Humans are very, very bad at biological categories. We focus on general shapes and ecotypes and miss the biologically significant details that truly trace the history of life on our planet, and again and again our colloquial terms fall short of the expansive splendor of reality. I’ve written before about how the basic categorization schemes humans use don’t quite capture the way turtles versus tortoises, frogs versus toads, and other dichotomous pairings relate to one another, and today, we dive into a still-deeper morass: what is an antelope?

This seemingly simple question is actually such a mess of corner cases and evolutionary accidents that it not only defies an easy answer, but drags the concept of “deer” down with it. Let’s have a look.

“Antelope” is a word of complicated etymology used to refer to a variety of hoofed mammals around the world, principally in Africa and Asia. These animals are nearly all lightly built, agile, and fast. Some of them are among the largest hoofed mammals in their ecosystems, such as the giant eland, while others are tiny, such as the dik-dik.  They are all even-toed ungulates, uniting them in long-range kinship with pigs, peccaries, hippopotami, and whales, and they are all ruminants, closer to each other than they are to any of the previous.

But within that, they are a bit of a mess, and the mess of the antelope also manages to take deer down with them into the realm of biological incoherence.

The Cast of Our Show

In order to get a handle on this conversation, it is necessary to lay out which animals get called “deer” and “antelope.”

“Deer” in general refers to a group of ruminants that usually have antlers on at least one of their sexes at least part of the year. Some deer have tusks, either in addition to or instead of antlers, and are included with other deer based on general physical resemblance. Most deer live either in forests or in the transition zones between forests and plains. Although typically much smaller than cattle and larger than goats, deer include the moose, Alces alces, one of the largest ungulates alive today.

“Antelopes” are a group of ruminants that usually have annulated rather than smooth horns. Most antelopes are relatively small and agile, balancing thin frames with herbivores’ characteristic large guts for fermenting their low-density diet. Antelopes are diverse and highly speciose, including the cattle-like nyala and nilgai, the goat-like oryxes and saigas, small gazelles, and giant elands.

All ruminants, including deer and antelopes, are artiodactyls, or even-toed ungulates, and kin to pigs, peccaries, camels, hippopotamuses, and whales.

There are many ruminants, including giraffes, cattle, goats, and sheep, that are generally not considered to be either deer or antelopes, at least in common thinking.

Although it is relatively easy to point to an animal and say with confidence “that’s a deer” or “that’s an antelope,” the actual evolutionary descent of these animals does not follow these tidy human-centric definitions at all.

The Antelope Tree

The most primitive ruminants are the Tragulidae, known commonly as mouse deer or chevrotains. These tiny ungulates generally lack horns or antlers, instead bearing tusks, and tend to look more like gangly pigs than like the deer they are named after. Despite being colloquially called deer, the Tragulidae are not closely related to any other animals with “deer” in their common names. Their shape is generally in keeping with fossils of primitive ruminants and indicates that the starting anatomical position of this group is small, lightly built forest dwellers who have not yet developed other ruminants’ trademark cranial weapons.

Other ruminants are grouped together as the Pecora, or “horn-bearers.” Ironically, this group has three different kinds of cranial weapon among its members, only one of which is technically horns, in addition to also having a few members with chevrotain-like tusks.

The Pecora divide into two primary extant lineages. One contains the giraffe and okapi of Africa as well as our first antelope, the American pronghorn antelope. The pronghorn is the animal bearing the common name “antelope” that is least closely related to any of the others, such that many push for it to lose that moniker and become simply the pronghorn, and it is the one that presents the greatest challenge when trying to rescue “antelope” as a word with biological, cladistic meaning. Incidentally, the giraffe family is where one finds one of the two non-horn kinds of cranial adornments among the ruminants, in the form of their skin-covered bone ossicones, and the pronghorn is also the only mammal with branched horns and the only one that sheds its horns seasonally, by growing a whole second horn under the first until the first falls. Pronghorns are weird.

The other main lineage within Pecora is the Bovimorpha, which includes true deer (the Cervidae) as well as the tusked musk deer (Moschidae) and cattle, sheep, goats, muskoxen, and the rest of the antelopes (Bovidae). Within this lineage, the musk deer are actually more closely related to Bovidae than they are to true deer. It was detailed genetic study that revealed deer as three distinct lineages that only superficially resemble each other, only one of which (the true deer, Cervidae) has all the classic deer traits, including bony, temporary antlers (the second non-horn cranial weapon within the Pecora).

Antelopes, somehow, are still more tangled, as we’ll see within the Bovidae.

Prattle About Cattle

Antelopes are all over the bovid family tree. Nearly all members of the Bovidae are colloquially thought of as antelopes, and the ones that aren’t all count supposed antelopes among their close relatives.

Cattle correspond to the group Bovini, which includes domestic cattle; their wild and semi-wild cousins such as Cape buffalo, American and European bison, and Asian water buffalo; and a mysterious animal from Asia known as the saola. The saola and a few of the others have the slim builds and small stature associated with antelopes but tend not to be referred to with this word. The closest relatives of the Bovini, sharing the larger subgroup Boodontia/Bovinae with them, are various antelope species, including the nyala, kudu, and nilgai.

