Sunday 11 July 2021

All the World's Deer: White-tailed and Mule Deer

White-tailed deer
When I started this series on deer species, I asked some non-experts how many different kinds of deer they were aware of, to see which ones were best known. I got roughly (but not entirely) what I expected, with red deer and fallow deer topping the list. But that's a reflection of where I live; I suspect that, had I been asking Americans, few could have failed to mention the white-tailed deer.

The white-tailed deer (Odocoileus virginianus) is the most widespread and common species of deer in the Americas, and well-known to anyone familiar with the American wilds. They live throughout the whole of the contiguous US, except for the arid south-west, across southern Canada and almost the whole of Mexico. They are also found right across Central America, and into Colombia and Venezuela beyond, reaching the Guyanas and far northern Brazil in the east and as far as Peru in the south.

That's one heck of a range and, as you might imagine for an animal that's equally at home in the Yukon Territory and the northern fringes of the Amazon jungle, implies a fair degree of adaptability. Well over 30 different subspecies are recognised across this vast area, although few are backed with any sort of rigorous evidence and they're most likely just the product of various local naturalists having come up with different names over the last couple of hundred years or so. 

In fact, it's surprising that white-tailed deer didn't appear in Linnaeus' original list of scientific names in 1758 - either he hadn't heard of them, or he just assumed they were a kind of red deer. Instead, they were first described as a distinct species by Eberhard von Zimmerman in his catchily titled 1780 work Geographische Geschichte des Menschen und der Allgemein Verbreiteten Vierfüssigen Thiere in which he described them as native to what Britain then claimed as the Colony of Virginia (although there was some debate about its exact legal status at the time). 

White-tailed deer are medium-sized deer, roughly similar in size to the fallow deer of Europe, although with greater variability across their range - those in the north are larger than those elsewhere. They have a relatively plain brown coat, with a richer, warmer, colour in the summer and fading to a greyer shade in winter. The tail is longer than in some other deer species, and is pure white on the underside, allowing the animal to flick it upwards and display a bright flash when it flees a predator, warning others of its kind.

The antlers are quite different in shape to those of Eurasian deer species. There is no brow tine, with the first branch being a very short basal snag that doesn't develop until the third year. The growth pattern of the bone suggests that this develops in the same place as the main beam of a red deer's (or elk's) antler, with the beam of the white-tailed deer instead developing where the trez tine would. Whether that's a real evolutionary signal or not, the beam then arcs upward and back before curving round to face the front again, with at least three further tines branching off it to the rear. In some older bucks, these can become quite impressive, and serve as an apparently honest signal of genetic fitness.

White-tailed deer are most common in what's called the "forest ecotone" - that is, the region where the woodland gives way to more open pasture. This allows them the benefits of both habitats, and the deer will eat pretty much any kind of available browse, with the details depending on where in the world they're found. It also means that, in modern times, they are often found close to farmland, making them comparatively easy targets for hunters. 

In fact, the population of white-tailed deer dramatically crashed after the arrival of Europeans in the Americas, and continued to decline as they expanded westward and as human populations grew through the 19th century. By the early 20th century, it was on the verge of being wiped out across much of the US, leading to some outright local bans on hunting that later gave way to established "deer seasons" as populations started to recover. And recover they have; the US white-tailed deer population was estimated to be over 50 times higher at the end of the century than it was at the beginning and may arguably be overabundant in some areas. 

It has, however, declined again over the last 20 to 30 years, with literally millions being hunted annually. This doesn't mean that there's any real risk to their continued survival, or even of a return to anything like the 1900 levels. Specific populations may be a different matter; the Key deer subspecies (O. v. clavium) inhabits only the Florida Keys and numbers only a few hundred individuals; hunting is banned, and the primary cause of death now appears to be vehicle collisions.

White-tailed deer are moderately social, and are most active around dawn and dusk. Females live in herds consisting of a matriarch, her adult daughters, and their recent young, while males live in much smaller bachelor herds where the members are mostly (although not always) around the same age. Herds may be larger where the deer live in more open country, presumably because of a greater risk of predators. They are, however, not usually territorial unless raising young fawns. At least eight different calls have been identified, signalling fear, distress, aggression, social bonding and so on.

In the US and Canada, the rut takes place around November, with the young being born about 200 days later, in the spring. This is less true further south, and breeding may occur almost year-round in populations living close to the equator. The rut is the only time that the sexes meet up, and is marked by the males thrashing vegetation and scraping the ground as they attempt to attract females. A common behaviour of rutting bucks, although not unique to the rut (or even males), is "rub-urination". This involves the deer spraying urine down their legs to mix with the secretions of the scent glands on their hind feet, providing information on their sexual readiness and genetic prowess. (They also have scent glands under their foreskin, but, oddly, these don't seem to do anything special during the rut).

Eventually, they approach the doe in a crouch, before leaping up and chasing her in a circle until she allows them to mate.

One would expect the older, fitter, bucks to monopolise the mating, and they certainly try, but they're a lot less successful at it than one might think. As many as a third of all fawns have fathers who hadn't even fully grown their antlers yet, although their mothers are often young and inexperienced, too. Moreover, older does are more likely to give birth to twins, and when they do, around 25% of the time the twins do not share a father.

