Bipolar disorder and the mammalian rut: an evolutionary perspective.

We propose that bipolar disorder evolved from seasonal rhythms that guided the lives of early men and women. In the animal world seasonal cycles are marked by the alternation of torpor/quiescence and rut/activation and remain ubiquitous in larger mammals, including primates, living in both temperate and tropical zones. The heightened sexual drive and aggressiveness stoked by the rut provide the means for a species to pass along the best genes at the best time. The defining and ancillary characteristics of hypomania/mania and the rut are remarkably similar. They are described and compared. We trace the evolution of these highly adaptive rhythms from the earliest mammalian life to their expression in Homo sapiens and speculate about why such traits remain in our gene pool, where they create a severe burden to many individuals. Finally, we address questions stimulated by our thesis and offer research suggestions.