Depression as an evolutionary adaptation: Implications for the development of preclinical models

Summary Several authors have suggested that rather than being a disease state, depression is an evolutionary adaptation to human social organization. Adaptations are produced in response to selection pressures and similar adaptations may easily have evolved in a range of other species. The current paper seeks to identify the social pressures that may lead to a species developing depression as an adaptation and the potential benefits that this may confer. It also examines whether rats and mice, the species most commonly used to model depression in the laboratory, have social organizations in which selection pressures towards the development of depression as an adaptation are likely to exist. It is proposed that depression is a useful adaptation in group-living animals, where there is competition for a social rank that gives reproductive advantage over others. The cluster of symptoms associated with depression include altered activity patterns, reduced sociability and appetite, and increased submissiveness. This combination strongly suggests that the function of depression is to reduce the likelihood of an individual being subject to further attack once they have lost social status and so increase their chances of survival in the period immediately following this. Successful transition from high to low status may provide further opportunities to reproduce. Hence, animals that become depressed during this critical period gain a reproductive advantage over those that do not, as these are either killed or expelled from the group. Wild mice do not have social hierarchies and are highly territorial. Wild rats do have social hierarchies however these only compete for reproductive advantage at the level of the sperm, and social status does not translate into significantly greater access to mates. Therefore, there are no selection pressures in these species towards the development of depression and so it is most unlikely that rats and mice have this adaptation. It is concluded that current models of depression based on rats and mice are in reality models of monoamine activity and that progression beyond this tautology requires future models to be developed using species selected on the basis of their social organizations in the wild. Hendrie and Pickles, Medical Hypotheses.