Reproduction in the Mountain Cottontail Rabbit in Oregon

Abstract

Reproductive tracts of 269 mountain cottontail rabbits (Sylvilagus nuttallii) collected in Deschutes and Jefferson counties, Oregon, between May 28, 1968, and August 28, 1969, were excised and examined. Sex ratios of specimens collected by shooting, and of embryos more than 25 mm in length, were not significantly different from 1:1. Adult males appeared to be in breeding condition in the interval January-July. Juvenile males apparently did not attain breeding condition in the year of their birth. Adult females began to breed in late February and ceased to breeid in early July in 1969. Most adult females probably procluced four litters in 1969; some may have produced five litters. Thirty-one adult females shed an average of 5.0 + 0.2 ova per pregnancy, of which an average of 4.6 + 0.2 were implanted, and an average of 4.3 + 0.1 were viable embryos at the time of collection. Only 1 of 15 juveniles collected during the breeding season was pregnant. Productivity of the population of mountain cottontails in central Oregon was considerably less than that reported for eastern cottontails ( Sylvi1ugus flDridanus) in Missouri or Oregon but apparently was considerably greater than heretafore thought possible for the species. This is a report on an investigation of the reproductive characteristics and potential productivity of a population of mountain cottontail rabbits in central Oregon. The mountain cottontail is native to much of the western portion of the contiguous United States (Hall and Kelson 1959:261). In Oregon, these rabbits occur in various abundance in the Upper Sonoran life zone ( Dice 1926: 17, Bailey 1936:107 ) but appear to be most common in the vicinity of lava buttes, low lava hummocks, and other rocky outcrops (Bailey 1936:107-108, Orr 1940: 10S106 ) . Although the mountain cottontail appears to possess many of the character1 Technical Paper 3058, Oregon Agricultural Experiment Station. This research was supported, in part, by funds provided by the Theodore Roosevelt Memorial Fund Committee, The American Museum of Natural History. 2 Present address: Oregon State Game Commission, LaGrande. istics of an excellent game animal, it is designated a game species in only 5 of the 14 western states consituting its principal range (Kozicky and Madson 1969). Knowledge of the biology of the mountain cottontail appears fragmentary in comparison with that of related species. We were unable to find a single published report devoted solely to the ecology or life history of the mountain cottontail. Information on the life history of mountain cottontails consists primarily of that in unpublished theses (Janson 1946:51-52, 63-65) or in short summaries published as parts of regional works on mammals ( Bailey 1936: 107-109, Davis 1939:360463, Hall 1946: 607-612, Dalquest 1948: 387489, Ingles 1965:149149, and others) and in monographs devoted principally to the taxonomy of the Order Lagomorpha ( Nelson 1909: 199-211, Orr 1940:98-109, Hall 1951: 161-