Reproductive adaptations to high densities in social mammals

Abstract

An overview of population density control in social mammals sets the framework for examining reproductive responses to increasing population densities. We begin by honoring John B. Calhoun who played a major role, starting about 70 years ago, in correcting the conventional wisdom of the time that population control was all about adjusting mortality rates. Reproduction was considered a constant, and emigration was viewed as a component of mortality. His research was mostly on domestic and feral populations of Rattus norvegicus and Mus musculus for which he built complex arenas that could house thousands of inhabitants. In addition, he constructed elaborate living quarters in the alleyways of Baltimore. For the captive populations, he would individually mark all the inhabitants so that he could identify them and observe their behavior. Eight examples of his research publications are described. He was able to document a list of reproduction related behaviors that are the main focus of this essay, and clearly demolish the simplistic notion that reproduction is a constant. A summary of some of the major issues that encompasses the science of population growth and regulation is provided. There follows a series of 19 examples of non-Calhoun publications that cover many more species of social mammals, including humans. A comprehensive list of the many negative adaptations involving reproductive processes are documented for the 27 publications reviewed confirming that in many species reproductive constraints are non-trivial. There follows a brief discussion that attempts to create a cohesive whole for this intellectual adventure.