Sunday 5 September 2021

All the World's Deer: Roe Deer and Reindeer

Roe deer
Of the three species of deer native to Britain, the one that's most often overlooked is probably the roe deer (Capreolus capreolus). In fact, it's a common and widespread animal, being found in every major country in Europe except for Ireland, Iceland, and Malta. It's absent from the other Mediterranean islands, too, and doesn't stray far into Russia, but it is found in northern Turkey, and both the Caucasus and Kurdistan regions further east. 

One of the reasons that roe deer aren't so easily brought to mind as fallow or red deer is probably just that they're so much smaller. In fact, they are smaller than any species of deer native to the US or Canada, and by some margin. A fully grown roe deer buck stands no more than 85 cm (2' 9") high at the shoulder, and most are smaller, as of course, are does. They weigh around 25 kg (55 lbs), and have a graceful and slender body compared with most other small deer. The hind legs are slightly longer than the front ones, a feature that's thought to help them creep through dense undergrowth.

As is typical of small deer, the antlers are relatively simple in form, another feature that makes them less immediately impressive than most larger species. When fully grown, they usually have just three points, with a forward-facing burr about halfway up, and a fork at the tip of the beam. A study of antler shape and development has shown that the first branch is technically a brow tine, although it's much further up than we'd expect. Another unusual feature is that the beam below this tine is covered in numerous rugged knobs or "pearling", rather than having the smooth contours seen in other species.

Roe deer typically live in places with thick vegetation in which to hide, which typically means younger forests where the undergrowth is still heavy. Indeed, it's been argued that they initially benefited from the rise of agriculture, which meant the creation of more forest edges and secondary woodland - although denser modern human populations are a different matter. Across Europe, they are found everywhere from the scrublands of the Mediterranean coast to the pine forests of central Scandinavia, and they live at up to 2,400 metres (7,900 feet) in the Alps. One of the main limitations preventing them from moving further north (or up) seems to be snow; with small and narrow hooves they sink into it easily and, not being very large, it doesn't take particularly deep snow to cause them a real problem moving about.

At first sight, roe deer appear to be indiscriminate browsers, since they'll eat most available foods. Closer examination of their diet, however, shows that they have distinct preferences, seeking out some plants over others; red deer, in comparison, eat plants in proportion to what's in front of them. Through the spring and summer, they eat the most nutritious young leaves and herbs, with a shift to dead leaves, acorns, and ivy in the autumn and winter. In part, this is because they have small stomachs for their size, and so want to concentrate on foods that they can extract the nutrition from quickly, but they also seem to like plants that contain tannins, which taste bad to many other animals. Their saliva seems to be particularly good at neutralising the mildly toxic properties of these chemicals and, well, if nobody else is going to eat it...

The small stomachs of roe deer also mean that they need to eat frequently, up to every couple of hours or so. As a result, they are active through most of the day and night, alternating between eating and resting, although there is a preference for dawn and dusk as times for feeding. Compared with some other deer, they are not especially sociable, living in family groups of up to four related females and their young. Sometimes a couple of unrelated families will travel together, but that's as close as they get to forming herds. When the family group does bed down in some sheltered place for a rest, the individuals try to stay two to four metres (6-12 feet) apart, rather than huddling together, even in a cold winter. This might help hide them from predators.

The bucks, however, are largely solitary, and form distinct territories around the rut, fighting off other males for the privilege. The fights are not especially aggressive, presumably since neither side really benefits from injuring itself, although they become more so if the contestants are evenly matched and a winner isn't obvious early on. As a result, younger males with small antlers don't experience much aggression from their larger counterparts, who know they have little to fear from them.

The rut takes place from July to August, when bucks mark their territory with secretions from glands between their antlers. Does are only fertile for a couple of days each year, but the male tends to stay around her after mating for a few days. For such a small animal, pregnancy seems to last an unusually long time, with the fawns - twins are most common, but triplets and singletons also occur - not being born until the following May or June. This is due to a state of embryonic diapause, with the embryo suspending all growth once it has become a structured, but microscopic, ball of cells. This suddenly changes in January when development restarts, so that the "real" pregnancy is only around five months.

We now know that this sort of thing is common in smaller mammals that, due to seasonal constraints, want to breed not long after they give birth, but it was first described in the roe deer, way back in 1854. Given the technology of the time, this was quite hard to prove, so it was an impressive piece of work for its day.

During the Last Ice Age, roe deer lived only in a few locations along the Mediterranean coast, recolonising Europe around 9,700 years ago once the ice sheets had retreated. The ancestors of modern roe deer seem to be descended primarily from those that had sheltered in Greece, and some modern authors don't consider there to be any identifiable subspecies (although this is far from universal).

