A platypus sleeping in a cool burrow looks like a furry ball: it tucks its legs up against the body, drops its bill against the chest, and then wraps its tail over its head and bill. This posture reduces the amount of heat lost from the platypus’s body surface during sleep and, in particular, means that the bare skin of the bill is kept warm under its tail. However, if the temperature inside the burrow rises substantially (for example, during a heat wave), a sleeping platypus may partly or entirely stretch out while reclining on its side, stomach or back.
Mammalian sleep is typically divided into two distinct and alternating stages: REM (“Rapid Eye Movement”) sleep and non-REM sleep. In REM sleep, nerve cells in the brain fire in rapid bursts, the face, eyes and limbs twitch and (at least in humans) dreaming occurs. Interestingly, researchers have found that a platypus engages in more REM sleep than any other adult mammal in which this has been studied (more than 8 hours per day on average) (Siegel et al. 1998). There are some differences between platypus REM sleep as compared to most other mammals (for example, the platypus brain cortex has a higher voltage when monitored using an electroencephalogram or EEG) (Siegel et al. 1998). However, the platypus resembles other mammals in vigorously twitching its bill and head and rapidly moving its closed eyes during REM sleep (Siegel et al. 1998). It remains unknown if a platypus actually dreams.
Photo courtesy of Erin Whitford
LITERATURE CITED
Siegel JM, Manger PR, Nienhuis R, Fahringer HM and Pettigrew JD (1998) Monotremes and the evolution of rapid eye movement sleep. Royal Society of London Philosophical Transactions B 353, 1147-1157.