Lagomorpha

Lagomorpha (from the Ancient Greek lagōs, "hare," and morphē, "form") is an order of herbivorous placental mammals comprising approximately 110 extant species divided cleanly between two living families: Leporidae (~75 species of rabbits and hares) and Ochotonidae (~35 species of alpine and steppe pikas, genus Ochotona). Together with their sister order Rodentia (rodents), Lagomorpha forms the cohort Glires within the superorder Euarchontoglires, having diverged from rodents during the latest Cretaceous or early Paleocene (~65–62 million years ago) in Asia (Mimotona).

Although historically classified as a suborder inside Rodentia ("Duplicidentata") until James Gidley elevated them to a separate order in 1912, lagomorphs are distinguished from rodents by four universal anatomical and physiological synapomorphies:

1. Duplicidentate Dentition ("Four Upper Incisors"): Whereas rodents (Simplicidentata) possess only a single pair of chisel-like upper incisors (I²), all lagomorphs possess a second pair of small, peg-like upper incisors (I³) positioned directly behind (lingual to) the primary chiseling incisors, against which the lower incisors occlude at rest.
2. Fenestrated (Lace-Like) Maxillary Bone: The lateral wall of the maxilla rostral to the orbit is perforated into a lightweight, sponge-like lattice of bony struts (maxillary fenestration) that reduces cranial mass and rotational inertia during high-speed bounding and sharp zigzag turns.
3. Double-Layered Enamel and Transverse Cheek-Tooth Chewing: Unlike rodent incisors (which bear enamel solely on the anterior labial face), lagomorph incisors bear enamel on both faces (thicker anteriorly) and have a longitudinal labial groove; their rootless (hypselodont) premolars and molars are wider than the lower jaw and chew via transverse (side-to-side) ectolophid shearing of one jaw side at a time.
4. Obligate Cecotrophy via the Fusus Coli: All lagomorphs are small-bodied hindgut fermenters that separate fine, bacteria-rich digesta from coarse indigestible fiber in the proximal colon and excrete nutrient-dense, mucin-coated cecotropes ("night feces") that are re-ingested directly from the anus.

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Order Lagomorpha (Clade Glires; Early Paleocene Asian origin, ~62 Ma) ├── †Stem Duplicidentata (*Mimotona*, *Gomphos*; Paleocene–Eocene of Mongolia & China) ├── †Family Prolagidae (*Prolagus sardus*: Sardinian pika, survived on Mediterranean islands until ~1774 CE!) │ ├── Family Ochotonidae (Pikas; Single Extant Genus: *Ochotona*, ~35 spp.; Holarctic talus slopes & Asian steppes) │ └── *Ochotona princeps* (American pika), *O. curzoniae* (Plateau pika), *O. dauurica*, *O. collaris* │ └── Family Leporidae (Rabbits and Hares; 11 Extant Genera, ~75 spp.; Global except Antarctica) ├── Basal Relict Lineages (*Nesolagus* [Sumatran/Annamite striped rabbits], *Pronolagus* [red rock hares], *Romerolagus diazi* [volcano rabbit]) ├── True Hares & Jackrabbits — Genus *Lepus* (~33 spp.: *Lepus europaeus*, *L. timidus*, *L. californicus*, *L. americanus*) └── True Rabbits (Polyphyletic grade of burrowing/altricial genera) ├── *Oryctolagus cuniculus* (European rabbit: ancestor of all domestic laboratory & livestock rabbits) ├── *Sylvilagus* (~20 spp.: New World cottontail rabbits, *S. floridanus*, *S. aquaticus*) └── *Pentalagus furnessi* (Amami rabbit), *Brachylagus idahoensis* (pygmy rabbit), *Bunolagus*, *Caprolagus* *`

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In fast-running leporids (Lepus europaeus and Lepus californicus, which reach sprint velocities of 65–72 km/h [40–45 mph] and single horizontal leaps of 3 to 6 meters), the skeleton incorporates two specialized shock-absorbing mechanisms:

1. Intracranial Hinge Joint (Cranial Kinesis): Unique among extant mammals, hares and rabbits possess an unossified, movable suture (intracranial joint) running transversely between the parietal/frontal bones anteriorly and the occipital/interparietal complex posteriorly. As demonstrated by Mariana Bramble (1989), when a galloping hare's forelimbs strike the ground during high-speed bounding, this intracranial hinge flexes slightly to dissipate peak deceleration shockwaves that would otherwise jar the brain and vestibulo-ocular visual stabilization system (whose large, laterally placed eyes provide a nearly 360° panoramic field of view).
2. Digitigrade Forelimbs and Plantigrade-to-Digitigrade Hindlimbs: The clavicle is vestigial or absent, allowing the scapula to rotate along the laterally compressed thorax to lengthen stride.

