Doe, it's Deer

Marsh deer Blastocerus dichotomus, copyright Jonathan Wilkins. An animal that just screams out, "Am I wearing the Chanel boots? Yes, I am."

I hardly need to explain what deer are, do I? Deers (Cervidae) are generally recognised as the second most diverse family of hoofed mammals (after bovids) in the modern fauna. Their most recognisable feature, of course, is the possession of antlers: bony cranial appendages that are shed and regrown every year rather than being permanently in place like the horns of a bovid. When antlers first grow, they are covered with a layer of skin (the velvet) that supplies them with blood, but this skin is later shed to expose the bare bone. In most species, antlers are only grown by males whose use them in conflicts during the mating season. The only genus of deer that grows antlers in both sexes is Rangifer, the reindeer. There is also one living species that lacks antlers, the Chinese water deer Hydropotes inermis; instead of antlers, males of this species possess large, dagger-like canines. In the majority of deer species, antlers are subcylindrical and often branched but broad palmate antlers have evolved on multiple occasions within the family. Antler morphology is generally significant in distinguishing taxa but it should be noted that variation within species is not unknown. For instance, the few recorded males of the small, now possibly extinct population of moose introduced to the south of New Zealand lacked the large palmate antlers generally associated with the species, probably due to poor nutritional conditions. Instead, they had more slender antlers that only became moderately palmate distally, like those of a fallow deer Dama dama.

One of the few photographs of moose from New Zealand, from here. I think this might be the one shot at Herrick Creek in 1952 but I could be wrong.

The first antlered deer are known from Europe back in the early Miocene, about 17 million years ago. They are not known from North America until some time later in the Pliocene, about five mya (Pitra et al. 2004), though these days they are every bit as diverse in the Americas as in Eurasia. They never made much inroad into Africa, only extending into the northernmost part of the continent, and they never made it into Australasia under their own steam, though a number of species have been dispersed to various parts of the world by humans. For instance, at least half a dozen species have become established in New Zealand, and until recently reindeer might be found wandering among penguin colonies in South Georgia.

Reindeer and king penguins on South Georgia, from here.

Recent decades have seen some pretty wild swings in cervid taxonomy, with the number of subfamilies recognised varying from two to seven, and some authors recognising a much larger number of genera and species than others. However, our general understanding of cervid interrelationships is pretty good these days, with many differences between systems being a question of ranking more than anything else. Recent studies have agreed that modern deer can be divided between two primary lineages that may be called the Cervinae and Capreolinae (Gilbert et al. 2006). The Cervinae include the majority of deer species in the Old World with a single species (the wapiti Cervus canadensis) extending its range into the New World. The remaining New World deer all belong to the Capreolinae, which also includes four genera (Rangifer, Hydropotes, the roe deer Capreolus and the moose Alces) found in Eurasia.

Male tufted deer Elaphodus cephalophus, copyright Heush.

The Cervinae can be divided between two tribes, the Muntiacini and Cervini. Muntiacini include the muntjaks of the genus Muntiacus and the tufted deer Elaphodus cephalophus. These are small deer native to southern and eastern Asia. Antlers are small and simple in all Muntiacini: muntjaks have antlers with only a single short anterior branch whereas the tufted deer has unbranched antlers that are barely visible under the large tuft of hair that this species has on top of the head. Muntiacini also resemble Hydropotes in their possession of large canines in the males. The other tribe, Cervini, includes larger deer species with more complex, multi-branched antlers. Some authors have historically placed all species of Cervini within a single genus Cervus; others may recognise nine distinct genera. Numbers of recognised species have also varied, largely due to phylogenetic studies finding that taxa previously recognised as conspecific subspecies may be more distantly related to each other or may not form monophyletic units. For instance, the wapiti has often been regarded as a subspecies of the red deer Cervus elaphus but recent studies have suggested that it is more closely related to the sika C. nippon and the white-lipped deer Przewalskium or Cervus albirostris, two east Asian species (Pitra et al. 2004). Difficulties in elucidating cervin phylogeny are probably best exemplified by the case of Père David's deer Elaphurus or Cervus davidianus, originally native to southern China but now only surviving in captivity. Molecular phylogenies associate this species closely with the brow-antlered deer Cervus eldi but it has many morphological features indicating a close relationship with C. elaphus, and it is widely suspected that Père David's deer originated from a hybridisation event between the two latter species.

Pudu (I think a northern pudu Pudu puda), copyright Neil McIntosh.

The Capreolinae can be divided between three main lineages. One comprises the roe deer and water deer; another comprises the moose (again, I'm making a point of referring to genera rather than species because the number of recognised species may differ between authors). Note that the position of the water deer suggests that the antler-less state of this species represents a secondary loss rather than retention of a primitive state. The majority of capreolines belong to the third lineage, commonly recognised as the tribe Odocoileini. Except for the reindeer, the species of this lineage are restricted to the New World, with the higher diversity in South America. The Odocoileini are perhaps the most taxonomically uncertain section of the deer family. There appears to be no question, at least, that Rangifer represents the sister group of all other Odocoileini. The remaining odocoileins have generally been divided between six genera: Odocoileus (including the mule deer O. hemionus and white-tailed deer O. virginianus), Mazama (brockets), Pudu (pudus), Hippocamelus (guemuls), the marsh deer Blastocerus dichotomus and the pampas deer Ozotoceros bezoarticus. However, except for the two monotypic genera, monophyly of all these taxa was placed in question by a recent molecular phylogenetic study of the group by Gutiérrez et al. (2017). The issue is particularly marked for the brockets, small deer with unbranched antlers, as not only the genus as a whole but also species within the genus have been indicated as non-monophyletic. Recent years have, as a result, seen something of a burst of new brocket species being described. It is quite probable that a similar taxonomic explosion may be in line for the genus Odocoileus, with both of the currently recognised species including a number of subspecies and each being of suspect monophyly. Matters are further complicated by the possibility of hybridisation between the two 'species'.

REFERENCES

Gilbert, C., A. Ropiquet & A. Hassanin. 2006. Mitochondrial and nuclear phylogenies of Cervidae (Mammalia, Ruminantia): systematics, morphology, and biogeography. Molecular Phylogenetics and Evolution 40 (1): 101–117.

Pitra, C., J. Fickel, E. Meijaard & P. C. Groves. 2004. Evolution and phylogeny of Old World deer. Molecular Phylogenetics and Evolution 33: 880–895.