The relationship between age of grouping and weight of selected organs of Prairie deermice

Abstract

Populations of several species of small mammals when maintained in the laboratory under conditions in which the physical environment is controlled or held constant and when provided with excess food and water, do not increase indefinitely, but growth is controlled as a result of factors thought to develop intrinsically to each population (BROWN 1953; CALHOUN 1950, 1952, 1962; CHITTY 1960; CHRISTIAN 1964 ; NICHOLSON 1957 ; SOUTHWICK 1955 ; TERMAN 1965, 1969). Increases in the density of such populations have been correlated with a number of changes within the populations. Among these are behavioral changes involving patterns of activity, increases in aggressive behavior, disturbance of maternal care, modifications of spatial distribution and alteration in the population social structure (CALHOUN 1950, 1962; STRECKER and EMLEN 1953; SOUTHWICK 1955; LLOYD and CHRISTIAN 1969). In addition, physiological changes have been shown to occur which increase mortality, decrease growth, and reduce reproductive function (see THIESSEN and RODGERS 1961, and CHRISTIAN et al., 1965, for reviews). While it is recognized that some of these changes may be species specific, others may be related to general mechanism of population control. Recent laboratory studies of Prairie deermice (Peromyscus maniculatus bairdii) have shown that populations of these mice also controlled their numbers in the presence of surplus food and water (TERMAN 1965). Males born into these populations exhibited significantly lighter testes and vesicular gland weights than bisexually paired controls of similar age. The development of the reproductive organs of females (ovaries and uteri) were also inhibited and were significantly lighter than those of controls. A further indication of the strength of the inhibitory forces in the populations is the fact that 95% of the females born into the populations failed to reproduce (TERMAN 1969). The experimental data obtained from these Peromyscus populations left unanswered the question of whether the significantly lighter weights of the reproductive organs of population males and females when compared to controls was due to inhibition of growth or to regression in weight or both. Further, it is not known whether the