The systematics of the lagomorphs of the northern Chihuahuan Desert is not well understood, and most species other than Sylvilagus audubonii are difficult to surely identify. Until recently, several probable species were all placed under Sylvilagus floridanus. Ruedas (1998) separated out populations often assigned to S. floridanus from the Guadalupe, Davis, and other montane situations in Trans-Pecos Texas as Sylvilagus robustus; he further indicated that S. floridanus cognatus and S. floridanus holzneri are species separate from S. floridanus. Frey (2004) treated these two populations as separate species. In view of the uncertainties surrounding these two species, they are not included in the key. Both are inhabitants of montane woodlands. Sylvilagus cognatus occurs in the central New Mexican mountains (Mount Taylor and the Sandia Mountains south to the San Mateo and Sacramento mountains and possibly onto the Mogollon Plateau (Frey 2004). Sylvilagus holzneri occurs in the mountains of extreme southwestern New Mexico north to the Mogollon Plateau; the systematics of cottontails on the Mogollon Plateau is unknown, but this species may be involved (Frey 2005).
1 Ear pinna as broad as long; no visible tail; premolars 2/2: Ochotonidae, Ochotona princeps.
1' Ear pinna very long; tail short but obvious; premolars 3/2: Leporidae, 2
2 Interparietal bone fused to parietals in adults; length of hindfoot usually 130 mm or more: genus Lepus, 3
2' Interparietal bone distinct; hindfoot usually 105 mm or less: genus Sylvilagus, 6
3 No anterior projection on supraorbital process, or if present, very small; skull length less than 80 mm; ear less than 75 mm: Lepus americanus.
3' Prominent anterior projection on supraorbital process; skull length more than 90 mm; ear more than 100 mm: 4
4 Whitish ventral color meeting dark dorsal color along a distinct line well up on flanks; ears without distinct black tip: L. callotis.
4' Light ventral coloration gradually blending into color of dorsum; ears with black tip: 5
5 Top of tail with black stripe that extends onto the back: L. californicus.
5' Top of tail without dark dorsal stripe: L. townsendii.
6 Hind foot usually 92 mm or less; bullar length usually 11.5 mm or more; hind foot/ear ratio 1.50 or less; ear/total-length ratio 0.14 or more; bullar length/basilar-length ratio 0.22 or more; sides of interpterygoid fossa slightly convex with slight shelf: Sylvilagus audubonii.
6' Hind foot usually 92 or more; bullar length 12.3 mm or less; hind-foot/ear ratio 1.35 or more; ear/total-length ratio 0.18 or less; bullar length/basilar-length ratio 0.23 or less; sides of interpterygoid fossa straight, little or no shelf: 7
7 Supraoccipital shield convex or pointed posteriorly; anterior segment of parietal-squamosal suture straight; basilar length usually 57 mm or less; mean ear length 65 mm or less; mean hind-foot/ear ratio, 1.48 or more; bullar/basilar-length ratio usually 0.21 or more: Sylvilagus nuttallii.
7' Supraoccipital shield truncate or emarginate; anterior segment of parietal-squamosal suture convoluted; basilar length usually 52 mm or more; mean ear length 65 mm or more: 8
8 Hind foot near 100 mm; ear near 70 mm; deepest (central) reentrant to P2 with complex and highly convoluted posterior wall; P3 with anterior wall of the deep reentrant of thin enamel (or if thick enamel present, discontinuous or continuous from anterior of tooth and not continuing throughout): Sylvilagus robustus
8' Hind foot near 90 mm; ear near 55 mm; deepest (central) reentrant to P2 with smooth enamel; P3 with anterior wall with continuous thick enamel continuing around the innermost end: *Sylvilagus floridanus.
Key modified from Findley et al. (1975).
* Sylvilagus floridanus of our region probably consists of three species (Frey 2004). If this is correct, S. holzneri occurs in mountains of southwestern New Mexico (Hidalgo, Luna, southern Grant counties) and presumably to the south in Mexico; S. cognatus occurs in the mountains of central New Mexico from Mount Taylor and the Sandia Mountains south; east of the Rio Grande it extends at least into the Sacramento Mountains and probably the Guadalupes. It's currently unclear as to which species occurs in the Mogollon highlands. S. floridanus (sensu stricto) occurs in the plains area of extreme eastern New Mexico.
Last Update: 27 Jul 2005