A coronal section image at 200 times magnification through the hypothalamus of the Congo Grey parrot, showing the cholinergic neurons in a region called the dorsomedial hypothalamic cholinergic cluster. These neurons are easy to see in the two parrots and the crow, but they are missing in the hypothalamus of the ostrich and the emu.Credit: Paul Manger.
Specialised neurons present in ostriches and the emus mean they enter a state that combines elements of slow wave sleep (SWS), and rapid eye movement (REM) at the same time. A recent study, in the Journal of Comparative Neurology, compared the hypothalamus of these flightless birds with other species that lacked the specialised neurons.
In the 1990s, Paul Manger, of the University of the Witwatersrand, one of study authors, observed strange sleeping patterns in the monotremes: egg-laying mammals. He had observed that these unique mammals’ brains have different sleep states at the same time.
Further research conducted by Manger’s two co-authors revealed similar sleep patterns in ostriches, prompted the team to investigate the cholinergic system, which is made up of neurons and plays a key role in regulating arousal levels in the brain.
"The cholinergic system acts as a connectivity axis throughout the brain," Manger explains. "Disruptions in this system can lead to the fragmentation of global brain states, and potentially contribute to sleep disorders and other neurological conditions.”
For this study, the researchers looked for cholinergic neurons in the hypothalamus of five bird species: Congo grey parrot, Timneh grey parrot, pied crow, common ostrich, and emu. While cholinergic neurons were clearly present in the hypothalamus of parrots and crows, the team found no convincing evidence of these neurons in the hypothalamus of the ostrich or emu. The unusual expression of the sleep states in the ostrich and the monotremes may then be explained by the lack of the cholinergic neurons.
For Manger, this result inspires further exploration of sleep physiology across diverse bird and mammal species to gain new insight into the underlying principles of brain function.
"Africa's geographic and biological advantage offers unique opportunities for advancing our understanding of sleep and brain states," says Manger.
