Sleep can be a blissful release, but it can also leave animals vulnerable to opportunistic attack. Yet, some creatures have come up with a remarkable solution to the problem: only one half of the brain sleeps while the other remains vigilant – a phenomenon known as unihemispheric sleep. John Lesku and colleagues from La Trobe University, Australia and the Max Plank Institute for Ornithology, Germany, explain that some birds and aquatic mammals that have the knack remain attentive by leaving the eye that is connected to the conscious half of the brain open to survey their surroundings, while the closed eye is connected to the slumbering half of the brain. Many sleeping reptiles also appear to keep an eye on events, although it is less certain whether they are half awake. Knowing that crocodiles are more closely related to birds than other reptiles, Lesku and his colleagues wondered whether crocodiles also nap with an eye open.
Filming young crocodiles over a day and recording their responses to other crocodiles and a potentially threatening human, the team could see that the animals usually slept with both eyes closed. However, during the early part of the day they often opened one eye briefly and when the team introduced another crocodile into the tank, the crocodile pinned the open eye on the intruder. ‘This is consistent with a vigilance function’, explains the team, adding that young crocodiles have a better chance of remaining with their crèche if they open one eye while sleeping , in much the same way that dolphins train an eye on their pod to stay in touch when taking half a snooze. And when a human entered the crocodile's enclosure while the crocodiles appeared to slumber, the animals really took notice, keeping their vigilant eye trained on the threat for several minutes at a time.
So, crocodiles keep an eye on events while they appear to sleep, suggesting that that one half of the brain may be active – although the team adds that measurements of brain activity are required to confirm whether the half of the brain connected to the closed eye is genuinely asleep while the other half is awake. They also say, ‘Unihemispheric sleep may have first evolved in the archosaur lineage with the appearance of birds by elaborating upon a pre-existing behaviour inherited from a common ancestor with non-avian reptiles’.