Current Problems in the Study of Infanticidal Behavior of Rodents

Infanticide in a variety of species, has been reported anecdotally in the scientific literature for many years. Hypotheses concerning its underlying causes have ranged from infanticide as a means of population control, and hence adaptive to groups of organisms, to infanticide as a maladaptive social pathology in populations subjected to severe overcrowding. Recently, infanticide has been viewed as an adaptive behavioral "strategy" that may enhance the reproductive success of individual animals, particularly males. This "sexual selection" hypothesis suggests that males benefit from killing unrelated young because they eliminate parental investment in offspring they have not sired. The loss of a nursing infant also removes the endocrine inhibition o estrus in potential mates and permits the siring of additional offspring sooner than would be possible if the female continued to lactate. Many researchers have studied the role of infanticide in nature, especially in primates. While many studies suggest that infanticide enhances the fitness of male primates, there are no data that demonstrates differences in lifetime reproductive success between infanticidal and non-infanticidal males. Many published accounts have attributed infant mortality to "presumed infanticide. " Female counterresponses to infanticide by males have been suggested, but have not been examined systematically. Whether any female primates normally kill unrelated young is unknown. Infanticide research in laboratories or zoos is excluded for most species (especially primates) because those environments can only approximate natural conditions, and because most scientists consider such research unethical. These problems have been overcome to some extent by examining infanticide in rodents in both the laboratory and field. Because both male and female rodents may be infanticidal, a more comprehensive approach to sudying and understanding this behavior is possible. Statistically reliable sample sizes, carefully controlled environments and manipulations, and comparisons of behavior among species and among genetically homogeneous strains of rodents have also contributed to a dynamic new effort to understand the causes and functions of infanticidal behavior. This article discusses the many studies of rodent infanticide that have emerged in this decade. We summarize this work and analyze the way in which different methodologies and perspectives may have contributed to the conflicting results and conclusions apparent in the literature on rodent infanticide. Data from laboratory studies are compared and placed in context with what is known about the social structures and population dynamics of rodents in nature. In this article we maintain that infanticide is an overt behavioral manifestation with many motivational bases; the same behavior may increase individual fitness of both males and females, but for different reasons. We demonstrate that generalizations about the function or evolution of infanticide in rodents based upon data from one species or strain of rodent are at present unwarranted. Finally, questions and interdisciplinary avenues for future research are suggested.