White-tailed deer can live for up 20 years in captivity, although the life expectancy in the wild is probably less than three years. A lot of that is due to fawns being caught by predators, but hunting and other man-made hazards are also a factor... it's the downside of being the single most common hoofed mammal in North America.

Mule deer

The closest living relative of the white-tailed deer is the other medium-sized deer of North America, the mule deer (Odocoileus hemionus). This is found only in the western half of the US and Canada, as well as parts of Mexico. Notably, this includes some arid areas where white-tailed deer are not found, and, where the two are found together, mule deer tend to stick to drier habitats, leaving the wetter ones to their cousins. 

Mule deer are similar in size and general coat pattern to white-tailed deer, but have shorter tails and longer ears. The tails are typically white with a black tip, but in two subspecies, found along the Pacific coast from northern California to southern Alaska, they are almost entirely black on the upper surface. These two subspecies are collectively known as "black-tailed deer" and descend from a single isolated population trapped somewhere in the Pacific Northwest by the advancing ice sheets during the Ice Ages while the ancestors of other living mule deer managed to migrate southwards. Nonetheless, they remain the same species and hybridise extensively where the two kinds meet in places such as the Cascade Mountains.

The antlers of mule deer start out growing in the same way as those of white-tailed deer, with a basal snag and a main beam that curves upward into a C-shape with the tip pointing forwards. In their case, however, the first tine off the beam grows to a much greater length, and branches off its own sub-tine, so that the antler essentially splits into two equal beams, each with its own forked tip. The total number of points is thus the same as in white-tailed deer (five, typically, although it can be more) but the arrangement is different.

In most respects, the habits of mule deer resemble those of their white-tailed kin. They are most active at dawn and dusk, although they are more adventurous at other times of day in the absence of predators. Females live in small family groups, while males are often solitary, or at best live with a few other, unrelated, bucks. They eat much the same food, although they do prefer slightly scrubbier and more rugged terrain, which can limit their food supply at certain times of the year - like white-tailed deer, they can significantly benefit from nearby agricultural land, even though this brings them closer to hunters. 

Some mule deer migrate significant distances over the course of a year, avoiding the worst of the winter snows by heading to lower elevations. Except in areas where snowfall is particularly bad, not all individuals necessarily migrate, and they are less likely to do so in mild winters. This doubtless protects them from the risks associated with long distance movement, and gives the smaller, year-round population more of a chance to find food during difficult times of the year. Nonetheless, distances of up to 160 km (100 miles) have been recorded, which implies quite a degree of effort.

Mule deer suffered similarly to the more widespread white-tailed species through the 18th and 19th centuries and were becoming scarce by around 1900. Their population increased as hunting bans were brought in, levelling off when discrete "hunting seasons" were brought in, and then booming in the early '40s when hunters were away shooting things other than animals. Unlike white-tailed deer, though, after reaching a peak around the '50s and early '60s, mule deer populations declined through the latter half of the 20th century and are not really recovering. At least in California, this has been attributed to overhunting due to overly cheap licenses, but it's likely other factors have also been at play.

Which is not to say that they are in any way rare, and they're still more numerous than they were at the dawn of the last century. For that matter, predators are a significant factor in their mortality, as is a naturally occurring chronic wasting disease similar to BSE ("mad cow" disease) that has been spreading amongst them since at least the 1960s.

The breeding habits of the two species of medium-sized North American deer are similar, and fertile hybrids between the two are sometimes reported. It's unlikely, however, that they are very common in the wild, especially as the two species tend to avoid one another, presumably to limit competition for resources. That they look rather similar may imply that some reports of wild hybrids are misidentifications, since it's quite hard to identify them with confidence.

Deer closely resembling white-tailed and mule deer have been living in North America since at least the mid-Pliocene 3.5 million years ago. Their ancestors must have crossed over from Asia, and likely not long before that time. But, when North and South America collided towards the end of that epoch, heralding the coming of the Ice Ages, some deer headed further south, entering the newly available continent. Despite their shorter evolutionary history on the continent, today, there are many more species of deer in South America than there are in the north. Next time, I'll be looking at some of them.

[Photos by "Tcolby6" and Constantine Kulikovsky, from Wikimedia Commons.]


  1. Nice to see a shoutout to my local deer species. White-tailed deer are one of the only large wild animals I've been able to see out and about where I live (the other being the american black bear), so they hold a special place in my heart.

  2. I enjoyed reading about both species of deer I'm most familiar with. Growing up in California, I could visit Mule Deer in the mountains. I don't recall them visiting the neighborhood of my youth on the fringes of Los Angeles, even though they lived in the Santa Monica Mountains a few miles away.

    On the other hand, White-tailed Deer visit where I live now in the Detroit suburbs all the time. Before the pandemic, I would see them once or twice a month. After the pandemic began, they became a near-daily sight as traffic declined, people stayed inside, and the area became quieter and safer for them. Even as human activity returned, they have stuck around and I can see them in both front and back yards from inside my house.

    That's the good news about the pandemic and deer. The bad news is that Nature reported at the beginning of this month that coronavirus is rife in White-tailed Deer based on a preprint of a paper from Cold Springs Harbor Laboratory. The most likely mechanism is that they picked it up from surface water contaminated by sewage. I don't know of any direct human-to-deer or deer-to-human transmission. I hope it stays that way.