Odd-toed ungulates, named for their unique foot structure, have either one or three hoofed toes on each of their hind feet. They are part of the Perissodactyla order, which sets them apart from Artiodactyla, or even-toed ungulates, that have two or four toes. These animals also differ in how they process cellulose. Odd-toed ungulates break it down in their intestines, whereas even-toed species rely on one or more specialized stomach chambers for digestion.
These mammals were historically widespread but are now restricted to Central and South America, as well as parts of Africa and Asia. They are strict herbivores, primarily grazing or browsing on grass, leaves, and other plant parts.
Currently, this order, having 17 recognized species, is divided into three families: Equidae (equids), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs), which include horses, zebras, and the rhinoceroses. This order was more diverse in the past, which included several extinct groups like brontotheres, palaeotheres, chalicotheres, and paraceratheres.
Living odd-toed ungulates range from medium to large. The largest among them are the Indian and square-lipped rhinoceroses, which can reach lengths of 13 to 16.4 ft (4 to 5 m) and weigh more than 3,500 lb (1,600 kg). The largest known land mammal to date is the now-extinct giraffe rhinoceros (Paraceratherium or Indricotherium), which stood about 18 feet tall at the shoulders.
The smallest representatives of the order, tapirs and asses, are around 6.6 to 8.2 ft (2 to 2.5 m) long and weigh around 550 to 660 lb (250 to 300 kg).
Most of these ungulates are gray or brown in color, though zebras and young tapirs are striped. Rhinoceroses have a notably thick epidermal layer with sparse or no hair, whereas horses and tapirs have short and dense coats. Contrastingly, Sumatran rhinoceroses possess a coat of crisp, black hair.
They have an elongated head with a long upper jaw, a high temporomandibular joint (the joint connecting the jawbone to the skull), and an enlarged mandible. The snouts in different families of these ungulates vary because of the differences in the structure of the premaxillary bone. Another bone, the lacrimal, is connected to the nasal bone and possesses projecting cusps in the eye sockets.
In rhinoceroses, there are one or two horns of epidermal origin encased in a keratinized sheath, unlike even-toed ungulates whose horns stem from a bony core.
One distinctive feature of these ungulates is the presence of a small out-pocketing of the auditory tube known as the guttural pouch. This pouch serves as a cooling mechanism by helping to lower the temperature of the blood in the arteries that carry it to the brain.
Teeth
These ungulates have different forms and numbers of teeth based on the diet they prefer. In the two African species of rhinoceros, both the incisors and canines are either extremely reduced or absent, while in horses, mostly the males possess canines. Asiatic rhinoceroses usually lack upper canines and have one or two upper incisors in each half of the jaw. Tapirs generally have brachydont (low-crowned) premolars and molars and lack the secondary cement on the crowns, whereas horses have hypsodont teeth (high-crowned).
There are three to four cheek teeth on each half of the jaw.
The dental formula found in this group is: (0-3. 0-1. 2-4. 3/1-3. 1. 2-4. 3) x 2 = 30 – 44.
They are characterized by high dorsal spines on the thoracic vertebrae. The cervical vertebrae are opisthocoelous (bodies of the vertebrae hollowed towards the rear end), a feature that helps these ungulates freely rotate their necks. Additionally, there are up to 22 trunk vertebrae in the column.
Most of these mammals have reduced the number of weight-bearing toes from the original five to either one or three, although tapirs still have four toes on their front feet. The third toe is the largest and serves as the main support, with the front and rear feet aligning along this axis. Non-weight-bearing toes are either positioned toward the back as vestigial remnants or are completely absent.
The toes of modern equines (subfamily Equinae) are covered with hooves. However, in rhinos and tapirs, the hooves cover only the leading edge of the toes. Equines with single-hooved toes are more agile and have higher running speeds.
In the forefoot of these ungulates, the third distal carpal bone is enlarged and lies interlocked with the proximal carpal bones. The third metacarpal is elongated, pushing firmly against the interlocked carpal bones. This arrangement is a key feature of this group.
In horses, the ulnae and fibulae are reduced, and the astralgus or ankle bone, scaphoid, and cuboid bones are fused. When compared to the other odd-toed ungulates, their thighs are relatively short, and they lack clavicle.
Odd-toed ungulates were traditionally classified with other mammalian groups, like hyraxes, elephants, and even-toed ungulates or artiodactyls. A close relationship with hyraxes was particularly predicted based on similarities like the anatomy of their ears and the course of the carotid artery.
