58 BRAIN MECHANISMS AND LEARNING 



very strong. As Kollcr (1955) has shown, this is due to the corpus-hiteum 

 hormone, progesterone. All three females built immediately; their 

 behaviour is shown in the film. 



Forty-two inexperienced females (group [c)) were tested for retrieving 

 and nest building immediately after they had given birth in the experi- 

 mental cage. Thirty-five of these females retrieved a nestling taken from 

 the corner where they were suckling and deposited in another corner. 

 Only seven of the females did not carry back their babies. In six of these 

 seven cases the nestlings, which were not protected by nesting material, 

 were so cold, that they seemed nearly lifeless, and did not squeak. Squeak- 

 ing is, however, one of the sign stimuli for the females, releasing search 

 and retrieving in the mother (Zippelius and Schleidt, 1956; Eibl-Eibesfeldt, 

 1958). Those retrieving behaved like normally raised mothers, with the 

 only difference that they showed some hesitation in grasping the nestlings 

 and often lost them during transport and had to pick them up again. 

 When the nestling squeaked while being grasped, the female changed the 

 grip, a behaviour which is now being studied in more detail. 



All forty-two females, including those that did not retrieve, started 

 immediately with the construction of a nest when, after the retrieving 

 test, nesting material was offered. Furthermore they showed the covering 

 of the nestlings typical for the breeding mother. This will be shown in the 

 film. 



Our experiments have shown that the handling of solid objects during 

 ontogeny is not a prerequisite for the development of nest-building and 

 retrieving behaviour. Riess did not realize this and he furthermore over- 

 looked that nest building is a complex behaviour, and therefore did not 

 look for the elements which compose it. 



The several motor patterns which, on the basis of a comparative study 

 of many species of rodents, had been assumed to be unlearned 'fixed 

 patterns' appeared completely unchanged in the rats reared in the manner 

 employed by Riess. One important difference was, however, found 

 between experienced and inexperienced rats. The sequence in which the 

 above-mentioned motor patterns are used was considerably better adapted 

 to the function in experienced rats, inexperienced rats often employing 

 motor patterns which can develop their function only at an advanced 

 stage of the newly built nest, at a very early stage of building at which the 

 patterns in question did not yet perform their function. The question 

 whether 'nest building' must be learned or not cannot be given a simple 

 answer. Certain essential motor patterns and taxis components are 

 completely independent of learning. The proper sequence in which 



