PARENTAL BEHAVIOR 



1337 



experimental birds. In such cases, the par- 

 ent birds often chase and peck at young 

 which were unlike the ones they had 

 adopted, even though the "strange" young 

 are of their own species. Tinbergen (1939a I 

 found that parent herring gulls react to all 

 young gulls indiscriminately until their 

 young are about 5 days old, after which 

 they recognize their own and attack others. 



It is thus apparent that individual recog- 

 nition of, and response to, particular young 

 animals frequently develops on the basis 

 of originally relatively undifferentiated re- 

 sponses, based in part on the i)hysiologic 

 condition of the parent. 



Possible effects of earlier experience. 

 Experience gained earlier in life, before par- 

 turition, may of course have an effect on 

 the development of parental behavior, and 

 several attempts have been made to demon- 

 strate this. Riess (1950, 1954) reared female 

 rats without access to anything manipulata- 

 ble; their food was finely pulverized, no 

 nesting material or bedding was permitted 

 in the cages, and the floor was constructed 

 of wide-mesh wire, so that the feces would 

 fall through the floor and be unavailable 

 for carrying. The animals were isolated 

 from other rats. Riess found that animals 

 so reared, when made pregnant and given 

 nesting material, showed no nest-building 

 behavior and decreased retrieving behavior ; 

 there was an infant mortality of 75 per cent 

 due to the absence of nursing behavior. The 

 experimental animals did tear the strips of 

 paper (provided as nesting material) from 

 their holders, but carried them about and 

 left them at random on the floor of the test- 

 ing chamber. The pups were likewise car- 

 ried about the cage without being gathered 

 into one area. In a less drastic limitation of 

 the animals' environment during develop- 

 ment. Kinder (1927) had found that rats 

 reared in cages without paper did as much 

 nest building, when later tested, as those 

 reared in cages with paper, but that the 

 amount of nest-building on the first day of 

 testing was less for the animals reared with 

 restricted experience. 



In a previous discussion of this prol)lem 

 (Lehrman, 1953, 1956b) I implied that Riess 

 showed that the animals must learn to carry 

 nesting material, and that practice in carry- 

 ing food pellets is, for this purpose, equiva- 



lent to practice in carrying nesting material. 

 Eibl-Eibesfeldt (1955a, 1956) has shown 

 that this conclusion is incorrect, and has 

 thrown further light on the nature and limi- 

 tations of the learning process involved. He 

 reared female rats in a manner similar to 

 that devised by Riess, and tested them at the 

 age of 10 to 12 weeks by placing nesting ma- 

 terial in their living cages, instead of by 

 moving the animals to a test cage, as had 

 Riess. He thus avoided any interference of 

 exploratory behavior with maternal behav- 

 ior. Of 29 animals so tested, 8 began build- 

 ing immediately, whereas 3 additional ani- 

 mals began within the first hour. Eleven 

 acted like Riess' rats, carrying nesting ma- 

 terial to and fro in the cage, eventually 

 dropping it at random. These animals, how- 

 ever, began to restrict themselves to a single 

 location in the cage after a few hours of such 

 activity, and had built nests by the follow- 

 ing morning. Five animals carried nesting 

 material to a single corner of the cage, and 

 there gnawed and played with it, but did not 

 build a nest until the following day. Two of 

 the 29 animals did no building at all. Ob- 

 servations carried out before the introduc- 

 tion of the nesting material showed that 9 

 of the 29 had established sleeping places in 

 the cage, although the remaining animals 

 had no fixed sleeping locations. The 8 ani- 

 mals which began to build immediately be- 

 longed to the group which had fixed sleeping 

 locations. For a further group of experimen- 

 tal animals, Eibl-Eibesfeldt placed a small 

 vertical partition in a corner of the cage so 

 as to make a cubicle open to the rest of the 

 cage. Of the 19 animals tested with this par- 

 tition, 18 built in this corner immediately 

 after nesting material was introduced for 

 the first time. 



It appears from Eibl-Eibesfeldt's data 

 that the failure of nest-building by some of 

 the animals in his and Riess' experiments 

 was due not to a lack of development of the 

 basic responses of picking up and carrying 

 nesting material, but to the failure of the 

 animals to develop an attachment to a par- 

 ticular place in the cage. Further, the more 

 differentiated the living space, and therefore 

 the more stimulation it can offer to attract 

 the animal into one part of the cage, com- 

 pared to other areas, the less necessity there 

 is for an extended period of development of 