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Because small herbivores (<5 kg) have a high mass-specific basal metabolic rate relative to their gut volume, they cannot retain fibrous grass in a rumen for 40–70 hours like a cow (Artiodactyla). Lagomorphs solve this allometric constraint by accelerating indigestible lignocellulose out of the digestive tract while selectively retaining and fermenting soluble cell contents and fine particles:

1. Proximo-Colonic Haustral Washing: Digesta leaving the small intestine enters the junction of the large cecum (which holds ~40–50% of total gastrointestinal volume) and the proximal colon. Anti-peristaltic muscular contractions of the colonic haustra pump water and dissolved/fine particles (<0.3 mm, including starch, soluble protein, and bacteria) backward into the cecum for microbial fermentation into volatile fatty acids (acetate, butyrate, propionate), while central peristaltic contractions rapidly propel coarse, indigestible lignified fiber (>0.5 mm) forward to be excreted as dry, hard fecal pellets within 4 hours of feeding!
2. The Fusus Coli Pacemaker and Cecotrope Production: Once or twice daily (typically at dawn or night, regulated by adrenal aldosterone and circadian autonomic input to a thickened, densely innervated vascular muscular segment of the colon called the fusus coli), anti-peristaltic sorting pauses. Contents of the cecum are extruded into the fusus coli, which encapsulates soft clusters of fermented microbial biomass in a protective lysozyme- and mucin-rich envelope.
3. Direct Re-Ingestion (Cecotrophy) and Cardiac-Fundic Secondary Fermentation: Guided by olfactory cues from anal-gland secretions, the rabbit or pika bends down and swallows the cecotropes whole directly from the anus without chewing. Inside the cardiac fundus of the stomach, the intact mucin coat shields the cecotrope bacteria from gastric acid for up to 6 hours while endogenous bacterial α-amylase continues fermenting carbohydrates into lactic acid, before lysozyme and pepsin digest the bacterial cells in the small intestine—recovering 20–30% of the animal's daily nitrogen/protein intake and 100% of its daily vitamin B₁₂ and vitamin K requirements.

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Following the introduction of 24 wild European rabbits (Oryctolagus cuniculus) into Victoria, Australia, in 1859, the population expanded in the absence of native predators to over 600 million rabbits, denuding arid rangelands. In 1950, Australian authorities released Myxoma virus (MYXV)—a Leporipoxvirus endemic to South American tapeti (Sylvilagus brasiliensis), in which it causes only benign cutaneous fibromas, but which proved 99.8% lethal (Grade I virulence, killing within 10 days) in naive Oryctolagus cuniculus.

As documented by Frank Fenner and later by genomic sequencing of archival viral and rabbit strains (Kerr et al., 2012; Alves et al., 2019), this release became the textbook empirical model of host–pathogen coevolution:

• Viral Attenuation to Intermediate Virulence: Because MYXV is mechanically transmitted on the mouthparts of biting mosquitoes and fleas—which only feed on living rabbits—hypervirulent Grade I strains that killed their hosts in <10 days had a shorter transmission window than attenuated Grade III strains (70–90% case fatality, 20–30 day survival), which rapidly displaced Grade I strains across the field.
• Polygenic Host Resistance and Immunosuppressive Arms Race: Simultaneously, surviving Australian, British, and French rabbit populations underwent parallel positive selection on standing genetic variation across innate immune genes (interferon-alpha IFNA, MHC Class I, CD200R, and antiviral FCRL3). In response to heightened rabbit innate immunity in the 1990s–2000s, field MYXV strains evolved a novel immunosuppressive syndrome (causing acute pulmonary edema and neutropenic bacterial sepsis rather than classic cutaneous myxomatosis), joined after 1995 by a second biocontrol agent, Rabbit Hemorrhagic Disease Virus (RHDV, a Lagovirus calicivirus).

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1. Asher, R. J., et al. (2005). Stem Lagomorpha and the antiquity of Glires. Science, 307(5712), 1091–1094. Google Scholar
2. Bramble, D. M. (1989). Cranial specialization and locomotor habit in the Lagomorpha. American Zoologist, 29(1), 303–317. Google Scholar
3. Hirakawa, H. (2001). Coprophagy in leporids and other mammalian herbivores. Mammal Review, 31(1), 61–80. Google Scholar
4. Alves, J. M., et al. (2019). Parallel adaptation of rabbit populations to myxoma virus. Science, 363(6433), 1319–1326. Google Scholar

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• Rodentia
• Mammalia
• Fermentation
• Microbiome
• Natural Selection