It is true – crocodiles can and will sleep with an eye open, according to a new study.
Australian legends said that crocodiles sleep with one eye open, but no substantial scientific evidence has been found to confirm or disprove this, until now.
Researchers from Australia's La Trobe University were able to confirm that crocodiles can engage in unihemispheric sleep, or putting only one-half of the brain to sleep while the other stays alert, as shown by keeping one eye open. Unihemispheric sleep has been observed in dolphins and some birds, but limited research has been done about this phenomenon on crocodiles.
"These findings are really exciting as they are the first of their kind involving crocodilians and may change the way we consider the evolution of sleep," said lead researcher Michael Kelly.
Kelly and co-authors, Richard Peters and John Lesku are from La Trobe's School of Life Sciences while Ryan Tisdale is from Max Planck Institute for Ornithology, Germany. The group observed the behavior and sleeping habits of juvenile saltwater crocodiles day and night using infrared cameras fitted on custom-made aquariums. The crocodiles were between 40 to 50 cm (16 to 20 inches) long.
Researchers noted that when humans entered the room, the crocodiles kept a close eye on them, and would fall asleep with one eye focused on the spot where the humans were last seen.
This is in agreement with findings by researchers from Max Planck Institute for Ornithology in Germany, who observed that crocodiles can actually sleep with both eyes closed but would more likely to sleep with one eye open when humans are present.
This behavior was also found to be true when younger crocodiles were introduced, seemingly as a way to look out for the younger ones and protect them from predators.
Researchers, however, still need to look into the reptiles' brainwave activity while sleeping. Lesku is currently working with his team to attach electrodes to the crocodiles' heads in preparation for this new experiment.
The team members believed that unihemispheric sleep is an evolutionary product of the shared ancestor of birds and reptiles. Mammals and humans, on the other hand, have brains that completely shut down during sleep, with the notable exception of aquatic mammals like dolphins.
The researchers concluded that if some birds, aquatic mammals and reptiles can sleep unihemispherically, then the whole-brain sleep of humans and other mammals can be considered an evolutionary oddity.
"What we think of as 'normal' sleep may be more novel than we think." Kelly said.
The study's findings were published in the Journal of Experimental Biology.