Sunday 25 June 2023

Pennsylvania Elk with a Wyoming Accent

We're familiar enough with the idea that humans in different parts of the world speak with different accents and vocabulary, even if they are otherwise speaking the same language. ("Faucet, railroad, trashcan, truck, don't say 'sidewalk' or you suck.") This is true of all languages that are spread over a sufficiently large geographic area; for instance, European and Brazilian Portuguese are at least as different as UK and US English. Even within a single national dialect, we can often distinguish local accents and some differences in spoken grammar and terminology - few native English speakers could confuse a blue-collar Texan with a New Yorker.

Since animals don't have language in the human sense, we might not expect the same thing to be true of them. Animals have distinct vocal repertoires, but these are largely instinctive, and a cat goes 'miaow' regardless of where it lives. (Well, arguably it goes 'meow' if it's American and 'nyan' if it's Japanese, but you get the point). It's perhaps not surprising that there is variation between individual songbirds of the same species, or among cetaceans, given the complexity of their calls, but we might not think of it among terrestrial mammals.

To be fair, if all the animal does is make a grunting or snarling sound, that may well be true. But there are exceptions. The general term used by biologists when they identify different sounds made by different populations of the same species is 'dialect'. These have been observed in several primate species, but also in seals and rodents, among others, so they're widely distributed across the mammal family tree. It's less clear why animals with instinctive calls should have dialects at all. This is not because we don't have any idea, but for precisely the opposite reason: we have several possible explanations.

Some of these are similar to the reasons behind the differences between British and American English (or European and Mexican Spanish, or Cockney and Geordie). For example, it could just be that there is a general drift with increasing geographic distance as populations become separated. Similarly, the emphasis could be more on something akin to cultural learning. In naked mole rats, for example, individual colonies have distinct dialects that they learn from their queen, perhaps helping them to identify strangers. (A human analogy here might be that, while British dialects have tended to borrow from the London one in recent decades, perhaps partly due to the influence of EastEnders, those of the neighbouring cities of Manchester and Liverpool have apparently drifted further apart from one another).

Other explanations are less likely to be relevant to humans. For example, it could be genetic, with isolated populations being at least some small way towards changing enough to form a new species. Another possible explanation is that, if the animals live in habitats with different features (such as tree density), that may affect the way sound is transmitted, effectively forcing them to change their calls if they want to be sure they will be heard at a distance. 

Moreover, there is no reason to suppose that two or more of these explanations could not be simultaneously true for any given species, perhaps interacting in complex ways. 

Elk (Cervus canadensis) have a distinctive "bugle" call. Although females do bugle, the call is much more commonly used by males, which employ it to attract mates and deter competitors. The call is so-named partly because of its pitch, but also because it is particularly loud and relatively prolonged - again, more so in males than females. It requires a fair amount of energy to make effectively and frequently so the volume and duration of the call may reflect the size and physical fitness of the elk in question, as well, perhaps, as its degree of motivation. But it's also the sort of thing that might be complex enough to vary between populations so that the elk could, perhaps be said to have a local dialect.

While the fact that elk bugle has, obviously, been known for a long time, scientific studies specifically aimed at understanding the details are relatively recent. Structural analysis of the call shows that it consists of a comparatively slow build-up of increasing pitch, followed by a prolonged high-pitch 'whistle', and finally a rapid decrease in tone that may sometimes be followed by a short yelp. This leaves a number of possible variables by which the call could differ: volume, pitch, duration, and the relative speed at which the sound either builds up or tones down. It would be unsurprising if there were not variation between individuals - being able to prove you can produce louder, longer, calls is probably part of the point - but do they also vary geographically, and if so, why?

Elk once lived across much of the contigious US, but native populations are now largely restricted to the western half of the country, with the entire eastern subspecies having been driven to extinction, probably in 1877. Since that time, however, there have been various attempts to introduce elk from the surviving, western, subspecies to suitable habitats in the eastern US. Many failed, but there have been some notable successes.

This gives us the opportunity to tease apart genetic and environmental factors in any variation in elk bugle calls that may exist. Any animals living in the eastern US must, after all, descend from comparatively recent populations still living in the west. If differences exist, and can be tied to habitat, that's indicative of rapid change that's more likely to be due to that than to genetic drift or learned experience.

This is what a recently published study did, taking recordings of the mating calls of wild elk in two introduced populations, one in the open terrain of Colorado and one in a much more forested location in Pennsylvania. They then compared these with calls recorded from elk in Wyoming, the source population from which both the Colorado and Pennsylvania elk had been transplanted in the early 20th century.

Using audio analysis software, they were able to look at multiple fine features of the calls, allowing them to pick up minor differences that might not always be obvious to the human ear. The results showed, as one would expect, that individual elk vary from one to the other, but statistical analysis showed that the variation trended in different ways in the three populations. Sure, there is some overlap, and it was only possible to tell which population a given call came from around 60% of the time. But that's a good deal better than the 33% you'd get from random guessing; the calls do have a lot in common, but there are differences at a population level that don't exist at the individual one. 

So elk, it seems do have dialects, if not necessarily as distinct as English language accents from, say, Alabama and Glasgow. But why?

Here's where it does get a little murky. What we would expect is that the Pennsylvania elk, in the densely forested habitat, would have calls that were longer and lower-pitched than their counterparts elsewhere. Which, as it turned out, was true when they were compared with those in the open terrain of Colorado. But, as it turned out, not when compared with the elk on the plains of Wyoming. And it makes no sense that the Colorado elk, transplanted to a habitat not too dissimilar to their original home, would be the ones that changed the most.

There, the difference was more based on the speed with which the tone rises and falls at the beginning and end of the call. You'd probably also expect this to be quicker in the forest-dwelling elk, and it was... but with the other two predictions squashed, that may not mean much. 

So a messy picture, with some differences but not enough to strongly support the hypothesis that environment is the main driver of dialect drift in these animals. What is going on?

Well, the elk in Colorado and Pennsylvania, being descended from a small immigrant population, are not as genetically diverse as those back in Wyoming. So it could be that small genetic differences seeded the changes in dialect, which were then passed on by young elk hearing older ones calling and learning from them. Habitat could still have an effect, promoting some changes over others, but not enough to make a clear difference across the board; it could plausibly be a bit of both.

Elk, it turns out, do have regional accents, and that's something we didn't know before this study. But nature isn't always forthcoming in giving us an explanation of what it's up to.

[Photo by Jakub Fryš, from Wikimedia Commons.]

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