Does Colder Mean Bigger?

Amne Machin, Qinghai

In 1847, German biologist Christian Bergmann formulated what has since become known as Bergmann's Rule. Over the following 165 years, the Rule has been somewhat modified and re-interpreted from Bergmann's original statement, but a common modern version is that warm-blooded animals tend to be larger the colder the environment they live in. Bergmann was originally thinking of closely related species; for example, polar bears tend to be larger than most brown bears. However, the rule is now often applied to individuals and populations within species; for instance, Siberian tigers are larger than those that live in the tropics.

The rule has, as I've noted before, been the focus of some controversy. Is it, in other words, actually true? Certainly, there are many counter-examples - snow leopards are the smallest cats in their genus, for example. But are these rare exceptions to a general rule?

For a start, the rule is only intended to apply to warm-blooded animals. There is, for example, no evidence that it applies to amphibians, and, if anything, lizards and snakes (but not turtles) seem to do the exact opposite. Opinions as to whether or not it applies to mammals have, historically, been somewhat divided. Studies in the 1930s seemed to show that the rule was substantially correct, but were subsequently criticised, while a major study in the early 1970s apparently showed that it wasn't.

The general consensus today is that the rule probably does apply to mammals, but only to a point. In North America, at least, there is evidence that something like two thirds of mammal species follow the rule, and perhaps a slightly higher proportion of birds do the same. That still means, of course, that a substantial number of species don't follow the rule at all, and in some cases even reverse it. If the rule applies - as it seems to - to significantly over half of all mammal species, it's probably fair to say that it is true, if only as a general rule of thumb. That there are so many exceptions, on the other hand, means that it's difficult to use as a predictive measure, as you might want to, for example, if you're looking at fossils.

But if we accept that the rule is true more often than not, why should that be so? Why should mammals living in colder climates be larger than those in warm ones? Bergmann himself thought that this came down to conservation of heat. A large object, such as a polar bear's body, has a smaller surface-to-volume ratio than a smaller, but similarly shaped, object, such as the body of a temperate-dwelling black bear. Because heat is lost through the skin, a larger animal finds it easier to retain internally generated body heat. That's why, when he formulated the rule, Bergmann only applied it to warm-blooded animals.

Bergmann's explanation may have a lot going for it, even now, although there is probably more to it than that. Other factors are probably also involved, or the rule would apply more often than it does, and it's also less obvious why it might apply to turtles, or, apparently, ants. Another plausible explanation, for instance, at least for mammals, is that food is harder to acquire in colder environments, and that larger animals can have more fat reserves - that the animal might have some other tactic for dealing with that problem might help explain why the rule doesn't always apply.

One way to disentangle such possible explanations is to look at animals that are relatively unaffected by temperature. Among land-dwelling animals those least affected by the weather are those that rarely experience it directly because they spend their entire lives underground. Sheltered in their burrows, the soil above them insulates them from the outside climate, at least to a greater extent than animals up on the surface. So the question of how Bergmann's rule applies to subterranean species might help shed light on how the rule works.

Zokors are burrowing rodents belonging to the blind mole rat family. They are notably distinct from the other members of their family, although exactly how they are related is still somewhat unclear. They are not quite as physically modified as the blind mole rats themselves, but, like them, they spend virtually their entire lives underground, only rarely venturing to the surface. They inhabit relatively forbidding terrain in northern and central China, Mongolia, and south-eastern Siberia, and, physically, they are somewhat mole-like animals, with short stubby limbs and tail, very small eyes, and brown or grey fur.

Which brings us to this week's study, by Tongzuo Zhang, of the Chinese Academy of Sciences, and co-workers. They collected over five hundred plateau zokors (Eospalax baileyi) from various locations across the Qinghai region of the Tibetan Plateau, and measured them to see whether or not they followed Bergmann's rule - which, after all, is supposed to apply to individuals within a species as well as to differences between related species.

There is, incidentally, some debate as to whether or not plateau zokors constitute a species, or are simply a subspecies of Chinese zokor (E. fontanierii). Most official sources regard them as a subspecies, but a genetic analysis in 2008 showed them to be identifiably distinct, and, while that's not cast-iron, given how little zokors have been studied, I'm happy to go along with it.

Many studies trying to confirm Bergmann's rule have used latitude as a proxy for temperature - the closer to the poles you are (and that's usually the north pole, since most studies are in Europe or North America), the colder it tends to get. Using that guideline, the zokors in this study followed the rule; those that lived further north were larger.

So, is the rule proven for this species? Well, no, because it turns out that, after examining climate records of the area, the researchers concluded that it isn't really any colder in the north than the south; other factors played a larger role than latitude in determining temperature in this particular part of the world. Since the rule is actually about temperature, not latitude, it apparently fails for these animals. Yet they still are larger further north, so if it's not temperature that causes that, what is it?

The authors point out that while the climate may not get noticeably colder in this region as you head north, it does get drier. They suggest that, for the same reason a larger body lets an animal retain heat, it may also help it conserve water. After all, living underground may protect you from extremes of temperature, but, since you can't drink from streams or the like, it doesn't protect against a lack of water - you're reliant on what soaks through the soil from rainfall, and on whatever is in the food you're eating. Well, maybe, but it's worth noting that other studies haven't found the same pattern.

More significantly, perhaps, because this is a mountainous region, the temperature may not change with latitude, but it certainly does change with altitude. So, if Bergmann's rule holds, the animals living higher up mountains should be larger than those further down. But they weren't. In fact, they were smaller.

Part of the reason for examining zokors is that we wouldn't expect temperature to have much effect, but clearly, since there is a pattern - albeit in the wrong direction - something is going on here. If creatures in colder climates needed more fat reserves to survive longer when food is scarce, that would still apply up mountains. Zokors are herbivorous, feeding on roots and tubers, and there will be less of those high up mountains, just as there will be in cold polar regions.

It might be that the food is so scarce, they just don't have the opportunity to grow as large. Effectively, they're just suffering from poor nutrition. But there are other possibilities. Since they're burrowing animals, the fact that the soil is frozen for more of the year might well be a problem for them, making it harder to find what little food there is.

There's also another issue with altitude that doesn't apply with latitude: the air gets thinner. When we're talking about the Tibetan plateau, that can be a serious issue. Even the lowest collection sites in the study were at around 3,000 metres (9,900 feet), where air pressure is just 70% of that at sea level, and humans can already start to suffer from altitude sickness. The highest sites, at around the maximum that it is believed zokors can comfortably live, were at around 4,500 metres (15,000 feet), when air pressure drops to below 60% of that at sea level, which is significantly less. Evidently, the animals can live here for extended periods without harm, but it might put further limitations on their ability to dig and actively search for food.

It's this sort of complexity that results in Bergmann's rule having so many exceptions in the first place. As a general guideline, it's fine, and it does seem to be true more often than not. But there are so many confounding factors that it can never be more than that. There are a great many creatures that, for all kinds of reasons, either ignore or actively flout the rule, and, just as there is probably no one reason why the rule is true when it does apply, there are probably even more reasons why it is false for the exceptions.

[Picture by "Nupgong6" from Wikimedia Commons]