LIFE PROCESSES IN CAPTIVE GRAY RATS 51 



female had access to them as soon as they had replaced her 

 own offspring. 



Two of the outstanding traits exhibited by individuals in 

 early generations still persisted at the twenty-fifth generation. 

 Adults continued to show pronounced antipathy to individuals 

 from other litters placed in their cage, and promptly attacked 

 and usually killed them unless special precautions were taken, 

 as described in the previous report. Even these devices were 

 not always effective. This treatment of strangers seems to 

 indicate a retention of the primitive instinct to protect the 

 nest from all entrants not members of the family group. The 

 'killer' instinct, discussed in a previous section, has been 

 shown by gray rats of all generations, especially by large 

 males. This trait is doubtless of advantage to the species, 

 since the elimination of smaller and weaker individuals in- 

 sures that the largest and most vigorous animals will be the 

 progenitors of the succeeding generation. In late generations 

 it was no longer necessary, though often expedient, to stupefy 

 rats with ether when they were removed from the cage for 

 weighing, as they could be picked up by the tail with long 

 forceps but not with bare hands. 



As this investigation was designed to study the effects of 

 captivity on gray rats, no attempts were made to tame any 

 of the rats used in this work so that they could be handled 

 as are the rats of various strains maintained for general 

 laboratory purposes. While captivity over many years had 

 accustomed these rats to receiving their food from man and 

 to his presence, their innate fear of him did not decrease to 

 the point where they would willingly submit to being held by 

 bare hands. Gray rats can be tamed more easily, perhaps, 

 than any other feral animals, if one has the required patience 

 and understanding of their temperament. Two young rats 

 from the third generation were taken to another colony by 

 Miss Ruth Meeser, of The Wistar Institute, and rendered so 

 tame that they showed no vicious traits when adult, although 

 they were always nervous in the presence of strangers and 

 could be handled only by Miss Meeser. More recently, a num- 

 ber of young gray rats were given to Miss Meeser for taming, 