Siberian roe deer

However, what were once thought to be eastern subspecies of roe deer are now regarded as an entirely separate species. Most commonly called the Siberian roe deer (Capreolus pygargus) this is by no means unique to Siberia, so that the broader term "eastern roe deer" is sometimes preferred. The western edge of their range is generally said to be in Russia, although seems to be some overlap with the western species in Ukraine today and, historically, in Poland. From here they are found eastward across Russia, reaching into Mongolia, parts of central Asia and northern China, and throughout Korea. 

Siberian roe deer are slightly larger than the western species, and have a more uniform coat, which is greyish in winter and reddish in summer. Examination of their genetics, however, reveals some significant differences. While both species have 35 pairs of regular chromosomes, the Siberian one has a large number of smaller "B-chromosomes" consisting of genetic fragments not attached to the main ones. This isn't enough to absolutely prevent the two species from cross-breeding, but it does make it difficult and it seems to be something that rarely happens in the wild, even where the two come into contact. They probably last shared a common ancestor over two million years ago, becoming separated during the Ice Ages.

Siberian roe deer are found in tall grasslands as well as forest, and venture to higher altitudes than western roe deer in the summer. These latter populations, however, migrate downhill in the winter, sometimes travelling up to 200 km (125 miles) to do so; their larger size makes them slightly more tolerant of deep snow, but it's still something they want to avoid. Although such migrating groups can be quite large, in general, Siberian roe deer are even less sociable than the western sort, not even associating much as families. In most other respects, the two species appear to be similar.

One species of deer that you would expect to have prospered during the Ice Ages is the reindeer (Rangifer tarandus). While even they were forced to retreat south by the advancing ice sheets, this still left them with plenty of suitable habitat and today's populations descend from those living in no less than three refugia, the most important of which was particularly large, stretching from Siberia across the Bering land bridge into Alaska. Today, wild reindeer live right around the Arctic Ocean, from Norway and Finland through Russia and northern Mongolia to Alaska, Canada and the west coast of Greenland, with a few reaching south into the northern part of Washington state. In the north, they reach as far as Svalbard and Ellesmere Island - almost as far as it's possible to go on land.

In the US and Canada, they are instead known by the name "caribou", while the word "reindeer" is usually reserved for the semi-domesticated form of the animal, but in Europe, it's just what the species as a whole is called. The "rein" part of the name comes from the Old Norse name for the animal; the first part of the name survives on its own in many European languages other than English. "Caribou", as one might expect, comes from one of the First Nations languages, specifically Mi'kmaq, in which it means "beast that shovels" (snow, presumably).


Reindeer are large deer, about the size of red deer or elk. Their hooves are wide and the dewclaws behind them are longer and reach closer to the ground than those of other deer, all to increase the surface area and help them walk on soft snow. Other visible adaptations to cold climates include unusually thick fur, which extends onto what's usually a hairless nose in other species. Without the need for camouflage in dappled woodland sunlight, the calves are born without spots. Their ears are short, likely as protection against the cold, although their hearing is no weaker than that of other hoofed mammals.

One of the most distinctive features of reindeer, however, are their antlers. For one thing, they're heavier in proportion to the animal's body mass than those of any other kind of deer, reaching 15 kg (33 lbs) in the largest males - over 6% of its total body weight. Unlike most other capreoline deer, they have a clear brow tine, and one which is unusually long. Immediately above this is an even longer branch that gives rise to several tines at the tip; this is typically called a bez tine but seems to develop in a manner closer to that of the trez in other deer species. The beam then branches off another small tine to the rear before branching into two and giving rise to multiple tines, which are often "palmated" into broad sheets at the base.

More remarkable than this unusual shape is the fact that, unlike every other living species of deer, it isn't just males that grow antlers. The antlers of reindeer cows are typically smaller and less well-developed than those of bulls, but not dramatically so. They also shed them later in the year, perhaps because their lower weight is less awkward.

Reindeer are primarily adapted to living on the tundra where the deep soil is too frozen to allow plants with deep roots, such as trees, to grow. Nonetheless, they do venture into open coniferous woodlands at the southern edge of their range and, for that matter, into barren mountain ranges. Which of these three habitats they prefer is reflected in subtle differences in their physical appearance and genetic makeup, suggesting that they have an ancient history. As one might expect, reindeer love temperatures that are cold to us, being able to survive at ridiculously low temperatures down to -60° C (-76° F). In fact, they start to pant at temperatures above 0°C and become truly uncomfortable above about 15°C (60°F).

As with most other animals, heavy snow does eventually slow them down, but it has to be over 60 cm (2 feet) deep, or especially difficult to dig through, to really inconvenience them. Of course, there are some places where snowfall that heavy isn't unusual in winter, so some reindeer do migrate over the course of the year, travelling quite rapidly when they do.

Among the many disadvantages of being a large animal living in such a harsh environment is that there isn't much to eat. Reindeer feed on grasses, sedges, and the low-growing bushes that dot the tundra, but lichen seems to be a particularly important component of their diet, especially in winter. Most animals won't eat lichens, since they can't extract much nutrition from them, but reindeer have an unusual enzyme in their stomach that breaks down a unique carbohydrate they possess. Even so, it's low in protein, and, even in winter, reindeer supplement their lichen-based diet by eating dead shrubs and the like, having gut bacteria that are unusually good at digesting low-quality, high-fibre plant material.

