The mammalian order Lagomorpha represents a group of animals that includes rabbits, hares, and pikas. Though they are often mistaken for rodents due to their size and gnawing behavior, lagomorphs possess a unique set of anatomical and physiological features that warrant their classification as a separate order. These herbivores share a common ancestry with rodents within the superorder Glires, yet their evolutionary path diverged long ago. The defining characteristics of lagomorphs are centered on adaptations for processing a highly fibrous diet, structural differences in their dentition, and distinct reproductive strategies.
Classification and Diversity
The order Lagomorpha is divided into two living families: Leporidae (rabbits and hares) and Ochotonidae (pikas). These families include over 100 species distributed across nearly every continent, inhabiting diverse terrestrial habitats.
Rabbits and hares are distinguished by reproductive development and shelter use. Rabbits are born altricial; their young (kits) are helpless, lacking fur and having closed eyes. They utilize underground burrows for nesting. Hares produce precocial young (leverets), which are born fully furred, eyes open, and capable of movement shortly after birth. Hares rarely dig burrows, relying instead on above-ground depressions called forms. Pikas are the smallest lagomorphs, characterized by short, rounded ears and no visible tail, typically inhabiting mountainous or rocky areas.
Defining Anatomical Characteristics
The most significant anatomical feature separating lagomorphs from all other mammals is their distinctive dental arrangement. Unlike the single pair of upper incisors found in rodents, lagomorphs possess two pairs of upper incisors. The primary pair of large, chisel-like teeth is backed by a second, smaller pair of peg teeth, which are hidden from view.
The presence of four upper incisors defines the order, historically leading to the name Duplicidentata (“double-toothed”). Both the incisors and the cheek teeth are hypselodont, meaning they grow continuously throughout life. This is a necessary adaptation for wearing down tough, abrasive vegetation. The absence of canine teeth creates a space called the diastema, which allows lagomorphs to manipulate food with their lips and cheeks before it reaches the molars.
The lagomorph skull reflects specialized herbivory, featuring characteristic fenestration (a lattice-like pattern of openings in the bone of the rostrum). During chewing, the lower jaw movement is predominantly lateral, or side-to-side, which facilitates the grinding of plant matter. Limb structure varies; hares have long, powerful hind limbs adapted for speed, while pikas have shorter limbs suited for scampering over rocks. All lagomorphs share the trait of having paws completely covered in fur, lacking the distinct pads found in many other terrestrial mammals.
Specialized Digestive Process
Lagomorphs employ a specialized digestive strategy known as cecotrophy to maximize nutrient absorption from their high-fiber diet. As hind-gut fermenters, they rely on a large cecum, a pouch connected to the junction of the small and large intestines, which acts as a fermentation vat. The cecum houses dense populations of microorganisms that break down cellulose and other structural carbohydrates indigestible by mammalian enzymes.
The digestive tract has an internal sorting mechanism, largely controlled by the fusus coli, that separates large, indigestible fiber particles from smaller, more fermentable ones. The larger fibers are quickly passed through the colon and expelled as hard, dry fecal pellets that are discarded. The smaller particles, however, are shunted back to the cecum for prolonged microbial action.
This microbial fermentation produces short-chain fatty acids, B vitamins, and microbial protein. These nutrients are packaged into a soft, nutrient-rich pellet called a cecotrope. Cecotropes are encased in mucus and expelled directly from the anus, where they are immediately re-ingested without chewing. Re-ingestion allows the stomach acid and small intestine to break down the microbial protein and absorb the newly synthesized nutrients, providing a second opportunity to extract maximum nutrition from low-quality forage.