However, data from molecular research revealed that these mammals are polyphyletic, which means some of their similarities with other mammalian groups may be due to convergent evolution rather than shared ancestry. These ungulates, along with their counterparts, the even-toed ungulates of the order Artiodactyla, are currently placed under the broad clade Euungulata.
The classification of these ungulates, however, has undergone multiple modifications with time.
Currently, 17 species remain under these suborders, which are further subdivided into 3 families: Equidae, Rhinocerotidae, and Tapiridae.
Although once widespread across the globe (excluding Antarctica and Australia), today, wild odd-toed ungulates are confined to regions in Central and South America, Eastern and Southern Africa, and Central and Southeast Asia. However, domesticated species like horses and donkeys are now found worldwide.
While tapirs typically inhabit tropical rainforests, horses live in grasslands or semi-desert regions. Rhinos are usually found in dry savannas, but in Asia, they prefer living in wet marshes and forests.
These ungulates are exclusively herbivorous, consuming grass, leaves, and other plant parts.
These ungulates are primarily grazers or browsers, traveling long distances in search of food. Browsers, in particular, have access to food throughout the year and tend to be more sedentary than grazers. Rhinoceroses often use their forelimbs to reach higher branches, breaking down trees and shrubs in the process. They pluck leaves and twigs with a finger-like extension on their upper lips. Similarly, tapirs use their upper lips, which are fused with their snouts, to gather food.
Mountain and plains zebras form family groups consisting of a single adult male, two to three females, and their foals. These groups follow a dominance hierarchy, with the stallion at the top (alpha) and a ranking system among the mares. Other males either remain solitary or form bachelor groups of up to ten members. The social behavior of other equids is less documented.
Rhinoceroses, on the other hand, exhibit a different social structure. Dominant males control territories where they may tolerate subadult or older subordinate bulls. Females, along with their calves, live within the territories of about six to seven dominant males. However, black rhinoceroses are typically solitary.
Tapirs are generally solitary or live in pairs, but little is known about their territorial behavior.
Equids communicate with their conspecifics through vocal calls and facial expressions. Zebras emit around six different sounds, including the whinny of two- or three-syllabic ‘ha’ sounds that help maintain cohesion among group members. Other sounds include alarm snorts and squeals expressing pain or fear.
They also exhibit a variety of facial expressions. For instance, they open their mouths and hold their ears back to threaten others while they droop their ears in submission. To greet other members of their groups, they open their mouths and hold their ears up.
Tapirs and rhinoceroses also produce a wide range of vocalizations, such as snorts, bellows, and squeals.
Equids, being less armored than other ungulates, primarily fight when competing for mares in estrus. Males engage in behaviors such as circling, neck-fighting, biting, and kicking. In contrast, rhinoceroses, equipped with horns, charge their opponents and often accompany this with vocal threats.
Many of these ungulates are long-lived, with rhinoceroses surviving up to 50 years, especially in captivity.
Equids usually engage in simple courtship rituals; however, in some, like the true ass, these rituals are quite elaborate. Males typically detect females in estrus by tasting their urine. They dominate the females through chasing, biting, and kicking.
In wild horses and zebras, the stallions often groom the females before mounting them. These mares keep their legs apart and slightly lift their tails, making a distinct facial expression called the ‘mating face.’
Tapirs and rhinoceroses undergo relatively complex courtship rituals. After chasing the female, the male engages in a low-intensity fight with her, following which he lays his head on the female’s rump and mounts her to copulate.
They have a long gestation period of about 330 to 500 days. Rhinos have the longest gestation among this group, with an average of 15 to 17 months, whereas in equids and tapirs, it is almost a year.
At the end of this phase, these ungulates typically give birth to a single offspring. The newborns are precocial and relatively independent; for example, young horses start following their mothers within a few hours of birth. Tapirs and horses generally reach sexual maturity between two to four years of age, while rhinoceroses take longer, maturing around eight to ten years.
Apart from the animals that are domesticated, most odd-toed ungulates in the wild are declining owing to hunting and habitat destruction. According to the IUCN Red List (2012), the African wild ass, the Javan, the Sumatran, and the black rhinoceroses are Critically Endangered (CR). Some, like the mountain tapir, the Central American tapir, the Malayan tapir, and Grévy’s zebra, are Endangered (EN), whereas the Indian rhinoceros, the mountain zebra, and the South American tapir are Vulnerable (VU). The Near-Threatened (NT) species include the onager, the plains zebra, and the white rhinoceros.
Przewalski’s horse, once wiped out in the wild, has been recently revived in numbers, but the quagga is completely extinct.
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