Volume 2 of the Handbook of Mammals of the World (HMW) covers 17 families in six orders. Although united here, their grouping as “Hoofed Mammals” is more practical than phylogenetic. Certainly the bulk of the species covered are related (Artiodactyla and Perissodactyla), but some of the smaller groups (Tubulidentata, Pholidota, Hyracoidea, and Proboscidea) are more distantly related, and included here more as a matter of convenient comparison rather than evolutionary relationships. The volume contains some of the most conspicuous and well-known mammals, as well as some smaller, more secretive, and less well-studied forms.
We begin the volume with one of the most curious of all mammals, the Aardvark (Oryctoropus afer). Probably best known as the initial animal in countless bestiaries, Aardvarks are unique, as recognized by their placement in the monotypic family Orycteropodidae, the only family in the order Tubulidentata. They are Africa’s representatives in a worldwide group of mammals known colloquially as anteaters. Through ecological convergence on a lifestyle of feeding on ants and termites, members of several different orders have evolved on all the southern continents. We also cover the order Pholidota (Pangolins) in this volume, an otherwise unrelated group of anteaters found in both Africa and Asia.
Two additional orders (Hyracoidea and Proboscidea) are in fact closely related, despite their dissimilar external appearances. Hyraxes and elephants have long been recognized as close relatives and they, too, are now found only in Africa and Asia. Hyraxes range from solitary, nocturnal, arboreal forms to gregarious, diurnal, rock-dwelling species that are conspicuous members of many habitats. Their resemblance to other small mammals led to inappropriate names, both scientific and common. The family name Procaviidae is based on the misperception that they were somehow antecedents of guinea pigs, and the common name Hyrax literally means “shrew-mouse.” The third member of this group, the Sea Cows (order Sirenia) will be covered in Volume 4 of HMW on Sea Mammals.
Elephants (family Elephantidae) on the other hand are the largest living land animals, and arguably the world’s most charismatic megavertebrates. Although traditionally divided into just two species, one in Africa and one in Asia, here we follow the more recent arrangement of dividing African elephants into Forest Elephants and Savanna Elephants, recognizing three species. Although the evidence for this separation is abundant, with long-recognized morphological differences recently confirmed with modern molecular analyses, the exact boundaries between the two African elephants and the exact ranges of each are difficult to determine. Hence, the range maps are not done with our normal precision, and should be regarded as indicative rather than authoritative.
The order Perissodactyla, the odd-toed ungulates, is represented by three families. The first, Equidae (Horses and relatives) contains seven species in a single genus, Equus. Although horses, zebras, and wild asses are well known, we continue to refine our understanding of their evolutionary relationships. The most up-to-date scientific evidence suggests the scheme used here, with the Plains Zebras and the extinct Quagga united under the oldest available name for the two, Equus quagga.
The family Rhinocerotidae (Rhinoceroses) contains five species in four genera, all well known, and unfortunately, all endangered. Because of the drastic reductions in the original ranges of these species, this is another group where it is difficult to map their current ranges. In addition, we have intentionally left the range maps of the African species especially vague, in this case for conservation reasons. The current situation for each species is clearly outlined in the text, however.
The final family of Perissodactyla, Tapiridae (Tapirs), consists of a single genus (Tapirus) with four species. The family represents an ancient lineage dating back some 50 million years and the distribution of the surviving four species is curiously disjunct. There are three species in the New World tropics and a single species in South-east Asia. Although the Old World form looks very different from its New World congeners, sporting a contrasting black and white color pattern, the group is thought to comprise only a single genus. The close relationship is confirmed in the color patterns of the young, however, as all four species look very much alike as youngsters.
By far the bulk of the volume is devoted to the order Artiodactyla, the even-toed ungulates, represented by ten families. The first of these, Camelidae (Camels) represents an interesting conundrum for compilations such as HMW. In general, we are working diligently to summarize what is known about each and every species of wild mammal. This implies (correctly) that we put less emphasis on domesticated forms, and normally we do not provide separate species accounts for domestic species. However, with the family Camelidae, there is ample evidence of the need to make an exception to this rule. Of the six species of camels we recognize here, three are known only as domesticated varieties. Had we been rigorous in including only wild species, we would have had to omit a tremendous amount of valuable information about all forms of this family. Dromedary Camels (Camelus dromedarius) no longer exist as non-introduced wild populations. However, they are a distinctive species found throughout Asia and Africa, with a well-documented history of domestication from a once-wild ancestor. Furthermore, there is an introduced wild population in Australia, a continent far from their ancestral home!
Similarly, the South American representatives of this group occur as four quite distinct forms: the Guanaco (Lama guanacoe), a wild species; the Llama (Lama glama), a domesticated animal derived from the Guanaco; the Vicuña (Vicugna vicugna), a wild species; and the Alpaca (Vicugna pacos), a domesticated form derived from the Vicuña. Careful scholarship has revealed a fascinating history, both evolutionary and recent, for all of these mammals. Ignoring half of this information solely because it relates to now-domesticated forms would be unfortunate, indeed. In general, we provide information about domesticated animals in the family accounts under “Relationship with Humans,” but we will use unique solutions to provide the maximum information in the most easily retrievable fashion. In addition, we have used a unique pattern to indicate the current range of the domesticated forms on the range maps accompanying the species accounts.
Like Camelidae, the family Suidae (Pigs) contains both wild and domestic forms. Because there are still extant populations of the wild ancestors of the domestic forms, the problem is more easily handled here. The reader will find a fascinating amount of information on the domestication and transport of pigs around the world both in the family account, under “Relationship with Humans,” and in the appropriate species accounts. The closely related family Tayassuidae (Peccaries) comprises a small group of three extant species, all restricted to the New World and all occurring as wild species.
