Victor B. Scheffer

The Origin of the Mima Mounds of Western Washington

The Mima mounds occur on certain prairies of glacial outwash in western Washington. The mounds are closely spaced, round or oval, from 10 to 40 feet in diameter, and from 1 to 7 feet in height. The typical mound is a double-convex lens of loose, unstratified, black silt-gravel set in a shallow pit in stratified yellow outwash gravel. Mounds are found only where a thin layer of soil overlies a compact bed of gravel, not on deep prairie soils. The Mima mounds are formed by pocket gophers (Thomomys talpoides) over long periods of time. Gopher activity in any particular place destined to become a mound site starts with intensive burrowing, such as that required in the construction of a nest, which loosens the soil and stimulates the growth of vegetation. The vegetation, in turn, furnishes food for the gophers and encourages them to concentrate their activities in the vicinity. A stage is reached where the gophers find sufficient food on the mounds to maintain them the year around, making it unnecessary for them to forage, except at rare intervals, into the intermound depressions. In deep burrowing to create living quarters gophers dig a shallow pit in the stratified gravel beneath each mound. The smaller elements in the stratified gravel removed by deep burrowing become mixed with silt to form the substance of the mound lens. Stones too large to be moved by the gophers are undermined and settle to the bottom of the mound. In shallow exploratory burrowing in the peripheral zone the gophers do not undermine large stones but remove soil from about them, eventually leaving them exposed on the surface of the ground.

Body Size with Relation to Population Density in Mammals

The first purpose of this paper is to present certain evidence of a decrease in body size in the Alaska fur seal ( Callorhinus ursinus ) with relation to a measured increase in population size; second, to review briefly the relationship reported to exist between body size and abundance in the brown rat, house-mouse, vole, muskrat, gray squirrel, raccoon, deer, and man. The body size: abundance relationship in mammals has not, we believe, received the attention it deserves. A third purpose of the paper, then, is to stimulate inquiry into the mechanics of body size regulation by density dependent factors in the environment. With the exception of an albino form of house-mouse, the mammals referred to above are wild species of economic importance. The present advantage of using such animals for study will disappear as more is learned about methods of suppressing natural fluctuations in their numbers. Thus, as the wildlife manager becomes progressively more skillful in controlling irruptions of deer and muskrat, the less opportunity will he have of studying the influence of population pressure, and the more will he turn to captive animals for evidence. The necessary conditions for a study of the size: abundance relationship are that both the subject population and the body size of its members shall be responsive to measurement from time to time. In measuring the population, the observer shall make certain that he is dealing continuously with the same genetic group; not now with one and later with another that has invaded the study area. And in measuring body size he shall be able to identify sex and age as a prerequisite to selecting comparable individuals. For example, references in the literature to “decrease in average body size” may refer only to a decrease in proportion of older and larger members of the population, …