Like roe deer, reindeer are neither truly nocturnal or diurnal, instead operating on an activity cycle of four hours or so. In their case, this probably isn't because they need to eat so frequently that they just have to have that midnight snack but more likely because they live at such high latitudes that sunlight and darkness are near-permanent at certain times of the year. In what may be a unique adaptation, the tapetum lucidum, a membrane in the eye that reflects light back inside it and that is responsible for the "eyeshine" seen in many mammals at night, changes colour over the course of the year. In winter, it is more effective, keeping more light inside the eye, increasing the ability to see in low light levels at the expense of being able to see fine detail. 

In another visual adaptation, reindeer are able to see into the ultraviolet part of the spectrum. This is thought to be helpful to them during long winter nights, especially since snow reflects UV light particularly well, so that other objects will stand out against it when viewed with that wavelength. It's possibly in relation to this that analysis has shown modifications to genes responsible for the functioning of the retina in reindeer, along with those for vitamin D metabolism, the circadian rhythm, and the ability to feel pain from intense cold.

Whereas roe deer, like most other small deer, escape from threats by leaping, reindeer simply run, as most other medium to large deer do. They are, in fact, the best adapted of all deer for running, which makes sense given their open habitat, and can reach an impressive 80 km/h (50 mph) for short periods. They are also effective swimmers, which may come in handy during their long migrations.

Reindeer are particularly sociable, with their basic family groups assembling into larger herds. During migrations, these can gather into still larger groups, with tens of thousands of the animals moving rapidly through an area over the course of a single day. Within the herd, cows seem to be dominant over younger bulls outside of the rut, perhaps partly because they retain their antlers for longer, and older cows are dominant over younger ones, regardless of their physical size, enhancing their ability to successfully raise calves.

The rut takes place from September to October, with most of the actual mating taking place over a 7-10 day period. Because they are so mobile, bulls don't defend a fixed territory, driving away rivals who get too close, but moving the area defended about as the herd travels. A single calf is born about eight months later. Whereas roe deer hide their fawns, which remain as still as possible while the mother feeds some distance away so as not to draw attention, young reindeer calves, having nowhere to hide, start to follow their mother about within around six hours of being born. Reindeer milk is particularly rich in fat and proteins; calves double their weight in the first two weeks of life and are weaned by five months.

Although they are not especially close relatives within the group, both roe deer and reindeer belong to the capreoline subfamily, which has most of its species in the New World. Before I head across to look at the remaining capreoline species, however, there are still a few cervine deer I need to look at, and it is to some of those that I will be turning next...

[Photos by Jerzy Strzelecki, Andrey Giljov, and Ryan Hodnett, from Wikimedia Commons.]


  1. Another fun thing about reindeer. The antlers are asymmetrical: ONE antler has a low branch, extending forward above the snout, with up-and-down palmation at the end: it looks very much like the snow-scrapers Canadians use to clear their automobile windshields of snow and ice in the winter, and I have long assumed, though without definite knowledge, that it is used to clear snow when browsing. ... ONE antler: it can be either the right or the left. I have asked whether an individual deer would have the "snow scraper" on the same side when growing a new set of antlers each year: one zoologist I asked said yes, but in wording that suggested that this was derived from general principles and not something specifically verified by observation.

    1. I did come across something about antler asymmetry while researching for this, but nothing definite enough to feel that it was worth including, so thanks. Doing a quick search now reveals a couple of papers (with the same co-author) that say it has something to do with parasite load - more parasites, more asymmetry. Which, if true, would make reduced asymmetry a fitness signal. On the other hand, if they really do grow on the same side each year, that would seem to count against the parasite theory, at least as the whole story.

  2. Living in suburban southern Sweden, roe deer are one of the commonest wild mammals I see, after hares and squirrels.

  3. Thanks for reply! Looking at the article you linked, it wasn't clear to me that the asymmetry w.r.t. "snow scraper" was a major contributor to the "fluctuating" asymmetry the article was concerned with. I also found "Asymmetry in antlers of barren-ground caribou, Northwest Territories, Canada," by Frank L. Miller in the 1986 volume of "Rangifer" (an open access Norwegian journal of reindeer and related studies), which (i) found that left-scraper and right-scraper forms were roughly equally common (but that there were also a fair number with both brow-tines prominent, and some with neither) (ii) nothing about constancy from year to year (iii) idea that the snow scraper was just that, and used to uncover food, though mentioned as a speculation, was dismissed because (a) prominent snow scraper brow tines are common in mature bulls but less so in other individuals (b) the antlers fall off in late autumn, so wouldn't be there at the times when there is the most snow to scrape! Concluding that the function of the asymmetrical brow scraper was unclear, but likely to be somehow related to courtship.