A Blog by Ed Yong

Ostriches sleep like platypuses (and look wide awake when they do)

How does an ostrich sleep? Almost imperceptibly, it seems. Even though an ostrich might be sound asleep, it can look wide awake or, at most, a little drowsy. John Lesku from the Max Planck Institute of Ornithology discovered this by fitting six ostrichers with “Neurologgers”, electrode-laden helmets that measures their temperature, brain activity, eye movements and neck muscle contractions.

The video above shows three of the birds cycling through two different types of sleep. The first is called ‘slow wave sleep’ or SWS, where the ostriches’ brain waves are slow and strong. Even though this is typically known as deep sleep, the birds look alert. They stay still, but their eyes are open and their necks upright. Nonetheless, the readings from the Neurologgers clearly showed that they were asleep.

In the second phase, known as ‘rapid eye movement’ or REM sleep, the ostriches’ brain waves are fast and weaker. Now, the birds shut their eyes, which move rapidly behind closed eyelids. They necks also start to droop and sway, righting themselves with awkward jerks like people falling asleep at a talk. Biologists have previously interpreted this as a sign of a tired ostrich. That’s partly right, although the animal is already asleep rather than on its way.

Many other animals share the ability to sleep with their eyes open, and almost all birds and mammals have REM and SWS phases to their sleep cycle. But Lesku found that the ostrich’s pattern has unique features that are shared by some truly bizarre counterparts – the duck-billed platypus and the echidnas.

These ancient mammals lack any obvious distinction between SWS and REM sleep. Instead, they combine features of both into a single sleep state, fusing the slow waves of SWS with the rapid eye movements, relaxed muscles and twitchy heads of REM sleep. Something similar happens in the brains of sleeping ostriches. During REM sleep, their brains would flip between fast, shallow REM-style brain waves and slower, stronger SWS-style brain waves. Ostriches and platypuses also spend more time in REM sleep than any other bird or mammal.

As Lesku wryly points out, “There appear to be few traits unique to ostriches and [platypuses and echidnas] that could explain such an unusual REM sleep state.” They do, however, have one thing in common. Ostriches belong to the most ancient group of living birds, and platypuses belong to the most ancient group of living mammals. Their unusual slumbering patterns might reflect an early stage in the evolution of sleep, before REM sleep segregated into a distinct entity.

It is an interesting idea, and Lesku wants to test it further by looking at other birds and mammals, as well as other groups, such as crocodiles. In the meantime, the ostrich-platypus connection raises and obvious question: if distinct REM sleep evolved shortly after the rise of birds and mammals, why did this happen? In humans, REM sleep is said to be important for strengthening memories and boosting creativity. Did these or other REM-related skills play a role in the early evolution of birds and mammals?

Reference: Lesku, J., Meyer, L., Fuller, A., Maloney, S., Dell’Omo, G., Vyssotski, A., & Rattenborg, N. (2011). Ostriches Sleep like Platypuses PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0023203

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