Another very well-known and conspicuous family is Hippopotamidae (Hippopotamuses). Although traditionally placed near the pigs and peccaries, recent molecular work has revealed that hippos are more closely related to members of the order Cetacea (Whales and relatives). This finding has led to classifications uniting Cetacea and Artiodactyla in an order called Cetartiodactyla, with hippos forming the link. Because HMW will treat Sea Mammals as a separate volume (Volume 4), we have retained the classical scheme here. Regardless of where they fall in the phylogenetic scheme of things, hippos are fascinating animals, and their natural history is well documented in the literature and nicely summarized here.
The family Tragulidae (Chevrotains) contains ten species divided among three genera. The name Chevrotain means “little goat” in French, and is curiously inappropriate for these animals. However, “mouse deer,” the only widely used alternative, is perhaps worse, as tragulids are neither mice nor deer. Found in both Africa and Asia, these diminutive forest dwellers represent the end points of another ancient lineage, dating back 50 million years. The curious African Water Chevrotain (Hyemoschus aquaticus) has been separated on that continent from its Asian counterparts for about 35 million years.
Musk-Deer (family Moschidae) are another enigmatic group. Restricted to the hills and mountains of China, Siberia, and the Himalayas, they were once thought to be a subfamily of Cervidae (Deer). Subsequently they were shown to represent a separate lineage, thought to be intermediate between Tragulidae and Cervidae. More recent molecular studies have questioned even this arrangement, suggesting that they may in fact be closer to the family Bovidae. The single genus contains seven species, all antlerless and with the males endowed with large canines that form tusks. The males also have abdominal musk glands, making at least half of the common name of Musk-Deer appropriate.
The second-most diverse family of artiodactyls is Cervidae (Deer). These familiar animals are found in Holarctic, Neotropical, and Oriental Regions and are divided into 18 genera containing 53 currently recognized species. This number should certainly be regarded as tentative, as our understanding of species limits in this group is changing rapidly with the application of new methods of study, including molecular analyses. Among the most common animals on several continents, deer have long provided humans with meat and hides, as well as sundry other products. Thus, there have been economic incentives to better understand the evolution and natural history of the group, resulting in a rich literature describing their various life histories.
Probably the single most eye-catching aspect of the current volume is the explosion of species recognized in the family Bovidae (Hollow-horned Ruminants). In 2005, the third edition of Mammal Species of the World listed 143 species in 50 genera of Bovidae. That list, prepared by the late Peter Grubb, was somewhat traditional and provisional, as he was engaged with his long-time colleague, Colin Groves, in a substantial revision of ungulate taxonomy. Their work, which will be published later this year, is the culmination of years of study of this important and wide-ranging family by these two venerable authorities. Colin Groves is the lead author for Bovidae in this volume of HMW, and in it we recognize all 279 species in 54 genera that are documented in his and Peter Grubb’s ground-breaking work.
At the root of this expanded number of recognized species is our changing view of the modern species concept. Like a growing number of taxonomists, Groves favors a phylogenetic species concept, which he defines as the smallest population or aggregation of populations that has fixed heritable differences from other such populations or aggregations. This is in contrast to the traditional biological species concept, which requires reproductive isolation between such populations. The difficulty in determining that reproductive isolation led to an underrepresentation of the number of species in many groups. Clearly there remain problems in determining which differences between populations are heritable, and the system used here undoubtedly will continue to be tweaked as our understanding grows. For now, this greatly expanded version of Bovidae species limits seems the best answer. One of the goals of HMW is to provide an up-to-date summary of the conservation status for every species of mammal, and this expanded species concept better enables us to explore the true conservation situation of each.
The family Antilocapridae is unique in several ways. It is the only monotypic family of Artiodactyla. The single species, the Pronghorn (Antilocapra americana), is restricted to North America. The entire fossil history of the group is from North America. The single remaining species is the only remnant of a once diverse family to survive the extinctions at the end of the Pleistocene that decimated the ungulate fauna of North America. Fortunately, Pronghorns continue to thrive on western grasslands of North America.
The final family in the volume is Giraffidae (Giraffe and Okapi). The two species in this family are among the most distinctive of ungulates. Giraffes (Giraffa camelopardalis) are widespread and conspicuous members of the African mammal fauna. Okapis (Okapia johnstoni) are much more secretive and elusive denizens of the forests of the DR Congo. In contrast to bovids, Giraffes are represented by widespread, fragmented populations, some of which have trenchant morphological differences, but all of which can interbreed, as far as we know. Therefore, authorities recognize nine subspecies, some of which may ultimately prove to be separate species.
Hoofed mammals have a rich and varied fossil history, with far more species having lived and gone extinct than survive today. Unfortunately, the rate of extinction is increasing as human populations continue to increase and modify natural habitats. HMW treats only living species; recently extinct species may be mentioned in the family accounts under the “Status and Conservation” section, but there will not be a separate species account for them. Species that are extinct in the wild, but still extant in captivity or semi-captivity, do have species accounts.
Although a detailed explanation of how to use these volumes is available in Volume 1, we should mention one nomenclatural detail that may seem strange to practicing systematists. The normal convention for listing authorities for scientific names is to place them in parentheses if the species in question was described in a genus different than the current name. We do not use these parentheses in HMW because of the varied readership we serve. For the non-technical user, the parentheses raise more questions than they answer. However, those wishing to pursue the basis of the name further will find a complete listing of the references to the original descriptions of every name used in the volume.