Sheep and goats are both within the Caprini, a subgroup of bovids that includes them, numerous wild relatives, and seemingly unlikely kin such as the bulky, tundra-dwelling, cattle-like muskox. Outside of the core goat and sheep genera, Capra and Ovis, the species in this group are variously referred to as sheep, goats, goat-antelopes, antelopes, and various local names, with the same animal often bearing several depending on the culture or authority consulted.

Surrounding the Caprini are numerous other antelope lineages, containing well-known creatures such as the impala, the klipspringer, the various gazelles, the Siberian saiga, the migrating wildebeest, and the majestic oryxes. With the Caprini, these groups make up the Aegodontia, sister to the Boodontia mentioned above.

Rescuing the Antelope

Antelopes are a “wastebasket taxon,” into which most hoofed mammals with horns were dumped before genetic testing could sort out their relationships to one another. The result is that “antelope” has since proven taxonomically incoherent as a concept and trying to define a group that contains some or all creatures commonly referred to as antelopes has expansive and sometimes silly results.

With the pronghorn included, “antelope” can become a valid biological category, encompassing the last common ancestor of all creatures called antelopes and all the descendants of that ancestor, only if “antelope” is equivalent to the whole of Pecora. That means that the giraffe and okapi, all true deer, all musk deer, all cattle, all goats, and all sheep are antelopes, and the only ruminants that are not antelopes are the chevrotains.

Since the pronghorn and its giraffid cousins are so clearly distinct from the other contenders, they are often excluded, and keeping them out of our thoughts here makes the rest of the story easier to manage. This means that animals outside the Bovidae are all definitely not antelopes, which helpfully removes the true deer and musk deer from consideration, but it does not make the Bovidae any easier to sort out. Cattle (the Bovini) are relatively easy to define, and they can even be defined separately from antelopes if the rest of the Boodontia (the nilgai, four-horned antelope, and several spiral-horned antelopes of the group Tragelaphini) are declared Not Antelopes, Actually Cattle. This leaves the other part of the Bovidae, the Aegodontia, as The Antelopes, but this subgroup inescapably includes goats and sheep. Goats and sheep are easy to define cohesively, whether narrowly as their core genera Capra and Ovis or broadly together as the Caprini, but both are so deeply embedded in the grand assemblage of (Aegodontian) antelopes that neither option creates the possibility of defining antelopes without including goats and sheep as subsets.

“Deer” are similarly impossible to rescue as a single monophyletic group in their fullness. The three subgroups of deer are so far apart genetically that the only group that includes all of them is ruminants as a whole. To take the most expansive view makes all ruminants, including the mouse deer and the entire conversation about Pecora above, into deer, and “antelope” thereby becomes a subset of “deer.” Deer are not quite as difficult to disambiguate as antelopes, however, because they form those three distinct groups, which can simply be regarded as separate from one another. If mouse deer and a musk deer are both Not Actually Deer, then “deer” refers only to the Cervidae, and order is restored.

A Primitive Name

The messes above are the clear sign that a name describes a primitive condition. Much like “turtle” tends to be used to describe any aquatic or semiaquatic chelonian regardless of lineage, “antelope” has been used to describe any ruminant that is not obviously something else and “deer” is only a little more coherent. This is similar to how the word “reptile” is useful in a colloquial sense, where it unites a diverse assemblage of mostly ectothermic, mostly scaly vertebrates, but can’t be defined in evolutionary terms without including birds. Most hoofed mammal groups alive today, including the artiodactyls (even-toed ungulates), perissodactyls (odd-toed ungulates, including horses, rhinoceroses, and tapirs), and proboscideans (elephants) have similar-looking deep ancestors, which were almost always small, forest-dwelling animals built to nimbly navigate complicated underbrush. Antelopes are not quite the same as these ancestors, since the ruminants are themselves fairly deep within the even-toed ungulate family tree, but a similar idea applies. The “antelope” shape is not a derived condition, marking a cohesive group, but a primitive one, whose descendants have taken various other shapes as they spread around the world into different niches. Those that evolved to attain large size and become grazers usually got called cattle; those that took up residence in mountainous regions often became called goats, sheep, or goat-antelopes; and their close cousins with different habitats or habits usually kept the general name “antelope” or received a specific name used by the people who live near their homes.

That’s the trick: cattle, goats, muskoxen, and sheep are all derived, specialized bovids, and “antelope” is deployed as a catch-all for bovids (and sometimes just things that look like bovids, such as the pronghorn) that don’t register as special cases to English speakers. The most natural solution, then, is to consider “antelope” generally synonymous with the Bovidae. Cattle, goats, muskoxen, and sheep are, in this view, all special cases of antelope: highly derived, specialized for unusual habitats, and (other than muskoxen) having a special place in human history as animals whose domestication changed the path of human history.

It’s not quite as elegant as an impala prancing across the landscape, but it’ll do.