PARENTAL BEHAVIOR 



1319 



guish between the two cages: young rats 

 sprayed with oil of hivender, and subse- 

 quently dried, were retrieved by most fe- 

 males less efficiently and less rapidly than 

 untreated young. Visual stimulation is also 

 significant: when female rats were placed 

 in cages each containing a pair of glass bot- 

 tles sealed to the floor of the cage, only one 

 of which contained a young rat, 17 of the 

 25 females which investigated the bottles 

 spent more time at the bottle containing a 

 live young rat than at the empty bottle; on 

 retrieving tests in which intact females 

 were oftered normal young and motionless 

 young, there was a tendency for the active 

 young to be retrieved more rapidly. Al- 

 though freshly killed young were retrieved 

 almost as quickly as normal young, young 

 refrigerated for a short time after being 

 killed were retrieved significantly more 

 slowly. It is not possible to say whether 

 this difference in reaction is based upon 

 olfactory, temperature, or tactual cues. Al- 

 though a number of stimulus modalities are 

 clearly important, the rats could retrieve in 

 the absence of any one of the modalities 

 tested {i.e., vision, olfaction, cutaneous sen- 

 sitivity around the mouth). Animals de- 

 prived of any 2 of these sensory capacities 

 retrieved less efficiently than animals de- 

 prived of only 1, and animals rendered 

 blind, anaptic, and anosmic were the poor- 

 est retrievers of all. It is clear from this 

 and other work {e.g., McHugh, 1958) that 

 a variety of different stimuli in different 

 sensory modalities may contribute to the 

 stimulation of retrieving behavior. 



Younger mice apparently stimulate more 

 retrieving and other aspects of maternal 

 care than older animals (Wiesner and 

 Sheard, 1933). Leblond (1940) found that 

 0- to 1 -day-old mice were retrieved by lac- 

 tating females 83 to 85 per cent of the time, 

 whereas 15-day-old young were retrieved 

 only 11 to 15 per cent of the time, the de- 

 crease being a steady and gradual one, as 

 revealed by tests using young of intermedi- 

 ate ages. Younger animals provide stronger 

 stimuli for retrieving than older ones even 

 for lactating mothers whose own young are 

 older. Indeed, Wiesner and Sheard (1933) 

 report that, at the end of lactation, the 

 grown-up litter no longer represents an ade- 

 quate stimulus for retrieving, although the 



mother may vigorously retrieve newborn 

 young presented to her. Menzel and Menzel 

 (1953) replaced older puppies with very 

 young ones, and found an increase in ma- 

 ternal care by the mother dog. Presented 

 with newborn puppies, she acts just like an 

 immediately postpartum female. This re- 

 placement of older by younger puppies can 

 be effected twice in one lactation period, 

 with the same results. 



The most exact and quantitative study 

 of this problem is the very recent one by 

 Rowell (1960c), who studied the retrieving 

 responses of lactating female golden ham- 

 sters at various ages postpartum, in re- 

 sponse to young of various ages. She found 

 that variations in the amount of maternal 

 retrieving are influenced by three types of 

 variables: (a) the time since the birth of 

 her own litter; (b) similarity between the 

 age of her own young and that of the test 

 young; and (c) variations in the stimuli 

 coming from the test young, which elicit 

 retrieving responses when they are 7 to 10 

 days old more efficiently than they do at 

 any other age, regardless of the kind of fe- 

 male being tested. Rowell further found that 

 the amount of time spent licking the young, 

 after having retrieved them, decreased 

 steadily and reliably during lactation. Us- 

 ing a litter-replacement technique, she kept 

 various groups of mothers with young of 

 different ages, so that, e.g., some mothers 

 constantly had 2- to 6-day-old litters, some 

 10- to 14-day-old litters, etc. Under these 

 conditions, the decrease in licking time, 

 characteristic for normal litters, did not oc- 

 cur. Further, these females licked their pups 

 for approximately the same amount of time 

 as was characteristic, in a normal litter, for 

 young of the same age as the test pups. 

 Thus the change in licking time, which al- 

 ways occurs postpartum, is a function of 

 growth changes in the ability of the young 

 to stimulate licking behavior, rather than 

 of physiologic changes in the mother. 



The stimuli inducing retrieving behavior 

 are not necessarily specific to the species, 

 as there are many instances recorded of 

 small mammals retrieving young of other 

 species. The laboratory rat readily retrieves 

 young mice (Wiesner and Sheard. 1933; 

 Shadle, 1945; Herold, 1954; Beach and 

 Jaynes, 1956b). When mother laboratory 



