Reproductive Variation in Cotton Rats

Abstract

Variation in reproduction and in sexual organs was studied for 1461 cotton rats, Sigmodon hispidus, between October 1959 and September 1961 in Payne County, Oklahoma. Seasonal variations in the size of the sex organs were more pronounced in years with cold winters than during years with mild winters. During the cold winter of 1959-60, under crowded conditions, cotton rats did not reproduce. Some reproduction did occur, however, during the mild winter of 1960-61 when the population density was low. Under favorable environmental conditions cotton rats can breed and produce litters at any time during the year. Uterine litter size varied from 1 to 11 per female, averaging 5.9. Uterine litter size was smaller in small or young females and larger in large or old animals. Litters were smaller and less frequent during the cold months of the second year than during the warm months of that same year. The size of testes and ovaries varied seasonally regardless of the size of the individual. In the population, at all times, at least some males and females had enlarged gonads and apparently were capable of reproduction. The reproductive data indicate that cotton rat populations are potentially irruptive throughout the year.