Testing thermogenesis as the basis for the evolution of cetacean sleep phenomenology.

Cetacean sleep phenomenology consists of a combination of unihemispheric slow wave sleep and a massive reduction in the amount of rapid eye movement sleep. Despite various proposals, the selection pressure driving the evolution of this combined sleep phenomenology is unknown. It was recently suggested that the need to produce heat in the thermally challenging aquatic environment might have been the selection pressure. Mechanisms of heat loss and heat production can be measured directly or indirectly. The present study was designed to test the thermogenetic proposal by recording indirect measurements of heat loss (surface area to volume ratio) and heat production (tail-beats per minute). A strong correlation was found between these two parameters, such that increases in potential heat loss were matched by increases in potential heat production. This result suggests that the need to compensate for heat loss can provide an evolutionary rationale for the appearance of extant cetacean sleep physiology.