Nuttall’s cottontail (nuttallii subspecies) COSEWIC assessment and status report: chapter 6

Biology

Because of possible life history differences in subspecies, biological information presented here is mostly limited to studies done on S. n. nuttallii. Only two studies have been done on the Canadian population. They assessed general distribution, habitat, and population ecology (Sullivan et al. 1989; Carter et al. 1993). Data on reproduction and food habits are based on research done in Oregon and Idaho. Remarkably, there are no data available for movements by this species.

Life cycle and reproduction

S. n. nuttallii is herbivorous. Studies in Oregon and Idaho (Johnson and Hansen 1979; MacCraken and Hansen 1982) have shown that it feeds mostly on grasses such as Wheatgrasses (Agropyron sp.), Needle-and Thread (Stipa sp.), and Cheatgrass (Bromus tectorum). Forbs and shrubs such Big Sage and Common Juniper are also consumed.

In Oregon, the breeding season extends from January to late July (Powers and Verts 1971). Pregnant females were found from February to July. Breeding among females appeared to be synchronous, with females breeding immediately postpartum. Consequently, the summer population contains several well-defined cohorts of young from different litters. In Oregon, females produced three to four litters, and a few produced five (Powers and Verts 1971). Mean litter size was 4.6 based on embryo counts with litters smaller early and late in breeding season. Females rarely reach sexual maturity in the summer of their birth and breeding in young-of-the-year S . n. nuttallii is rare. The generation time is about one year (McKay and Verts 1978). Reproductive data for the Canadian population are scanty. The breeding season probably extends from March to July. Cowan and Guiguet (1956) reported litter sizes of only two with females producing two or three litters per year, but they did not give the source of their data.

S. nuttallii are solitary–adults only interact during the breeding season (Orr 1940; Verts and Gehman 1991). Sullivan et al. (1989) found that sex ratios (proportion of males) varied from year-to-year with estimates of 0.57, 0.44, and 0.16 for 1984-86 on his study grids in the Okanagan Valley. Survival rates were higher for females than males. In Oregon, mortality in juvenile cohorts was high (40-80%) and was related to precipitation and the quality of summer forage (McKay and Verts 1978; Hundertmark 1982). Increased mortality in autumn and early winter was associated with low temperatures. Individuals may live up to four years (Verts and Carraway 1998).

Predation

Predation has not been studied in the Canadian population. Potential predators in the southern Okanagan-Similkameen valleys include: Great Horned Owl (Bubo virginiana), Red-tailed Hawk (Buteo jamaicensis), Golden Eagle (Aquilachrysaetos), Badger (Taxidea taxus), Bobcat (Lynx rufus), and Coyote (Canis latrans).

Physiology

Physiological requirements and adaptations of this species have not been studied. To assist with digesting cellulose, S. nuttallii re-ingests its faeces, an adaptation of most hares and rabbits. No research has been done on this species’ water requirements in arid environments, but Verts et al. (1984) suggested that water availability was a critical factor for survival of young.

Dispersal/migration

There are no estimates of home range size for S. nuttallii and no data on dispersal or long distance movements. The general pattern for most cottontail species is that males have larger home ranges than females and home range size increases during the breeding season.

Interspecific interactions

Two species of Leporidae were sympatric with S. nuttallii in British Columbia: L. townsendii and L. americanus. L. townsendii occupied similar shrub steppe habitats as S. nuttallii and had a similar diet, but appears to be extirpated from British Columbia (Nagorsen 2005). It is possible that declining populations of this large hare in the 1940s and 1950s contributed to the successful range expansion of S. nuttallii into British Columbia. L. americanus is found throughout British Columbia and is broadly sympatric with S. nuttallii. However, L. americanus is associated with forested habitats above the elevational range of S. nuttallii. Competition between these two species is unlikely because of habitat segregation.

Adaptability

Verts and Carraway (1998) noted several adaptations for coping with arid environments: tree climbing, crepuscular or nocturnal activity, and a solitary life-style. One of the more striking adaptations of S. nuttallii is its arboreal activity up to 3 m above ground. Verts et al. (1984) concluded that this rabbit climbed juniper shrubs during summer droughtsto obtain water droplets that condensed on the boughs and to consume the succulent foliage.

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