The fact that so many mammals, of widely different kinds, moult to at least some extent shows that it must be an evolutionary ancient phenomenon. In fact, it turns out that animals have probably been moulting since before they even had hair. We can tell this because it's not unique to mammals. For instance, birds moult their feathers, and the process is similar to hair moulting in mammals. More significantly, perhaps, moulting has the same underlying mechanisms as reptiles periodically shedding their skin and can be tied back to sloughing in fish and amphibians as well.
Come to that, the oldest fossil evidence of moulting of any kind goes back almost as far back as identifiable animal fossils exist, but it's a stretch to link that to what happens in vertebrates.
Moulting has received a fair amount of research attention in birds, partly because feather condition can affect annual migration in some species. In mammals, however, attention has primarily focussed on domesticated, rather than wild species - although that's mainly cattle, not dogs. That is not, however, to say that it has been ignored.
Perhaps the most obvious reason for moulting in wild mammals is the creation and then shedding of a winter coat. Thick fur keeps you warm in winter but might overheat you in summer, while, if you live somewhere that it snows in winter, a paler coat at that time of the year makes sense as camouflage. Even without this driver, however, it may make sense to concentrate the replacement of hair at a particular time of year. This is because it's energetically costly, and ideally, you want to do it when food is abundant, but you don't need resources for something else - such as breeding or raising young.
There is some evidence that seasons were less extreme before around 16 million years ago, becoming more intense as large ice caps developed at the poles. If so, many groups of mammals may have only begun to evolve thicker and/or paler winter coats around this time - something that requires them to moult twice a year, not just once.
The marmots (Marmota spp.) are one group of seasonally moulting mammals that have evolved over this particular time. There are fifteen recognised species of marmot, all of which live in cold environments, either in the far north or in mountainous areas. They first evolved in North America around 9 million years ago, during the Late Miocene. These early marmots, however, were smaller than the modern sort, and likely had short fur. The features we associate with marmots today may have appeared with the last common ancestor of all living species, which lived about 6 million years ago. They were not especially common animals for much of this time, being found much less frequently than other rodents in the areas where they lived, perhaps because the cold, marginal, habitats they were best suited for were not yet common.
Around 3 million years ago, however, as the world cooled and the Ice Ages beckoned, they suddenly came into their own, increasing in numbers and rapidly diversifying. It was also at this time that they crossed the Bering Land Bridge into Asia, eventually reaching as far as Western Europe. (The current population of Alpine marmots in the Pyrenees is a modern introduction, but they did live there during the Ice Ages).
The metabolic demands of being a huge fat squirrel that hibernates in winter have been the subject of significant study, as have marmot social lives. But, while species descriptions will naturally tell what colours the different species are, there has been less of a focus on their fur more generally. A recent review took a look through what we do and don't know about the subject, and how it compares with other mammals.
For a start, the hair fibres of marmots differ from those of animals in less hostile climes. At a microscopic level, hairs consist of a sheath of pigmented cortical cells around a core of spongy tissue and enclosed by a thin layer of protective scales. This spongy core, the medulla, is missing in some mammals, and in thinner hairs on some parts of the body but, perhaps by simply bulking up the hair volume, it is thought to play a role in insulation. Fitting that theory, in groundhogs, the most temperate dwelling species of marmot, the medulla makes up around 60% of the hair diameter, while it's about 87% in the Tarbagan marmot of Siberia and Mongolia. Even this is well short of the 98% in reindeer, but it's worth remembering that marmots hibernate through the winter in burrows as well as having a thick layer of insulating fat, so it may be less important for them.
One of the differences between an animal's winter coat and its summer one is that the former tends to be about twice as thick, as measured by the number of hairs per square centimetre in the main pelt. A study of the coat of Siberian marmots estimated that this had about 3,100 guard hairs per square centimetre, while others have found that, in most marmot species there about eleven times as many underfur hairs as guard hairs (the latter being, as I mentioned in a post a few weeks back, the longer, protective hairs projecting above the underfur). Taken together, this makes the coat of a Siberian marmot about the same density as the winter coat of a red squirrel, which makes sense given its environment.
Since the number of hair follicles isn't thought to change much throughout an animal's life, the only way that hair density could change in this manner is if some follicles are switched off, or at least have their activity greatly slowed down. In most mammals, hair only grows for a few weeks or months, and then stops, remaining at a set length until the moult. (This contrasts with humans, where the growth phase can last up to six years, and the hair only remains quiescent for about three months after that before it falls out... which is why cats don't need haircuts in the same way humans do). Rather than simply becoming loose and falling out, the hair follicle actively expels the hair when it is time to grow a new one, and it's the physiological ability to coordinate this process across the body that underlies the ability of animals to moult.
There are at least six different patterns in which moulting can occur. In most mammals, it progresses in a wave from one point to another and four of the patterns reflect this, depending on whether it starts on the head, tail region, back, or belly. In the fifth pattern, mosaic moulting, it starts at multiple points, expanding outwards until it has reached the whole body. In most cases, it's not clear what reason, if any, there might be for which pattern a given animal uses.
