Attachment behavior in rats

This thesis describes studies into the rat as an animal model for attachment, along the lines of Bowlby's attachment theory. First, the relation between attachment and human psychopathology is reviewed. The conclusion is that psychopathology is more frequent in insecure attached persons and that there is a relationship between specific types of insecure attachment and specific types of psychiatric disorders. Next, a reinterpretation of the literature on social preference experiments in rodents is given from an attachment theory point of view. Rodents distinguish their own mother by olfaction and prefer her to other conspecifics from birth. This preference also develops for non-natural odours that are offered in the presence of the mother. The pharmacological literature shows that oxytocin is a major positive influence on the development of preference for a specific partner. In the first two experimental chapters rat pups are offered a choice between different nests. A camera and a computer record the movements. A clear preference for the home cage develops, with a maximum at day 17 after birth. The importance of genetic and environmental factors is determined with the help of a cross-fostering experiment. On day 17 the rat pup does not distinguish between between two unfamiliar nests, in one of which it was born and cross fostered away from immediately. Genetic relations do not play a role in the preference. This result is discussed in the light of Hamilton's kin selection theory. The conclusion is that little is known about practical evolutionary advantages of recognition of genetically related animals in rats. The evolutionary advantages of a preference for a conspecific that offers food and protection is evident. Pharmacological experiments did not show a significant effect of oxytocin, while vasopressin reduced the preference for the home nest on day 17 after birth. The next experiment investigated if adult male rats developed a partner preference according to a protocol that is used in prairie voles. The male was put in a cage with a female for 24 hours and was offered a choice between this female and three others after the ensuing 24 hours of separation. A preference for the cohabitation partner developed although rats are polygamous in natural circumstances. However, then we carefully removed all possible stimuli the male could have left behind during the cohabitation period. If sawdust was exchanged and the female was shampooed the preference for the cohabitation partner disappeared. This suggests that partner preference is actually dependent on recognition of own stimuli, and thus a form of territorial behavior. In the last experiment pups were cross-fostered daily to a new nest during the first 20 postnatal days, while controls received exactly the same amount of handling and separation but were put back into their home nest. Then the effects of this prevention of attachment on adult behavior were studied. In a number of different behavioral tests the discontinuity in early environment resulted in an increase in anxiety-like and passive behavior and a slowing down of the sexual cycle.