Rhinoceroses are ungulates, which is the descriptive term for hoofed mammals. This classification places them within a broad group of animals defined by physical traits related to their locomotion and diet. These shared anatomical adaptations allow these large herbivores to thrive across various terrestrial environments.
Defining the Ungulates
Ungulates are a diverse collection of mammals characterized by possessing hooves, which are specialized digits used to support their entire body weight. This form of walking is known as unguligrade locomotion. This grouping includes nearly all large, terrestrial herbivores, such as cattle, deer, horses, and the rhinoceros.
Most ungulates share other physical traits related to their herbivorous lifestyle. They typically lack a collarbone and have specialized limbs adapted for speed and endurance, which helps them escape predators. Their teeth feature large, grooved molars designed to efficiently grind down tough, fibrous plant matter. Their digestive systems are also equipped to handle this diet, though the exact method of fermentation differs between major subgroups.
How Rhinos Fit into Classification
Ungulates are formally divided into two distinct taxonomic orders based on the structure of their feet. Rhinoceroses belong to the order Perissodactyla, known as the odd-toed ungulates. This order also includes extant species like horses, asses, zebras, and tapirs.
The Perissodactyla classification is defined by a foot structure where the main axis of the limb passes through the third, or middle, toe. This arrangement means the third digit bears the greatest portion of the animal’s weight. The five living species of rhinoceros, which are the only remaining members of the family Rhinocerotidae, all feature this odd-toed arrangement.
Rhinoceroses have three functional toes on each foot, both front and hind, all ending in a hoof. The closest living relatives to the rhino are the tapirs, which are also three-toed on the hind feet.
The Defining Characteristic of Odd-Toed Mammals
The defining feature of the odd-toed mammals is the mesaxonic foot structure. This condition means the central, third digit is significantly larger than the others and carries the axis of the animal’s weight. This contrasts with the paraxonic foot structure found in even-toed ungulates, such as deer and cows, where weight is distributed equally between the third and fourth digits.
In the rhinoceros, the three toes on each foot are splayed slightly to support its massive body weight, with the central toe being the largest and most robust. The hooves cover only the leading edge of the toes, leaving the sole of the foot relatively soft. This configuration is adapted for supporting immense mass and navigating varied terrain, representing a successful adaptation for a large, heavy-bodied browser and grazer.