Sunday 4 June 2023

Social Posting - Bear Style

Communication between animals is an important feature of all mammalian species. For those living in groups, it can help to maintain social bonds and provide urgent information, such as the unexpected arrival of a predator. If a mother has young, she may need to find them if they become lost or attend to them if are distressed, and so on. Even solitary adult animals need to communicate with others of their species, whether it be to mark out a territory and ensure that others don't poach their food source, or to find a mate at the appropriate time.

At least among mammals, there are two primary modes of communication that are generally studied by researchers. Perhaps the more obvious of these to we humans is vocal communication, since that's the main one we used in pre-literate societies. Not all species are especially vocal, but many are, and some are using sound outside of the human hearing range - such as the ultrasound squeaks of many small rodents such as mice. One estimate suggests that around 95% of mammal species use acoustic communication and this may be on the low side (it's 100% in birds and 90% in amphibians, but apparently less than 5% in reptiles, suggesting that it has evolved at least three times).

The second is scent-marking, which isn't a big deal to humans with our attenuated sense of smell, but is key to many mammal species. This is particularly useful for leaving long-lasting messages and therefore can be more important to solitary species who need to inform others of their kind that the territory they are passing through is already taken, or that they're ready and willing to mate. Humans may not be able to decode such marks (if they can even detect them) but the sort of message that a dog leaves on a post can provide plenty of information about sex, age, and physical fitness. All of which can be useful to other members of the species.

In comparison, at least among mammals, visual communication is often considered less important beyond, perhaps, adopting an aggressive posture. But even things such as clear stripes or bright markings can at least alert other members of the species of such things as sex or reproductive status and help to maintain social cohesion, and this is true even in animals that are largely nocturnal.

Longer-lasting visual communication, however, doesn't seem as if it's something that would be all that significant. After all, writing is clearly something unique to us - and even then, is, even if it's not always phonetic, it in some way represents spoken words. But, while, yes, animals don't write, they can still leave visual markings for one another. The question, perhaps, is whether this is merely an adjunct to something else, or even if the animals are paying any attention to such visual cues when scent is what really matters to them.

For example, some animals leave scratch marks on objects, as any cat owner will know. But if all they're doing is sharpening their claws, and other cats don't pay any attention to the marks, that's not really a visual signal. Similarly, visual marking could just be an unavoidable consequence of leaving a scent mark - even if the animal is literally gouging a chunk out of a tree with their teeth, perhaps that's just so they can leave scent from their saliva.

But then again, perhaps not. This seems to have first proposed as recently as 1983, for black bears in North America. Since then, there has been some evidence that such visual marks might convey more information than scent alone for tigers and howler monkeys... and, yes, quite possibly for that domestic cat accosting the leg of your dining room table. But there's perhaps little that's really conclusive here.

Brown bears (Ursus arctos) are solitary animals as adults but are not strictly territorial, being willing to share portions of their home range with other neighbouring bears. Their sense of smell is remarkable, perhaps even superior to that of dogs. As such, scent-marking is, as with black bears, an important means of communication for them. They can leave scent marks by urinating on some obvious object that other passing bears will happen to notice, but they also have scent glands on their feet and their posterior which they can use to deposit messages. 

Tree-rubbing is another common practice among bears, in which they rub their back and shoulders up against a tree, often stripping the bark from it in the process. There is little doubt that this leaves behind a scent mark, produced by sebaceous, and possibly also sweat, glands in the skin that can convey information about, for example, the bear's testosterone levels. It's less obvious that the bears can learn anything new by simply looking at debarked trees, but a study published a couple of years ago suggested that they might.

That was essentially a preliminary study, attempting to prove that anything was happening at all. Earlier this year, a follow-up was published following bears in the Cantabrian Mountains over the course of a full year. The idea that the researchers used was to find trees that the bears had marked, and cover up the visual signs with fresh bark; they had already demonstrated that bears won't remove bark coverings on random nearby trees that they hadn't used and so presume that this had to be a deliberate act to expose the scratch mark. And then they set up camera traps to see how the bears used those marking trees, and whether there was any pattern to their age or sex.

The bears in the study area left few scent marks on trees during the winter, when most of them are probably hibernating, but otherwise they were reasonably active throughout the year, ramping up their activity dramatically in May at the height of the mating season. However, going so far as to strip the bark off trees altogether - leaving an apparent visual signal besides the undoubted chemical one - was a different matter. All but two of the marking events recorded happened in April and May, with nothing happening at all for most of the year. Perhaps more significantly, only adult males ever stripped bark from trees, and they soon removed any coverings over their marks, apparently having returned to the tree to check on it.

In order to prove that the act of stripping the bark off the tree is a visual signal we'd have to show that it provides some piece of information that the mere leaving of a scent mark would not. Another possibility, for instance, is that it simply shows where the scent mark is, making it easier for other bears to find. If it's doing something else as well, then, given that it's only left by adult males, and mostly around the mating season, it presumably has to do with mating - and certainly, females and younger males did check the marks even if they never left any themselves. Depending on their circumstances, they might have been trying to avoid the bear leaving the mark, rather than seek him out as a mate, but that's unlikely to be the male's motivation or they'd do it throughout the year.

But what information might be provided by the debarking that could not equally well be provided by the scent deposited while doing it? The researchers speculate that this could be the size of the bear, since they are often (but not always) high up on the trunk. It's hard to know whether this is really true although the fact that, even if a given tree was by several males for scent-marking, only one would rip the bark off lends support to this - presumably, he'd be the dominant male in the area.

Personally, I'm not convinced that all of this amounts to hard proof that the visual signals are more than supplementary information to the scent marking. But it's certainly suggestive that that might be the case, and it's an entirely plausible idea. If nothing else, the fact that sexual active male brown bears leave such visible markings in the woods, whatever their reason, provides human conservationists with information about where they are and what they're doing.

[Photo by Jeffrey Pang, from Wikimedia Commons.]

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