example, when a flood or wildfire is approaching one’s neighborhood. Sometimes the stress is chronic, like in the growing number of young adults feeling anxiety over future environmental crises. Climate is not only altering our physical landscapes. It is compromising our health.
Ecologists warn that climate change is also stressing wildlife. Like humans, changes in weather take animals out of their comfort zones. Animals that survive natural disasters, like hurricanes and wildfires, certainly experience distress. Considering climate as a stressor in the wild raises a basic question: Do animals experience stress as humans do?
Perhaps stress is a human construct. Much of what people consider stress arises through very human circumstances. A work deadline causes stress, as does a political debate. Animals generally do not experience deadlines and do not have political systems. When drought limits water availability in an ecosystem, a frog may experience stress when they forgo their only annual chance at reproducing in ponds. Alternatively, the frog may simply cope with the adverse situation and stay in its burrow, without getting stressed about it. There are clear reasons to suspect that stress is a human phenomenon, and attempts at extending it to animals would be yet another case of anthropomorphism.
Robert Sapolsky settled any debate over the existence of stress in animals. He showed that animals do experience stress, and one can measure physiological variables, like metabolism and hormone levels, to definitively show this. Scientists define stress as a bodily response to perceived or physical threat to homeostasis (stable functioning of our internal systems), or anecdotally, ‘the body’s response to an insult’ (Sapolsky, 1990). A consistent component of the stress response in all organisms is the release of hormones, namely epinephrine and glucocorticoids (e.g. cortisol), that trigger a range of physiological and behavioral survival mechanisms. Measuring glucocorticoid hormones shows that levels spike in wild animals placed in threatening situations, just as they do in anxiety-ridden people. Animals clearly are subject to stress. Whether stress experienced by animals is comparable to stress experienced by people is questionable.
In the wild, stress is thought to mostly arises from brief, yet intense threats - predator attacks, competition for mates, etc. The threats are physical – caused by another organism or a harsh set of environmental conditions. The stress elicited in animals is acute – intense and temporary. The ‘fight or flight’ response is a classic example of an acute stress response. In the wild, acute stress is highly beneficial. More than that, it is essential; stress motivates actions that protect us from inevitable disturbances to daily life. Acute stress arising in the wild is different from the stress that people normally experience.
People normally suffer from chronic stress, that is, repeated and/or sustained for long periods, and the stressor is usually psychological (in our heads) rather than physical (in the environment). This is partly because people have successfully removed many acute physical stressors from daily living, which is a major achievement that affords longer lifespans. The price of this achievement seems to be a heightened susceptibility to chronic psychological stress. Anxiety over an upcoming deadline exemplifies psychological stress. Whereas physical stress in animals may happen a few times a month or week, psychological stress may occur many times in a single day. Chronic stress entails the same spikes in hormone levels and resulting physiological and behavioral changes as does acute stress. Unlike acute stress, spikes in hormonal and neural activity are slow to return to baseline levels. Staying in a chronically stressed state wears away at our bodily functions, just as long road trips wear away at vehicles. The result is decreased performance, lifespan, and immune health. The stress that wild animals experience (acute) does not seem comparable to human stress (chronic).
Climate change creates conditions that could cause chronic stress in wild animals. Much climate change is incremental, for example, gradual increases in temperature or increasing frequency of years with below-normal rainfall. The subtle changes are not immediately lethal, but they do have effects. Lizards in Mexico face more days annually of unfavorable heat, and in response, they spend less time breeding and more time sheltered. The amphibians that I study face increasingly frequent drought years, and so more of the aquatic offspring desiccate. Like amphibians, breeding conditions for the many wild animals with seasonal life cycles can be summarized as good or bad years. Increasing frequency of bad years due to climate change, which is reported for many species, has qualities of a chronic stressor. Chronic stress and its detrimental effects to health and lifespan may have historically been a civilized human phenomenon, but climate change permits the hypothesis that its reach now extends into the wild.
Further Reading
Boonstra, R., Hik, D., Singleton, G. R. & Tinnikov, A. The impact of predator-induced stress on the snowshoe hare cycle. Ecological Monographs 68, 24 (1998).
Boonstra, R. Reality as the leading cause of stress: rethinking the impact of chronic stress in nature. Funct Ecol 27, 11–23 (2013).
Clinchy, M., Sheriff, M. J. & Zanette, L. Y. Predator-induced stress and the ecology of fear. Functional Ecology 27, 56–65 (2013).
Sapolsky, R. M. Stress in the Wild. Sci Am 262, 116–123 (1990).
Sapolsky, R. M. Social Status and Health in Humans and Other Animals. Annu. Rev. Anthropol. 33, 393–418 (2004).
P.S. The above words express some ideas only. I have not tested them. They might be falsified. Please don't judge me by them.