Biological Mechanisms Balancing Torpor and Reproduction in Mammals.

Selective pressures have led to the development of innumerable unique strategies to maximize fitness-an organism's ability to survive and reproduce. One strategy is the ability to reversibly enter a regulated state of hypometabolism, known as torpor, to temporarily halt or slow important metabolic processes to conserve energy and avoid a lethal energy shortage. While torpor can be highly adaptive, it can also interfere with reproduction. As a result of this tradeoff, organisms have developed adaptations that balance energy homeostasis needs with the metabolic demands of reproduction, thereby maximizing their fitness. In mammals, numerous studies have described a bidirectional relationship between torpor and reproduction, with reproductive state influencing the pattern and frequency of torpor use and, conversely, the reproductive axis undergoing dramatic changes during periods of frequent torpor use. Here, we review the current knowledge of the signaling networks underlying this bidirectional relationship, weigh hypotheses on unresolved questions in the field, and discuss the value and larger implications of these studies.