Seasonal Changes in the Endocrine Organs and Behavior Patterns of the Muskrat

Abstract

Behavior patterns combine with the physical environmental conditions to control the population density and distribution of the muskrat, Ondatra zibethica zibethica (Linn.). Many of these behavior patterns are seasonal or at least seasonal in the intensity with which they are manifest and seem to be correlated with and probably controlled by the seasonal physiological changes which occur in the animal.In this study we attempt to correlate the seasonal behavior patterns with the physiological cycles, and suggest how they may affect the composition and density of the population in the Wisconsin marshes. The investigation includes anatomical data from 237 muskrats, from every month of the year, and field observations on seasonal behavior patterns. The number of animals examined was limited by the difficulty of obtaining them. Most of the muskrats were captured alive, while a few were freshly skinned carcasses obtained from trappers. The live muskrats were obtained from the Horicon Marsh Wildlife Area in Dodge County, Wisconsin, and from the University of Wisconsin Arboretum, with the carcasses coming from nearby areas. This is a contribution from the Department of Wildlife Management and the Department of Zoology, University of Wisconsin. The study was made possible through the co-operation of the Pittman-Robertson Division of the Wisconsin Conservation Department, and was supported in part by a grant from the Wisconsin Alumni Research Foundation.Live muskrats were brought into the laboratory where they were killed within a few hours and the pituitary, testes, epididymides, prostate, preputial glands, ovaries, uteri, adrenals, and thyroids were removed and weighed immediately on a torsion balance. The coagulating glands were separated from the prostate since there seems to be some disagreement as to whether these lobes are morphologically and physiologically different (Walker, 1910a and 191Ob;Price, 1936). All organs, except the uteri, were weighed with their fluids and the combined weights of paired organs were recorded. The uteri were nicked with a razor blade and any fluids that were present were removed. The uteri from pregnant females were not used as it was nearly impossible to separate the maternal from the embryonic tissues. Smears were made from one epididymis to determine the presence or absence of sperm.The animals were divided into four classes: adult males, adult females, im-

Population Dynamics of the Wood Thrush in Southern Veracruz, Mexico

“…Although avian evidence for differential mortality between sedentary individuals and wanderers is scarce (but see Tompa 1962;Carrick 1963;Jenkins et al 1963Jenkins et al , 1964Jenkins et al , 1967cf Catterall et al 1989), mammalian nomads are known to suffer high mortality (see Errington 1943Errington ,1963Beer and Meyer 1951;Hawkins et al 1971;Gaines and McClenaghan 1980, p. 184-185;Packard andMech 1980, Andelt 1985). Nomadic individuals lack intimate knowledge of an area, a fact that may be disadvantageous (Brown 1963).…”

Innate and Environmental Dispersal of Individual Vertebrates

“…By the time the populations in the earlier occupied habitats approached overcrowding, which also probably resulted in increased mortality and decreased fecundity, the range of the species had probably already been extended many miles. Beer and Meyer (1951) report on muskrats as follows: "The bulk of the reports on movements in the Madison area are received in March with a few in February and quite a few in April. This agrees with the period of increasing gonadotropic activity of the pituitary and the rapid development of the gonads.…”

Muskrat (<em>Ondatra zibethicus</em>) interference with aquatic invertebrate traps

“…We are uncertain why they did this, as there was no evidence of them feeding on the contents. Muskrats display strong territorial behaviour during breeding season; their aggressive behaviour reaches a peak from late April to late May and diminishes in early June (Beer and Meyer 1951). The frequency of our trap repairs was highest from late May into early June, when we suspect invasion of the territory by conspecifics prompted destructive territorial behaviour.…”