Are Animals Darker in the Tropics?

In 1833 Prussian ornithologist C. L. Gloger published the book Das Abändern der Vögel durch Einfluss des Klimas (roughly "the modification of birds by climatic influences"). In this, he put forward a proposal that the plumage of birds living in hot, humid, environments tends, on average, to be darker in colour than that of those which do not. In 1929, biologist Bernhard Rensch formalised this as "Gloger's Rule", one of a number of zoological "rules" that seemed to be true more often than not but for which there was not necessarily a clear explanation.

Crucially for this blog, it seemed to be true of the skin and hair of mammals as well. This observation raises a number of questions, of which the first is clearly "is it true?" 

As we'd expect for such a rule, it's by no means a universal truth; there are a number of exceptions. But, as a general principle, there have been a number of studies, even in just in the last few years that seem to support its reality. In 2020, a study examining coat colours across 137 different species of squirrel showed that they do, generally speaking, conform to the rule. So do opossums and, to show that it can apply even within a single species, house mice. There are many other examples, including several among primates.

Such studies typically look at the simple question of whether or not the animals tend to be darker in colour. Which was, essentially, all that Gloger had originally said. However, when Rensch formalised the rule in the 1920s and '30s, he gave a rather more detailed description of what he thought was going on. (Rensch, incidentally, is today better known for his own "rule", which concerns sexual dimorphism, and he also formalised Allen's Rule that animals living in cold climates tend to have bulkier builds. Trying to codify such principles was clearly an interest of his.). 

Rensch described the principle in terms of melanin. Melanin is not a single compound or even, in a chemical sense, a meaningful class of compounds. It's really just a general term for a natural pigment that tends to make living organisms darker; even plant pigments count. In mammals, there are three types of melanin found in the skin and hair (there are others elsewhere, for example in the brain). The one we typically think of is dark eumelanin, which is the one responsible for dark skin and hair in humans. But there is also "brown" eumelanin, which is paler in colour and responsible for lighter hair colours, and phaeomelanin, which is reddish and (if not masked by dark eumelanin) responsible for ginger hair.

The way Rensch described it, animals living in hot, dry, environments produce the most phaeomelanin, and those in cold environments the least. On the other hand, eumelanin (of both subtypes) is most common in hot, humid, environments and, again, disappears as the habitat gets colder. The two types of melanin are produced via different chemical pathways, so it's certainly possible for an animal to produce more of one without also producing more of the other, thus adjusting the proportion of dark to paler melanins in the skin and hair. This has even been observed within individual species, allowing them to adjust to the specifics of their local habitat.

Generally speaking, there are at least five hypotheses as to why Gloger's Rule tends to hold more often than not. As so often, they are not mutually exclusive, so it could be that the real reason is due to a combination of factors, but they are as good a place to start as any.

The reason that I suspect is the one most likely to spring to mind is that melanin protects against ultraviolet light. UV light, if it penetrates the skin, can damage DNA, leading to mutations and causing skin cancer - not least melanomas, which arise from the cells that are supposed to be producing melanin in the first place. The argument goes that, since there is less UV light at latitudes away from the equator, animals living in such environments need less melanin, and so tend to be paler.

There's no real doubt that this is the main explanation for variation in skin colour in humans. There's a reason that melanoma rates are highest among pale-skinned people living in sunny places such as Africa and Australia. But it's less clear whether or not this is the explanation - or at least, the whole explanation- amongst most other animals. That's because a hairy pelt should already protect against UV, even if it's relatively pale, making the addition of melanin less of a benefit.

A second possibility relates to how different colours absorb or reflect sunlight, thus warming the animal up or cooling it down. Since darker colours absorb more heat, you would expect that the reverse of Gloger's Rule ought to apply, and there is some evidence that this is true for "cold-blooded" animals such as reptiles, where this opposing principle is known as Bogert's Rule. Apparently, it's true for the eggshells of birds, too. However, this is thought to be much less of a factor in animals with thick fur (or feathers). Here, dark hair absorbs the heat and is able to radiate it away before it reaches the skin, so that darker fur might actually be advantageous to animals in hot climates.

Camouflage is a third likely factor. Obviously, it's beneficial to a polar bear for it to be white and this is going to apply to anything that lives in environments where there is plenty of snow, regardless of whether it wishes to hide from predators or sneak up on prey unseen. But it's also the case that tropical environments have darker, richer soil than places elsewhere, which would help explain why Gloger's Rule seems to relate to humidity, not just heat alone. For that matter, tropical jungles inevitably have a lot of tree colour, so it can actually be quite dark down on the forest floor, which it certainly isn't on the open tundra.

The remaining two possible explanations are a little more complex. Melanin in birds may protect against bacteria that degrade feathers and which are thought to be more common hot, humid, environments, but this doesn't seem to apply to mammalian fur. Finally, it could be a side-effect of selection for some other function of the genes that are responsible for melanin production that may be more relevant in hot environments.

It's difficult to pick apart all of these various explanations. They could all be true under different circumstances, and what may be true of one species isn't necessarily true of others. Most of the studies that have attempted to tease them apart have been on birds, where the effect may be stronger than among mammals. Nonetheless, at the end of last year, a study was published looking specifically at feral "razorback" hogs in the United States.

These are basically wild boar (Sus scrofa), the same species as the domestic animal. They were first introduced into America from domestic stock way back in the 16th century, largely so that settlers would have something familiar to hunt. The descendants of these deliberate early introductions have naturally been supplemented by escaped domestic pigs over the centuries since, and are widespread enough today to be a common game animal across the southern US, with smaller numbers in New England, Oregon, and Hawaii. This has doubtless been helped by the fact that the sort of predators that might want to eat them (wolves, cougars, etc.) often aren't common where they live, leaving humans as their main threat. 

Examining photographs of feral pigs shot by hunters, the researchers graded their coat colour on a standardised scale, not just in terms of how dark it was, but how reddish - indicating the balance between regular eumelanin and "ginger" phaeomelanin. The results showed that razorbacks living in parts of America that are hotter, sunnier, and wetter, have coats with more eumelanin than those living elsewhere. Phaeomelanin, on the other hand was more common in the pigs living in drier habitats and less so where it was sunny.

Camouflage seems unlikely to be a factor here, given the relatively small number of predators, and the fact that darker pigs don't seem to reside in shadier habitats (plus, it clearly hadn't helped hide them from the hunters...). That leaves temperature regulation as the likely main driver in this species, especially where high humidity renders sweating less effective as a means of cooling down. But UV might also be a factor, since phaeomelanin is poor at protecting against that, and was more common where it was less sunny. It may be significant that some of these pigs were living in areas that have only recently been colonised by razorbacks and still followed the rule, implying that it doesn't take many generations to establish such changes in coat colour.

It does not follow that the same will be true of other mammal species, or of variation between species; this applies only to feral pigs in the US. But these animals at least are darker in warmer, more humid environments, and the fine details of their colour suggest that both UV light and general temperature are the main drivers behind that. Gloger's Rule is doing pretty well for something proposed 190 years ago without a clear mechanism.

[Photograph by NASA, in the public domain.]