IRENAUS EIBL-EIBESFELDT 55 



legs, identical with those performed when pushing earth out ot the tuimel 

 and by shoving with the snout. In the centre of the heap the rat starts 

 scratchiiiii, forming a bowl and, furthermore, turning on its axis and 

 pyiishiiK^ the nest material towards the periphery, it forms a ring-shaped 

 mound around itself. Intermittently the rat reaches over the mound, 

 grasping scattered nesting material with its teeth, and deposits it on the 

 rim of the mound or inside the nest. It reminds one of the way geese 

 construct their nests, but whereas the latter are able only to lay the 

 material backwards over their shoulder, the rat shows more plasticity, 

 being capable of depositing nesting material from different positions in 

 relation to the nest. Coarse nesting material, like straw, is split with the 

 teeth longitudinally. The rat holds the material at both ends m its paws 

 and bites along it with the lower incisors. By a sudden lifting of its head 

 it splits the straw [splittiiKT). Straw, and even solid wood, are transformed 

 into soft nesting material. Similar movements of nest building are ob- 

 served in many other rodents and the patterns of splitting, scratching, or 

 pushing, are common to all those species mentioned above. Sometimes 

 additional movements are observed. Tree squirrels, for example, have 

 special movements for bundling the nesting material before transport. 



In addition, I was aware of certain environmental situations which were 

 obviously indispensable, for instance, previously explored environment 

 containing known nest locality, or else good cover. ^ At once I suspected 

 that the failure of Riess's rats to build was due to the fact that they did not 

 have a definite nesting place in the unfamiliar testing situation with which 

 they were confronted. 



When I took ten virgin rats experienced in nest building, duplicating 

 Riess's test situation, none oi them started building. After they had over- 

 come their shyness they first started exploring, in between retreating to 

 one corner where they cleaned themselves and rested. Some pulled out 

 paper strips and scattered them all over the Hoor, thus behaving like the 

 rats of Riess. But none built a nest within the first hour and only three 

 built within 5 hours. 



On the basis of my observations on different species of rodents, Riess's 

 statement of the problem, asking whether 'nest building' as a whole was 

 innate or not, seemed much too simple and consequently his conclusions 

 too strongly generalized. Therefore, I attempted to clarify the following 

 special problems, using experimental procedures as closely as possible 

 similar to those employed by Riess. The question was: 



1 The rats have furthermore to be fainihar with the presence of an observer, otherwise they 

 are irritated, showing curiosity or shyness. 



