When we think of hoofed animals, horses, zebras, and rhinos often come to mind. These fascinating creatures belong to a group known as Perissodactyla, or odd-toed ungulates.
While less diverse today than in the past, Perissodactyla includes some of the most iconic land mammals on Earth. In this post, we’ll explore what makes these animals unique, from their evolutionary history to their physical characteristics, behavior, and conservation challenges.
What are Perissodactyla?
The word Perissodactyla comes from Greek, meaning “odd-toed,” which is a defining feature of this group of hoofed mammals. While most animals walk on an even number of toes (like cows and deer), Perissodactyla walk on an odd number, typically one or three.
This unique structure distinguishes them from their close relatives, the Artiodactyla (even-toed ungulates), which include animals like cattle, sheep, and goats. Odd-toed ungulates rely heavily on their central toe for walking and bearing weight, a feature known as mesaxonic.
Perissodactyla can be broken down into three main families:
- Equidae: Horses, zebras, and asses.
- Rhinocerotidae: Rhinos.
- Tapiridae: Tapirs.
While the Perissodactyla group was once far more diverse, today only 17 species remain, spread across these three families.
A Rich Evolutionary History
Perissodactyla were not always so limited in number. Millions of years ago, they were among the dominant groups of large mammals, roaming vast landscapes during the Tertiary period. This era was marked by a much greater diversity of species, including some of the largest land mammals ever to exist. One of the most notable examples is the Indricotherium (formerly known as Baluchitherium), which stood over 16 feet tall at the shoulder and weighed up to 30,000 kg. This gigantic rhino-like creature dwarfed today’s largest land animals, including modern rhinos and elephants.
However, much of this diversity was lost during the late Eocene and Oligocene periods. Competition with the emerging group of Artiodactyla (even-toed ungulates) and changing environmental conditions led to the extinction of many odd-toed ungulate species. Today, the remaining species are mainly found in parts of Africa, Asia, and the Americas.
To read more about ungulates as a whole and how their hoofed feet influence their way of life, you can explore our detailed guide on ungulates here.
The Modern Families of Perissodactyla
The three remaining families of Perissodactyla include some of the most familiar animals, each with distinct characteristics:
1. Equidae (Horses, Zebras, and Asses)
The Equidae family is known for its strong, fast animals that have played a central role in human civilization. Horses, zebras, and asses are part of this family, and they are recognized for walking on a single, large hoofed toe. Horses, in particular, have been domesticated for thousands of years and continue to serve humans as transportation, companions, and work animals. Zebras, with their striking black and white stripes, live in the grasslands of Africa and rely on their speed and agility to avoid predators. Asses or donkeys are hardy creatures that can survive in harsh environments.
All members of the Equidae family share certain traits, including a long skull, large teeth adapted for grazing, and a single hoof on each foot.
2. Rhinocerotidae (Rhinos)
The Rhinocerotidae family, or rhinos, is one of the most endangered groups of mammals. There are five living species of rhinos, divided between Africa and Asia. Rhinos are famous for their large size, thick skin, and distinctive horns. Unlike the horns of many other animals, a rhino’s horn is made entirely of keratin (the same material as human hair and nails) and lacks a bony core.
Rhinos are known for their solitary nature and spend much of their time grazing on grasses, leaves, and shrubs. While they may seem slow due to their massive size, they are capable of short bursts of speed when threatened. Sadly, all five species of rhinos are in danger of extinction due to poaching for their horns and habitat destruction.
3. Tapiridae (Tapirs)
Tapirs are perhaps the least well-known members of the Perissodactyla group. They are large, herbivorous mammals with short, prehensile snouts that they use to grab leaves and branches. Tapirs live in Central and South America and parts of Southeast Asia. These nocturnal animals prefer dense forests where they can blend in with their surroundings.
Tapirs are shy, solitary creatures and are currently threatened by habitat loss and hunting. Their unique appearance and behavior make them a fascinating but vulnerable species within the Perissodactyla order.
Unique Physical and Anatomical Features
Perissodactyla have several distinctive physical features that set them apart from other ungulates:
- Odd-toed structure: The defining trait of Perissodactyla is their odd number of toes. Horses, for example, walk on a single large toe, while rhinos and tapirs walk on three.
- Hoof formation: Their hoofs are enlarged toenails that protect their toes and provide stability on various terrains.
- Simple stomachs: Unlike their even-toed relatives, Perissodactyla do not have a complex, multi-chambered stomach. Instead, they rely on an enlarged cecum for digesting plant material. The cecum contains bacteria that help break down cellulose, but the process is less efficient than the fermentation chambers found in animals like cows or deer.
- Cranial appendages: Rhinos are known for their horns, which, unlike the antlers of deer, are made entirely of keratin and grow continuously.
Habitat and Distribution
Today, Perissodactyla species are found in parts of Africa, Asia, and the Americas. Horses and asses thrive in open plains and grasslands, while rhinos and tapirs prefer forests and savannas. These animals have adapted to a variety of habitats, but many of them are now confined to protected areas due to habitat loss and human expansion.
Diet and Foraging Behavior
Most Perissodactyla species are herbivores, relying on grasses, leaves, and shrubs for nutrition. Horses and zebras are grazers, meaning they feed primarily on grasses, while tapirs and rhinos are browsers, feeding on a variety of leaves, twigs, and fruit.
Because they lack the multi-chambered stomachs of ruminants (like cows and sheep), Perissodactyla species rely on their enlarged cecum for digestion. This makes them less efficient at extracting nutrients from plant material, so they must consume large quantities of food to meet their energy needs.
Conservation Challenges
Sadly, many Perissodactyla species are now endangered. The rhino is perhaps the most well-known example, with all five species facing severe threats from poaching and habitat loss. Rhinos are hunted for their horns, which are prized in some cultures for their supposed medicinal properties.
Tapirs, too, are at risk, primarily due to deforestation and human encroachment on their natural habitats. Conservation efforts are underway to protect these species, but challenges remain, particularly in regions where wildlife conservation conflicts with agricultural expansion and poaching.
Ecological Importance
Perissodactyla species play an important role in their ecosystems. As large herbivores, they help maintain plant communities and influence the distribution of vegetation.
Their foraging habits can impact the landscape, preventing overgrowth in some areas and promoting the growth of certain plant species in others. In addition, animals like zebras and tapirs help disperse seeds, promoting plant diversity in their habitats.
For a broader look at the ecological roles of other hoofed mammals, check out our in-depth article on ungulates here.
Conclusion
Perissodactyla may not be as diverse as they once were, but they continue to captivate us with their unique characteristics and vital roles in their ecosystems. From the speed of zebras to the size and strength of rhinos, these odd-toed ungulates are remarkable in their own right. However, they face significant challenges in the modern world, and without continued conservation efforts, we risk losing these incredible animals forever.
