Individual recognition of social rank and social memory performance depends on a functional circadian system.

In a natural environment, social abilities of an animal are important for its survival. Particularly, it must recognize its own social rank and the social rank of a conspecific and have a good social memory. While the role of the circadian system for object and spatial recognition and memory is well known, the impact of the social rank and circadian disruptions on social recognition and memory were not investigated so far. In the present study, individual recognition of social rank and social memory performance of Djungarian hamsters revealing different circadian phenotypes were investigated. Wild type (WT) animals show a clear and well-synchronized daily activity rhythm, whereas in arrhythmic (AR) hamsters, the suprachiasmatic nuclei (SCN) do not generate a circadian signal. The aim of the study was to investigate putative consequences of these deteriorations in the circadian system for animalś cognitive abilities. Hamsters were bred and kept under standardized housing conditions with food and water ad libitum and a 14l/10 D lighting regimen. Experimental animals were assigned to different groups (WT and AR) according to their activity pattern obtained by means of infrared motion sensors. Before the experiments, the animals were given to develop a dominant-subordinate relationship in a dyadic encounter. Experiment 1 dealt with individual recognition of social rank. Subordinate and dominant hamsters were tested in an open arena for their behavioral responses towards a familiar (known from the agonistic encounters) or an unfamiliar hamster (from another agonistic encounter) which had the same or an opposite social rank. The investigation time depended on the social rank of the WT subject hamster and its familiarity with the stimulus animal. Both subordinate and dominant WT hamsters preferred an unfamiliar subordinate stimulus animal. In contrast, neither subordinate nor dominant AR hamsters preferred any of the stimulus animals. Thus, disruptions in circadian system result in an impaired individual recognition of social rank. A social recognition/discrimination task was used in Experiment 2 to quantify social memory performance. In a training session, the hamsters were confronted with two unfamiliar stimulus animals. In the test session, one of the two animals was replaced. The training-test interval was 2min or 24h. The times animals did explore the novel and the familiar stimulus animal were recorded, and the discrimination index as a measure of cognitive performance was calculated. Behavioral tests revealed that after 2min both subordinate and dominant WT hamsters were able to discriminate between familiar and novel stimulus animals but after 24h only the subordinate animals. On contrary in AR hamsters, only subordinates were able to perform the social recognition/discrimination task and only after a training-test interval of 2min. The results show that the social rank and the circadian system have an impact on the cognitive abilities of Djungarian hamsters. Disruptions of circadian rhythms impair individual recognition and social memory performance.