Effects of social interaction on monocular/unihemispheric sleep in male and female domestic chicks.

The monocular/unihemispheric sleep was studied in male and female domestic chicks reared socially. In first week post-hatching, chicks were reared in couples whilst during the second week, for half of the chicks the cospecific was maintained (Conspec-chicks) and for the other half the conspecific was removed (No-Conspec-chicks). During the first week, female chicks showed a bias for more left-eye closure/right unihemipheric sleep, whilst male chicks did not show any eye-closure bias. In the second week, both female and male Conspec-chicks showed a bias for right eye-closure/left unihemispheric sleep. Female No-Conspec-chicks did not show any eye-closure bias whilst male No-Conspec-chicks showed a bias for more left eye-closure/right unihemispheric sleep. Based on the role of the avian brain lateralization, a bias for more right or left eye-closure could be associated with a prevalent activation of left or right hemisphere during wakefulness. Eye-opening during sleep might be connected with the hemisphere that was not or was less activated during wakefulness or with lateralization of the environment monitoring against predation.