Often these signals need to convey more than the basic message, but also provide some information about the animal that left them. This often includes physical fitness, whether to intimidate potential intruders or to advertise oneself as a sexual partner. At close range, this can be achieved through such things as growing huge antlers, proving that you have the physical capacity to waste resources on something that's otherwise pointless. But, at longer range, a simple indicator of size may be all that's required.
We see this in vocal signals, where a deep, booming call might, for example, indicate the general size of an adult male seeking a mate. But it can apply to scent marking, and even to some visual signals.
For instance, when mongooses and civets leave scent marks, they do this by performing a handstand, spraying scent and/or urine onto a tree or other vertical feature. This unusual posture allows them to place the mark up as high as they can physically manage, an honest indicator of how big they happen to be. They can't, after all, place the mark higher than they can reach, so, like the deep call of a stag in rut, it's genuinely useful information to others of their kind.
With any such signal, there are payoffs to be considered. There's the obvious balance between how much effort it takes to leave the mark or make the call, and how big the benefit you expect to receive is. But there's also a question of honesty. Can the signal be faked somehow, giving an illicit benefit to the dishonest animal?
Let's take those mongooses again. Presumably, they didn't start out by performing handstands for the sake of it. They probably left marks on trees, with those who were bigger seeming more intimidating or sexually attractive to passers-by coming across the mark. But, over time, those who could manoeuvre themselves into a position allowing them to place the mark higher benefited from 'exaggerating' their size. Evolution took hold, until everyone was balancing upright on their forepaws, and the mark was as high as they could possibly make it - an honest signal once again.
Absent such long-term changes, however, a signal from one animal to another needs to be taken as reliable, or there's little point making it. If dishonesty is an option, it needs to be rare enough that, at least most of the time, the intended recipient doesn't consider it. So long as it's reliable significantly more often than not, the communication method can remain in use even if a few individuals benefit by cheating the system.
Brown bears (Ursus arctos) have long been known to use scent marks as a means of communication. Their usual method of leaving these marks is to stand up on their hind legs and rub their back against tree trunks. This leaves behind secretions from the sebaceous glands on their back, shoulders, and neck that, because these glands become enlarged in the presence of testosterone, provides useful information about their sex and virility. Bears also have regular anogenital scent glands and can also leave marks by urinating or rubbing their feet on the ground.
However, in just the last few years, it has become apparent that brown bears also leave visual signals. They do this by debarking trees with their claws, a behaviour previously attributed to leaving scent marks. That's undeniably a part of it, and it's only done by adult males during the mating season, so the reason for it is fairly clear. But it turns out that the bright, visual signal of leaving patches of bare wood on the tree is also an important part of the message.
Specifically, it gives passing females an idea of how large the male is, by virtue of how far he can reach.
Assuming, of course, he doesn't try to cheat by climbing a short way up the tree first. The question is, how often does this really happen, and is there any real benefit to the male bear in trying to do this, given that he'll inevitably be found out when the female sees him in the flesh? A recent study conducted in the Cantabrian Mountains of northern Spain aimed to find out.
The researchers set up camera traps at trees known to be used by the local bears. (These were, incidentally, once considered a distinct subspecies and, while that's no longer true, they are locally endangered, with an estimated total population of just 130 in 2009). To encourage the bears to cheat, they provided piles of stones or wooden platforms close by the trees that they could clamber on top of, allowing them to reach higher up the tree than they could while standing on the ground.
Over the course of the study, the researchers were able to capture 164 incidents of bears marking trees and it turns out that, on the whole, they did so honestly. Even when presented with a viable method of cheating, the bears only attempted to use them 4% of the time. This is good news for other bears relying on the height of tree markings as a measure of who else is in the area; far more often than not, the information is reliable.
This is what we'd expect of a good communication method, allowing the recipient to trust what they are being told. But, even so, it's worth noting that some bears were "dishonest" about their size, in a few cases even climbing trees without the platforms, demonstrating that this, at least occasionally, a natural behaviour, not one they need to be coaxed into performing.
But, here, one can wonder what the point is. It's exclusively a male behaviour, because the females don't bother to indicate their size in this way. They don't need to, because females don't compete for mates, and rarely come into conflict with one another. There's no reason to lie about your size when nobody cares about it anyway.
Among those bears who were apparently being deceptive, the majority were subadults or otherwise smaller (and typically younger) than average. Full-grown bears are apparently confident enough in their prowess that they don't need to exaggerate it, even assuming they can do so as easily as smaller bears - which is likely, but perhaps not a given. But if those smaller bears did come across a female, then, like somebody using a fake Tinder photo, they would immediately be found out.
It's possible that any opportunity at mating is worth the risk, but it's perhaps more likely that this is merely a secondary benefit. It may be more important to the bears that the markings act as intimidatory signal to other, larger males. A full-grown male, seeing that someone of similar size to (or larger than) himself is in the area, might decide it isn't worth the trouble to look for females there, and run the risk of fighting the mystery male.
Avoiding such fights might well be worth it for subadult or smaller males, giving them an incentive to occasionally lie about their size - so long as they don't do it so often that nobody believes them. This is, of course, just speculation, since the study merely shows that it happens, not any definitive reason for it. Still, something similar has been proposed for giant pandas, which seem to avoid areas where fake scent marks have been placed artificially high on trees, as if left by some mysteriously large, potentially hostile, panda.
With 96% of the bears in this study providing a genuine signal of their height, it would seem that honesty is often the best policy. But, every now and then, one of them plays the system, making himself out to be tougher, and perhaps sexier, than he really is.
[Photo by "Byrdyak" from Wikimedia Commons.]
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