Onset of Evening Activity of Swamp Rabbits and Cottontails in Relation to Sunset

Abstract

Onset of evening activity was recorded for ten swamp rabbits (Sylvilagus aquaticus) (1967 breeding season) and 22 cottontails (S. floridanus) (1960-1962 breeding seasons) confined in 1.8-2acre enclosures. As sunset was delayed toward spring, the onset of activity of individual swamp rabbits was delayed. The delay was greater for females and the lowest-ranking male than for the reproductively active males. The still later sunsets of late spring and summer were not accompanied by a further delay in onset of evening activity, and time of onset became highly variable but occurred well before sunset. Cottontail data indicated a pattern similar to that of swamp rabbits. Onset of activity for both species occurred during the twilight period following sunset in late winter and early spring but well before sunset in late spring and summer. Rowley (1957), in Tasmania, reported that activity of confined wild rabbits (Oryctolagus cuniculus) began nearly an hour earlier, relative to sunset, in autumn than in winter. He suggested that this difference was probably due to the increased grazing opportunity offered by the longer winter nights, making daylight emergence unnecessary. Based on 24-hr roadside counts of cottontails in Illinois, Lord (1961) concluded that activity began at 5:00 PM (all times are Central Standard Time) and ceased at about 7:00 AM, regardless of varying light conditions between winter and summer. He concluded that some factor other than sunset acted as the stimulus for onset of cottontail activity. Lord (1964) recorded activity of caged cottontails throughout the year and determined that activity lasted longer during summer when 1Joint contribution from the Department of Zoology, University of Missouri, and the Missouri Cooperative Wildlife Research Unit: U. S. Bureau of Sport Fisheries and Wildlife, Wildlife Management Institute, Missouri Department of Conservation, Edward K. Love Foundation, and University of Missouri cooperating. Special aid and facilities were furnished by Pittman-Robertson Project (Missouri) 13-R-15, and the Gaylord Memorial Laboratory, Puxico, Missouri. 2 Present address: Department of Zoology, University of Missouri, Columbia. nights were shorter. Lord (1964:41) suggested that the process of food assimilation may provide the endogenous clock for reglating rabbits' daily activity pattern. In contrast, Mech et al. (1966) suggested a relation between onset of activity a d sunset, and cessation of activity and sunrise for cottontails and snowshoe hares (Lepus americanus) in Minnesota. During a study of the social behavior of confined cottontail and swamp rabbits (Marsden and Holler 1964), it became apparent that onset of activity in both species occurred much earlier in relation to sunset in late spring and summer than in winter and early spring. In view of the contradictions in the literature, additional data on seasonal changes in activity onset times of swamp rabbits were collected during the breeding season of 1967. This paper reports on those data and data collected on cottontails during the earlier study. We express our gratitude to T. S. Baskett, U. S. B. S. F. W., and to C. H. Conaway, University of Missouri, for valuable advice and assistance during the course of the study. D. H. Hazelwood, University of Missouri, and K. C. Sadler, Missouri Department of Conservation, kindly reviewed the manuscript.