Although light is the main factor that influences circadian rhythms, social interaction may also have a role on their regulation. Here, the influence of social interaction on rat circadian behavior was investigated, addressing the question of whether cohabitation would induce the appearance of a circadian rhythm in arrhythmic rats due to constant light. To this end, circadian rhythms of motor activity and body temperature of male and female LL-induced arrhythmic rats were studied before, during and after a 20-day period in which rats stayed in the same cage with a rat of the same sex but with stronger rhythm. Results showed that the manifestation of the circadian motor activity rhythm of LL-induced arrhythmic rats increased after cohabitation. In the case of the expression of the body temperature rhythm, there was a progressive daily increase in the power content of a daily 24 hour pattern throughout the cohabitation days, which remained when animals were again isolated. Thus, the presence of a rhythmic rat increases the strength of the circadian behavior of rats showing a weak circadian rhythm.
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