A study of some effects of increased population density on reproduction in two inbred lines of mice : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science at Massey University of Manawatu, New Zealand

Abstract

The Theory of Regulation of Reproductive Rate within Populations The original hypothesis considered that soeiopsychological pressure might act as a stressing stimulus in some proportion to population density, which would activate the complex hypothalamo-hypophyseal-adrenocorticalgonadal system (Selye, 1939, 1954; Harris, 1956). This proposal was based on field data reported in the literature and on the results of experiments in the laboratory with the pituitary-adrenocortical system. A relationship between reproductive endocrine function and population density was foreshadowed by Pearl and Surface (1909), who demonstrated decreased egg production by domestic fowl with increased population size even though the area per bird was kept constant. Crew and Mirskaia (1931) and Retzlaff (1938) demonstrated a similar inverse relationship between population size and reproductive performance in albino mice. These authors recognized the importance of behavioural factors in producing these effects. Retzlaff observed that socially dominant female mice gave the best reproductive performance. Selye (1939) demonstrated that stimuli evoking a response of the pituitary-adrenocortical system also suppressed the reproductive function of female rats. He postulated that the decreased reproductive activity might result from a shift in pituitary function to produce an increased amount of adrenocorticotrophin (ACTH) at the expense of the production of gonadotrophins although an actual decrease in the production of gonadotrophins was not shown. [From the Introduction]