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Measuring Animal Welfare, Part Four: Acute Stress
Physiological measures of stress are less helpful than you'd suppose
“Acute stress” is a cluster of physiological responses that prepares the animal for dealing with certain situations that are, well, stressful. Acutely stressed animals are stronger, have more energy, and think faster. The animal’s body draws energy away from less urgent things—eating, digesting, trying to seduce other animals, exploring—and towards more urgent things, like fighting or running away. You’ve felt acute stress before: you feel it when you are running to catch a bus, making a speech in front of a crowd, talking to someone you want to impress, or half-watching your kid and half-watching TV and then you realize you haven’t heard anything for the past five minutes.
The most common way to measure stress is measuring the level of certain chemicals in animals’ blood, especially cortisol and glucocorticoids. But you can use other ways to measure stress, such as heart rate.
How do we use this in practice? For example, scientists measured cortisol and β-endorphin levels in hunted red deer. They used red deer who were euthanized for humane reasons (for example, because they were stuck in a barbed-wire fence) as a control. Red deer hunted with hounds had higher levels of cortisol and β-endorphin than euthanized deer, suggesting that red deer suffer horrifically when hunted by hounds.1
Mating season in seasonal tournament species.
Having a mate and thus having to drive off rivals in non-seasonal tournament species.
Dominance-related aggression in social species.
Hunger and failed hunts in carnivores.
Childbirth, visible sickness or injury, or being at the edge of a social group in prey species.
Seasonal and climactic changes.
Not all situations that would be stressful for a human are stressful for an animal. For example, king penguins fast for weeks on end as part of ordinary nesting behavior, and do not experience any rise in glucocorticoid levels. Animals are less likely to experience stress during normal life-history stages. However, that generalization is not always true: for instance, many species that only reproduce once, such as salmon, experience high glucocorticoid levels during mating.
We can also learn about what makes situations more or less stressful for animals. For example, animals tend to be stressed when they’re in situations that aren’t stressful yet but might turn stressful. The median amount of time that, say, a hyena chases a zebra is less than a minute, which is far less than the amount of time stress-related physiological changes take. So animals have evolved to become stressed in a preparatory way. You can observe this through introspection: you’re not just stressed during a high-stakes test, you’re stressed before.
Other factors affect how stressed an animal is. For example, many animals are less stressed if they perceive themselves as having control over a situation. Stress responses often increase if a situation is getting worse and decrease if a situation is getting better. Again, you can probably observe these phenomena through introspection.
So this seems great, right? Measure the heart rate and the cortisol level, you can tell how distressed an animal is by something.
The small problem is that nearly all animals find being handled and having their blood drawn stressful. This is a problem you can deal with—if nothing else, you can study dead animals—but it makes studying the stress response a bit more difficult.
The big problem is that stress is a preparation for action, not an indicator of distress. The stress response prepares an animal for short-term exertion. It doesn’t distinguish between running from a predator and running because you just saw some tasty food—even though these are two very different subjective experiences! Sex, play, exploration, and exercise are all pleasant experiences which sometimes lead to a stress response. Some scientists cause positive stress “eustress” and negative stress “distress.”
You can observe the distinction between stress and distress yourself. When you talk to someone you have a crush on, have sex, ride a roller coaster, dance energetically, go for a run, play a first-person shooter, watch a horror movie, or work on a difficult problem, you experience a stress response. But many people enjoy these experiences. We wouldn’t appreciate it if some aliens tested our cortisol levels and decided to improve our welfare by banning orgasms, roller coasters, and scary movies.
In general, very high levels of stress—such as those experienced by red deer hunted with hounds—indicate poor welfare: animals’ physiological responses to distress get much more intense than their physiological responses to eustress. However, we might want to know about whether animals enjoy or dislike things that aren’t plausibly one of the worst experiences of their entire lives.
You might say, “okay, we can use common sense. Orgasms don’t present a welfare issue. Dying animals don’t have a high cortisol level because they’re having such a great time.” But then we go back to list-based approaches to animal welfare; we don’t have the objective metrics that we investigated stress to find. Measuring acute stress is an important tool in our animal-welfare toolkit, but it’s just that—a tool. If we don’t have some guesses about what is good and what is bad, it just gives us nonsense.
This finding doesn’t generalize to other kinds of deer. In natural situations, red deer mostly stand, walk, and lie down, and almost never run for long distances.