Reproductive Performance of a Population of Woodchucks after a Change in Sex Ratio

Abstract

A reciprocal relationship between density and reproductive performance in mammalian populations is now well established (Crew and Mirskaia 1931, Retzlaff 1938, Davis 1951, Calhoun 1952, Christian 1955a, b, and 1956, Christian and LeMunyan 1958, Clark 1955, Kalela 1957, Crowcroft and Rowe 1957, Hoffman 1958, Helmreich 1960). This relationship apparently depends upon social interactions which increase with increased population density and which operate through endocrine mechanisms to affect reproduction (Clhristian 1959). A corollary to this thesis is that actual numbers (density per se) are not wholly determinant. The intensity of social interaction also should be related to factors such as the sex ratio and age composition of a population, because present evidence shows clearly that competitive behavior changes with age and differs with sex. The present paper describes the reproductive performances of 3 different populations of woodchucks (Marmiota m. monax L.). In one population, a surplus of males was maintained by selectively removing a portion of the females at intervals in 1957 and 1958. In a 2nd population, males and females were removed selectively in large numbers in 1957 and 1958 to reduce density and eliminate older animals. The 3rd populatioln was disturbed as little as possible during this time. The reproductive performance of each population was assessed in 1959 by examining the reproductive tracts of females collected during the breeding seasonl. The main purpose of this paper is to describe the way in which each population was man-ipulated and to present an analysis of the reproductive data collected subsequently. Observations oni mortality, movements, and social interactions, and data colncerned with compensatory responses to the