The reproductive capacity of most mammals is far greater than what would be required in the absence of predators. In a litter of six or eight, the typical offspring does not survive to reproduce, and the typical result of reproduction is to provide a food supply for the rest of the ecological system rather than to maintain the species. The purpose of this paper is to test further the hypothesis that the parameters of biological systems are chosen so that the entropy production is a minimum. This hypothesis has previously been applied in an analysis of breathing (Wilson, 1964), an internal mechanism, and in a discussion of seasonal migration (Prigogine and Wiame, 1946), a characteristic of the species as a whole. The parameter that is investigated here is the mortality rate, part of a mechanism that involves the interaction among species. For some mammals, it appears that the mortality rate that is chosen, that is, the one for which the population will remain constant, can be described as the one for which food is supplied to the ecosystem with minimum metabolism. A particular value of the reproductive rate is required in order that these two values of the death rate should agree. Some approximations are necessary in order to establish a workable model. A constant probability of mortality is assumed. Of an initial number of animals N. born at time t= 0, the number surviving N at time t is (1) N= N. e 8t