A Synopsis of the North American Lagomorpha

by E. Raymond Hall

Hare is a name applied to any lagomorph whose young are born fully haired, with the eyes open, and able to run about a few minutes after birth. The young are born in the open, not in a nest. All of the species of the genus Lepus are hares. The species of leporids of all genera other than Lepus, in North America at least, are rabbits. Their young are born naked, blind, and helpless, in a nest especially built for them and lined with fur. Considering the degree of development of the young at birth, the gestation periods are about what a person would expect: 26 to 30 days in Sylvilagus and 36 to 47 days in Lepus (see Severaid, 1950:356-357). Vernacular names are misleading because the names jack rabbit and snowshoe rabbit are applied to hares; also, Belgian hare is a name applied to a rabbit (genus Oryctolagus) that is commonly bred in captivity. There are many domestic strains and varieties of Oryctolagus and the animals are second only to poultry in some areas as a protein food for man. Also, the pelts are sold as a source of felt and many of the skins are dyed and processed for making fur coats and other fur-pieces that appear on the market under names not readily associated with rabbit.

Rabbits and hares are crepuscular and possibly more nocturnal than diurnal. So far as I know they do not store food as do their diurnal relatives, the pikas. Some leporids, however, have an unusual, and possibly unique, method of processing food: Two types of vegetable pellets are expelled from the anal opening of the digestive tract; the dark brownish pellets, from which the nutriments have been extracted, are feces, but the greenish pellets seem to be only slightly predigested foods which are re-eaten. Southern (1942:553), among others, has written about this. This system functionally resembles that in the ruminants where a cud of vegetation is returned to the mouth, from one part of the stomach, to be re-chewed and finally swallowed.

Because the causative organism of a disease that decimates dense populations of small mammals, and some other kinds of vertebrates, was isolated first in leporids, this disease, tularemia, is more associated in the popular mind with rabbits than with other kinds of mammals. Actually, many kinds of mammals are quite as likely to have tularemia as are rabbits. Now that streptomycin is available, cases of tularemia in persons are easily cured.

Key to Species of the Genera Sylvilagus and Romerolagus

• 1. Antorbital extension of supraorbital process more than 1/2 length of posterior extension; first upper cheek-tooth with only one re-entrant angle on anterior face; re-entrant angle of second upper cheek-tooth not crenate
• Sylvilagus idahoensis, p. [139]
• 1´. Antorbital extension of supraorbital process less than 1/2 of posterior extension or entirely absent; first upper cheek-tooth with more than one (usually 3) re-entrant angles on anterior face; re-entrant angle of second upper cheek-tooth crenate.
• 2. Anterior extension of supraorbital process absent (or if a point is barely indicated, then 5/6 or all of posterior process fused to braincase).
• 3. Tympanic bulla smaller than foramen magnum; hind foot more than 74; geographic range wholly in United States.
• 4. Ear more than 58 from notch in dried skin; basilar length of skull more than 63
• Sylvilagus aquaticus, p. [166]
• 4´. Ear less than 58 from notch in dried skin; basilar length of skull less than 63.
• 5. Underside of tail white; posterior extension of supraorbital process tapering to a slender point, this point free of braincase or barely touching it and leaving a slit or long foramen
• Sylvilagus transitionalis, p. [160]
• 5´. Underside of tail brown or gray; posterior extension of supraorbital process always fused to skull, usually for entire length but in occasional specimens there is small foramen at middle of posterior extension of supraorbital process
• Sylvilagus palustris, p. [147]
• 3´. Tympanic bulla as large as foramen magnum; hind foot less than 74; geographic range limited to southern edge of Mexican tableland at high elevations
• Romerolagus diazi, p. [138]
• 2´. Anterior extension of supraorbital process present, and posterior extension free of braincase or leaving a slit between the process and braincase.
• 6. Tympanic bullae large (see [fig. 26]).
• Sylvilagus audubonii, p. [162]
• 6´. Tympanic bullae small (see figs. [23],[ 25 and 27]).
• 7. Restricted to Pacific coastal strip from Columbia River south to tip of Baja California, west of Sierra Nevada-Cascade Mountain Chain; hind foot less than 81.
• Sylvilagus bachmani and S. mansuetus, pp. [143], [147]
• 7´. East of the Pacific coastal strip mentioned in 7; hind foot usually more than 81.
• 8. If north of United States-Mexican boundary:
• 9. In Arizona, New Mexico and southern Colorado posterior extension of supraorbital process free of braincase, and supraoccipital shield posteriorly pointed; from central Colorado north into Canada diameter of external auditory meatus more than crown length of last three cheek-teeth
• Sylvilagus nuttallii, p. [161]
• 9´. In Arizona, New Mexico and southeastern Colorado posterior extension of supraorbital process of frontal with its tip against, or fused to, braincase, and supraoccipital shield posteriorly truncate or notched; from central Colorado north into Canada, diameter of external auditory meatus less than crown length of last three cheek-teeth
• Sylvilagus floridanus, p. [154]
• 8´. If south of United States-Mexican boundary:
• 10. Geographic range restricted to Tres Marias Islands
• Sylvilagus graysoni, p. [169]
• 10´. Geographic range not including Tres Marias Islands.
• 11. Underside of tail dingy gray or buffy (not white).
• 12. Tail short (less than 30) and brown like rump; ear from notch (dry) less than 53; interorbital breadth less than 16.
• Sylvilagus brasiliensis, p. [141]
• 12´. Tail of moderate length (more than 30) and dingy gray; ear from notch (dry) more than 53; interorbital breadth more than 16
• Sylvilagus insonus, p. [168]
• 11´. Underside of tail distinctly white.
• 13. Total length more than 476; ear from notch (dry) more than 64; interorbital breadth usually more than 19.3; geographic range, southwestern Mexico north of the Isthmus of Tehuantepec.
• Sylvilagus cunicularius, p. [169]
• 13´. Total length less than 476; ear from notch (dry) less than 64; interorbital breadth usually less than 19.3; geographic range, Canada to Panamá
• Sylvilagus floridanus, p. [154]

Genus Romerolagus Merriam—Volcano Rabbit

1896. Romerolagus Merriam, Proc. Biol. Soc. Washington, 10:173, December 29. Type, Romerolagus nelsoni Merriam = Lepus diazi Diaz.

Total length 300 to 311; tail rudimentary; hind foot, 52; ear from notch (dry), 36; upper parts grizzled buffy brown or dull cinnamon brown; underparts dingy gray; anterior projection of supraorbital process absent; jugal projecting posteriorly past squamosal root of zygomatic arch more than half way to external auditory meatus. The two cranial characters mentioned are resemblances to pikas although the skull otherwise resembles that of the true rabbits. The genus contains only the one living species.

Living in well defined runways in the dense sacoton grass, these small rabbits are mainly nocturnal and crepuscular, but sometimes are active by day, especially in cloudy weather in the period of mating.