The present analysis is based on data first acquired incidental to personal fur-trapping (principally in South Dakota, 1915-28), later as a product of intensive year-to-year field researches in central and northern Iowa, 1934-42. In the intensive phases of the investigations, particular efforts were made to inquire into the mechanics of predation suffered by the muskrat at different densities, under different environmental conditions, and at different seasons.

It was found that strangeness of environment, intraspecific intolerance, and drouth were especially important in predisposing muskrats of north-central United States to predation, as by minks; and that kinds and numbers of wild predators doing the preying, with a few apparent exceptions, had little bearing upon the net mortality suffered by the muskrats. For reasons of intercompensatory (automatically adjusting) trends existing in reproductive as well as loss rates of the muskrats, severity of predation (or, at any rate, statistics on numbers of muskrats or on proportions of populations killed) may leave highly misleading impressions as to population effect. Even when locally nearly annihilative, predation rarely showed evidence of functioning as a true population depressant, insofar as it usually only took the place of some other mortality factor and as, in its absence, some other factor, particularly intraspecific strife, tended in its turn to become sufficiently operative to compensate for decreased predator pressure.