It isn't even always the same within a given species; in harvest mice, moulting is equally likely to start at the head or the tail. That it normally does neither in other mice species, instead starting on the belly suggests there's probably a reason for this, but we can only speculate as to what. It may, for instance, have something to do with minimising the time that thick fur on the belly gets in the way during climbing, but even if that's right it doesn't say much about mammals more generally.
The sixth "pattern" is to have no pattern at all, with hair falling out more or less at random. Arguably, this is what happens in humans and other mammals that have very little in the way of hair, and it is also seen in some domesticated animals, which tend not to have distinct summer and winter coats even if their ancestors did. Otherwise, in wild animals, such "diffuse" shedding is normally a sign of old age or illness, suggesting that whatever underlies the regular pattern is breaking down.
The pattern in marmots varies between species. In Asian species, it starts at one to two points in the middle of the back, and progresses outwards, moving towards the head more quickly than towards the rump, so that the tail is the last part of the body to moult. The groundhog does the exact opposite, with the moult starting on both the tail and face, and then progressing inwards until it meets in the middle. The only other American species for which this information is recorded live in isolated regions of the Pacific Northwest and show a more complex, mosaic pattern. Why there should be such a difference remains a mystery.
Perhaps surprisingly, marmots do not have distinct winter and summer coats and they only moult once a year. It's thought that this may be because they spend so much of their lives hibernating underground, which both removes the need for a thicker winter coat and reduces the time available to grow one (assuming they can't do it while asleep and conserving energy). Set against is the fact that the only other hibernating squirrel in the far north - the Arctic ground squirrel of Alaska - does, in fact, moult twice a year.
Nonetheless, marmots have apparently lost the spring moult at some point in their evolutionary history. We can say "lost" because at least two species of marmot shed the surface layers of their skin in the spring, even if their hair remains intact. They do this again during the moult, and similar sloughing of surface skin is often seen during the moult of other mammals, and is normally part of the same process. Here, it's worth noting that even some entirely hairless mammals, such as killer whales, have a seasonal "moult" that consists solely of shedding skin.
I should briefly mention the Olympic marmot of Washington state at this point, since it does have a notably blond coat in the spring and a black one later in the summer. It used to be thought that this meant the animal must surely moult twice a year, with one of the moults even occurring while it hibernated. But it turns out that the hair itself changes colour, apparently due to some bleaching effect from sunlight when the animal first emerges from its burrow.
The timing of the annual moult in marmots varies between species, but it usually begins one or two months after emergence from the burrow, and is clearly tied to the seasons. In other mammals, this is linked to the changing length of the day, and there's no reason to assume this would be any different in marmots. The way that this works is that levels of melatonin increase as the day shortens and one of the many functions of melatonin is to encourage hair follicles to produce new hair. In animals that do moult twice a year, this rise in melatonin induces the growth of the winter coat - mink injected with melatonin during the summer produce a winter coat six weeks earlier than they normally would.
Another function of melatonin is to reduce the production of another hormone, prolactin. As melatonin levels go down in the spring when the days lengthen, prolactin levels increase. This is mainly involved in triggering things like lactation in nursing mothers, but it also seems to be responsible for the growth of a summer coat in those animals that have one. In those that don't, including marmots, prolactin tends to prevent or delay moulting, presumably so that the mother doesn't waste resources on a new coat at the same time she needs them for producing milk for her young.
It's unlikely that these hormones alone control when an animal moults. Local weather conditions, or whether or not the animal lives in colder highland terrain, also play a part. This may well be tied to the availability of food and the resulting energy that the animal has to spare.
The very first moult that a mammal undergoes is often different to the others, marking the shift from a distinct juvenile coat to the adult one. In deer and pigs, for example, the young are often spotted, but the adults are not; the change is perhaps even more dramatic in seal pups as they shift from their fluffy white insulative coats to the waterproof adult ones. In the case of marmots, this happens between two and seven weeks after birth, depending on species. However, it is such a slow and gradual process that it hasn't always finished when the young marmot hibernates for the first time.
Prior to this, the coat is shorter and darker than it would be in adult, although this may partly be because it doesn't have as long to grow. Since they are born underground where we can't see them, we don't know for certain whether most marmot species are born hairless, as many other rodents are. However, we do know that while groundhogs are born with very thin short fur on the head, they are otherwise hairless, and that steppe marmots on Russian fur farms don't develop any hair at all until the second day after birth.
There is clearly more to be learned here even if, outside of domestic animal husbandry, it's likely of low priority. While the article I have used as the basis for this post focussed on marmots, and highlighted the gaps in our knowledge - which, surprisingly, are greater for the North American species than the Asian ones - the reality is that we don't know that much more about moulting most other wild mammals. We have some idea of what triggers it, and why some animals would want distinct winter and summer coats, but less about the pattern or why it should happen in animals that only moult once a year. Humans do well enough without it... but then, when it comes to hair, we're kind of odd.
[Photo by David Monniaux, from Wikimedia Commons.]
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