Reproduction in Eastern Cottontail Rabbits in Western Oregon

Abstract

Reproductive tracts of 486 eastern cottontail rabbits, Sylvilagus floridanus, collected between 1 January 1968 and 30 June 1969 from an introduced population in Benton Co., Oregon, were excised and examined. Sex ratios of adults, juveniles and embryos that were gestated 20 days or more were not significantly different from 1: 1. Male eottontails appeared to be in breeding condition from December to the following August. Some juvenile males attained breeding condition during the breeding season in which they were born. Adult females began to breed in January in 1968 but not until February in 1969; inclement weather and deep snow' were believed to retard breeding activities in 1969. Adult eastern cottontails produced up to eight litters in 1968; some adults either ceased to breed or failed to produce one or more litters in June and July. A mean of 5.58 + 0.14 embryos was implanted per pregnant female, and 5.10 + 0.15 appeared to be viable at the time the females were collected. Resorption of embryos was not significantly greater among females which had caseous abscesses from which Staphylococcus was isolated. More than half of 21 juvenile females older than 75 days when collected during the breeding season in 1969 had produced at least one litter. Breeding apparently ceased about the end of August 1968. Based on ages of cottontails collected during the first 5 months of 1968 and 1969, the breeding season terminated earlier in 1967 than in 1968; drought may have been responsible for the early cessation of breeding activities in 1967. Productivity of the introduced population was at least equal to that reported for the most productive populations of eastern cottontails sampled in the native range of the species.