The pygmy rabbit (Sylvilagus idahoensis) is a rabbit species native to the United States. It is one of the only two native rabbit species in North America to dig its own burrow (the other is the volcano rabbit). The pygmy rabbit differs significantly from species within either the Lepus (hare) or Sylvilagus (cottontail) genera, and was once considered to be within the genus Brachylagus, which includes one extinct species. One isolated population, the Columbia Basin pygmy rabbit, is listed as an endangered species by the U.S. Federal government, though the International Union for Conservation of Nature lists the species as lower risk.
Taxonomy The pygmy rabbit was first described by Clinton Hart Merriam in 1891, as Lepus idahoensis; the type specimen, in winter pelage, had been collected in the Pahsimeroi Valley the previous autumn. In 1900, Gerrit Smith Miller Jr. erected the new subgenus Brachylagus for the pygmy rabbit. Historically, the pygmy rabbit has also been placed in the genera Microlagus and Sylvilagus. A 2022 study of ultraconserved elements within the lagomorphs recommended that Brachylagus idahoensis be reclassified as a member of Sylvilagus.
Description The pygmy rabbit is the world's smallest leporid. with adults weighing between 375 and 500 grams (0.827 and 1.102 lb), and having a body length between 23.5 and 29.5 centimeters (9.3 and 11.6 in); females are slightly larger than males. The pygmy rabbit is distinguishable from other leporids by its small size, uniquely rounded ears, gray color, small hind legs, and lack of white fuzzy fur. Like other rabbits and hares, it has a dental formula of 2.0.3.31.0.2.3 × 2 = 28—two pairs of upper and one pair of lower incisors, no canines, three upper and two lower premolars on each side, and three upper and lower molars on either side of the jaw.
Distribution The range of the pygmy rabbit includes most of the Great Basin and some of the adjacent intermountain areas of western North America. Pygmy rabbits are found in southwestern Montana from the extreme southwest corner near the Idaho border north to Dillon and Bannack in Beaverhead County. Distribution continues west to southern Idaho and southern Oregon, and south to northern Utah, northern Nevada, and north-eastern California. An isolated population occurs in east-central Washington, and another in Wyoming. The pygmy rabbit has a range that overlaps those of the desert (Sylvilagus audubonii) and mountain cottontails (Sylvilagus nuttallii), and it lives in sympatry with both species. The elevational range of pygmy rabbits in Nevada extends from 1,370–2,135 meters (4,495–7,005 ft) and in California from 1,520–1,615 meters (4,987–5,299 ft). The last male purebred Columbia Basin pygmy rabbit, found only in the Columbia Basin of Washington state, died 30 March 2006, at the Oregon Zoo in Portland. The last purebred female died in 2008. A crossbreeding program conducted by the Oregon Zoo, Washington State University, and Northwest Trek is attempting to preserve the genetic line by breeding surviving females with the Idaho pygmy rabbit.
Lifecycle The breeding season of pygmy rabbits is very short. In Idaho it lasts from March through May; in Utah, from February through March. The gestation period of pygmy rabbits is unknown. It is between 27 and 30 days in various species of cottontails (Sylvilagus spp.). An average of six young are born per litter and a maximum of three litters are produced per year. In Idaho the third litter is generally produced in June. It is unlikely that litters are produced in the fall. The growth rates of juveniles are dependent on the date of birth. Young from early litters grow larger due to a longer developmental period prior to their first winter. Pygmy rabbits are capable of breeding when they are about 1 year old. The mortality of adults is highest in late winter and early spring. Green and Flinders reported in 1980 a maximum estimated annual adult mortality of 88% in Idaho. Juvenile mortality was highest from birth to 5 weeks of age. Pygmy rabbits may be active at any time of day; however, they are generally most active at dusk and dawn. They usually rest near or inside their burrows during midday.
Habitat Pygmy rabbits are normally found in areas on deep soils with tall, dense sagebrush which they use for shelter and food. Individual sagebrush plants in areas inhabited by pygmy rabbits are often 6 feet (1.8 m) or more in height. Extensive, well-used runways interlace the sage thickets and provide travel and escape routes. Dense stands of big sagebrush along streams, roads, and fencerows provide dispersal corridors for pygmy rabbits. The pygmy rabbit is the only leporid native to North America that digs burrows. Juveniles use burrows more than other age groups. Early reproductive activities of adults may be concentrated at burrows. When pygmy rabbits can utilize sagebrush cover, burrow use is decreased. Pygmy rabbits use burrows more in the winter for thermal cover than at other times of the year. Burrows are usually located on slopes at the base of sagebrush plants, and face north to east. Tunnels widen below the surface, forming chambers, and extend to a maximum depth of about 1 meter (3.3 ft). Burrows typically have 4 or 5 entrances but may have as few as 2 or as many as 10. In Oregon, pygmy rabbits inhabited areas where soils were significantly deeper and looser than soils at adjacent sites. Site selection was probably related to ease of excavation of burrows. In areas where soil is shallow pygmy rabbits live in holes among volcanic rocks, in stone walls, around abandoned buildings, and in burrows made by American badgers and yellow-bellied marmots. Some researchers have found that pygmy rabbits never venture farther than 60 feet (18 m) from their burrows. However, Bradfield observed that pygmy rabbits range up to 100 meters (330 ft) from their burrows. Some areas inhabited by pygmy rabbits are covered with several feet of snow for up to 2 or more months during the winter. During periods when the snow has covered most of the sagebrush, pygmy rabbits tunnel beneath the snow to find food. Snow tunnels are approximately the same height and width as burrows. They are quite extensive and extend from one sagebrush to another. Above ground movement during the winter months is restricted to these tunnel systems.
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