|Southern flying squirrel|
In fact, stress is such an integral part of life that, at a biological level, it works in more or less the same way in all vertebrates. Stress results in the release of stress hormones, of which the most important is cortisol. In mammals, this is produced in the adrenal cortex, the outer part of the adrenal glands, which sit on top of the kidneys, and produce a whole range of different hormones of which cortisol is only one. So basic is this that all non-fish vertebrates have adrenal glands of some sort, although which bits of them make the cortisol (and, in some cases, how many there are) does vary between different groups. Even fish, which usually don't have adrenal glands as such, do have the relevant sorts of tissue somewhere in the general vicinity of the kidneys, so that they too can make cortisol. It really is that universal.
Cortisol is a steroid, a term that refers to the nature of its chemical structure. (If you're a chemist, trained in terms of IUPAC terminology, it may help to think of it as (11β)-11,17,21-trihydroxypregn-4-ene-3,20-dione. If not, not). Release of cortisol into the bloodstream has a number of effects which, again, are pretty much the same regardless of the species of animal in question. It raises blood sugar levels - which is obviously a bad thing if you don't have insulin to counteract it - and alters a number of other aspects of metabolism, eventually doing things like breaking down muscles for their protein and reducing bone growth to divert resources elsewhere. It reduces the effects of inflammation, which can be a positive in the right circumstances, but does have the effect of slowing wound healing.
All in all, it's important stuff, and you'd die if you didn't have it. But, on the other hand, you clearly don't want to much of it sloshing about and keeping your body permanently stressed. In order to stop that happening, the body produces a protein called transcortin that mops up most of the cortisol (and other related hormones) in the bloodstream and stops it from doing its thing. Indeed, in the species that typically get used in biomedical research, as much as 70-90% of the cortisol in the blood is normally bound up in this way, remaining inactive until it's needed.
Unless, it seems, you're a flying squirrel.
There are at least 44 different species of flying squirrel, and most of them are found in southern and eastern Asia, with just a single species in the north of the continent. Two species (Glaucomys spp.), however, live in North America, and it's these that have had their stress hormones analysed. According to preliminary analyses, the cortisol levels of these two species are staggeringly high, among the highest ever seen in any animal. Moreover, less than 10% of the hormone is bound up in transcortin, which presumably means that it's all active. True, they aren't so high as to be unique (there are at least some monkey and bat species with a similar profile), and they're not so high that you'd otherwise expect the animal to be dead or something, but it is as if these animals are permanently stressed out.
One possible explanation is that they are. Well, okay, not "permanently", but think about it: how do you find out what the cortisol levels are in a wild animal, anyway? You capture the animal, take a blood sample, and have it analysed. All well and good, but isn't being captured and having a blood sample taken kind of, well... stressful?
To rule that out, we'd need to take samples from unstressed animals, and compare them. That might also give us more information about what's going on if turns out that the effect is real. But how exactly are you going to do this?
We're helped out here by the fact that it takes time for the body to produce and mobilise cortisol - the levels don't shoot up instantaneously the moment something bad happens. On the other hand, it really doesn't take very long, with values of about three to five minutes being typical before something starts to happen. So, if you can somehow take your blood sample within three minutes of the animal noticing there's a problem, you should be all right.
So, the researchers who had originally found the high cortisol levels in flying squirrels repeated the experiment with some tweaks to get the results more quickly. There are, as I said, two species of flying squirrel in North America. The southern species (G. volans) lives in deciduous woodland across the eastern US and in the highlands of Mexico and northern Central America. The northern species (G. sabrinus), however, prefers forests with more coniferous trees, and is therefore found across Canada and parts of Alaska, and in some highland regions of the US, especially in the west. There are relatively few areas where you can find both, but there are some, mostly around the eastern end of the US/Canadian border, and it was here, specifically in southern Ontario, that the researchers set up their traps.
Since the squirrels are nocturnal, what they'd done on the previous occasion was to put the traps up trees in the evening, and come back to see if they had caught anything the next morning. (You can see a picture of the trap here - it's about 18 inches [45 cm] long, so easily enough to fit a squirrel in without discomfort, although it's hardly going to be happy about it). This time, however, having baited the traps with tasty peanut butter, the researchers hid nearby and waited for up to five hours. The moment they heard the trap closing, they had to dash across, anaesthetise the squirrel, and take their blood sample - all before the three minutes were up. They then put the animal back in the trap, and came back half an hour later to take a second sample before releasing it.
If the two samples had come out the same, then that would have implied that the cortisol levels spike really quickly in this species, and even three minutes wasn't quick enough. But they didn't, and, in fact the samples they managed to nab within two minutes showed no difference from those just under three, so the stress response seems to take about the time you'd expect. Which isn't to say that the animal wouldn't be a bit freaked out by being caught, with increased heart rate and whatnot, but the stress hormone apparently takes longer to produce - and, of course, unpleasant experience though it might be, the animals aren't really harmed, and likely have similarly scary moments whenever they see a predator.
But the summary of the results is that the very high levels of stress hormones in these squirrels is real, not some artefact of the experimental set up. Flying squirrels are most closely related to "typical" tree squirrels, such as the reds and greys we are familiar in Europe and North America. At least some of these also appear to have unusually high levels of stress hormone in their blood, even at the best of times. But, even so, the common North American red squirrel (Tamiascirus hudsonicus) has, after a full 30 minutes of acute stress, about the same cortisol levels as the northern species of flying squirrels do within just three minutes of capture - and only about half what the southern species has within that time.
And, yes, after 30 minutes in an escape-proof cage, the cortisol levels of both species get quite a lot higher, rising, in the southern species, to about three times the maximum found in red squirrels. Which, for what it's worth, is itself three times higher than is healthy for a human.
It's worth noting that these studies were conducted in the autumn, so as not to interfere with the breeding season, mothers raising their young, and so on. Which means that it might have something to do with changes in metabolism in preparation for winter, helping the animals to stock up on nutrients in anticipation of the tough times ahead. This might also explain why it's highest in the southern species, which is, in the location studied, pretty much at the extreme end of its range, beyond which there isn't enough of the right kind of food for it to survive.
But that's just a guess, and, in any event, it's not apparent why it should apply so much less to American red squirrels. And it's one set of studies, in one geographical area, at one time of year. Might it be different elsewhere, or at different times? It certainly looks as if something odd is going on, but there's likely more to learn about why some squirrels seem, at first sight, to be permanently stressed.
[Photo by Ken Thomas, from Wikimedia Commons.]