Embryo Resorption in the Swamp Rabbit

Abstract

The purpose of this paper is to report some changes in the reproductive rates of swamp rabbits (Sylvilagus aquaticus) following, and apparently induced by, marked habitat alteration. The study was made in the Mingo Swamp, an abandoned valley of the Mississippi River in southeastern Missouri. Large acreages of swamp rabbit habitat occur here within the 22,000-acre Mingo National Wildlife Refuge and the adjacent 6,000-acre Duck Creek Wildlife Management Area.1 Swamp rabbit populations at the onset of the breeding season in February 1957 were very high. During the previous winter the density of swamp rabbits on a 70-acre study plot was one rabbit per 1.5 acres (Toll, et al., in press). Through this period and until the end of March 1957, water levels in Mingo Swamp were low and there was no major fluctuation in inundated acreage. During the first three days of April 1957, however, the total rainfall was 5.2 inches. Water levels rose rapidly and by the evening of April 4, the majority of previously occupied swamp rabbit habitat was inundated. Measurements from the Mingo Wildlife Refuge indicate that the water acreage on April I was 1,550 acres and when next measured on April 8, it was 12,000 water acres (Table 1). Swamp rabbits were driven from their former habitat to adjacent high ground where they were forced to exist under crowded and exposed conditions. Although we have no accurate measurement of population density during this flood period, an indication of the conditions that existed is that 20-25 swamp rabbits were observed at one time in a single 2-acre field. Rabbits were constantly seen during the day on the levees and most collecting was done by driving rabbits into the water and catching hem by hand. Flood conditions continued through June 1957; water levels remained high during the winter of 1957 and through the spring of 1958. Therefore, before and during the 1958 breeding season water levels were relatively high but not fluctuating. During April 1958, swamp rabbits were again collected. At this time th y were not so conspicuous as they had been in 1957, and collection was possible only with usual hunting effort. Few rabbits were seen during daylight hours, and no concentrations were found either in fields or on the levees.