PARENTAL BEHAVIOR 



1325 



vasoconstrictor action, which interferes 

 witli the access of oxytocin-carrying blood 

 to the alveoli. 



It should, however, not be concluded tliat 

 there is no central inhibition of oxytocin 

 secretion as a consequence of emotional 

 arousal. Cross (1955b) found that when 

 rabbits were suckled while forcibly re- 

 strained, the amount of milk removed (es- 

 timated from the weights of the pups) was 

 substantially reduced (by 20 to 100 per 

 cent in different animals). This reduction, 

 however, could be restored in about 80 per 

 cent of the 35 subjects by the injection of 

 oxytocin. This suggests that part of the 

 effect of emotional inhibition is the reduc- 

 tion of oxytocin secretion. Cross suggested 

 that the experiments involving exogenous 

 adrenaline probably involved much more 

 adrenaline than is usually present in the 

 blood as a result of emotional disturbance, 

 and that the vasoconstrictor effect of ad- 

 renaline in those experiments may thus have 

 been greater than that which normally 

 occurs. 



In any event it seems probable that both 

 peripheral effects of adrenaline on the mam- 

 mary glands and central inhibition of oxy- 

 tocin secretion are involved in the mech- 

 anism of emotional inhibition of milk ejec- 

 tion. 



3. Mother-young Relationships and the 

 Regulation of Lactation 



Sickling and the duration of lacta- 

 tion. Hammond and Marshall (1925) ob- 

 served that the mammary glands of rabbits 

 became inactive when the young were re- 

 moved, whereas lactation was prolonged 

 when the young were kept with the mother. 

 Selye (1934) removed young rats from 

 their mothers at 3 days of age, and found 

 that milk secretion disappeared within 3 

 to 5 days, and that the alveoli began to 

 disappear after 6 to 8 days. When young 

 were left with the parents, lactation con- 

 tinued for the normal period of aj^proxi- 

 mately 3 weeks (Nicoll and Mcites, 1959). 

 Similar results were obtained in the guinea 

 pig (Hesselberg and Loeb, 1937a). Selye 

 and McKeown (1934a) repeatedly replaced 

 young mice by younger litters, so that the 

 mothers were provided with yoimg of suck- 

 ling age for a much longer period than nor- 



mal. Under these conditions, lactation was 

 considerably prolonged, although the mam- 

 mary glands could not be maintained in- 

 definitely solely by continuously offering 

 new young: signs of degeneration of the 

 alveoli were to be noted by about 28 days 

 postpartum. Nicoll and Meites (1959) re- 

 peated this experiment with rats, and found 

 that they could maintain active mammary 

 glands during a 70-day lactation period 

 (compared with the normal 20-day period) 

 by repeatedly replacing the litters. 



It is apparent that the normal duration 

 of lactation is in part regulated by stimuli 

 provided to the lactating mother by the 

 suckling young. This implies, of course, that 

 the suckling stimulus must be capable of 

 stimulating not only the posterior pituitary 

 substances responsible for the milk ejec- 

 tion reflex, but also, directly or indirectly, 

 must play a role in stimulating the secre- 

 tion of the anterior pituitary hormones re- 

 sponsible for lactation (see chapter by 

 Cowie and Folley). 



Suckling and the onset of lactation. 

 Although it is thus clear that suckling 

 stimuli stimulate the maintenance of lac- 

 tation, it seems that the onset of lactation 

 at the time of parturition does not reciuire 

 suckling stimuli (Meites and Turner, 

 1942b). Klein and Mayer (quoted by 

 Mayer and Canivenc, 1951) tied off the 

 uterine horns of pregnant rats, so that no 

 parturition occurred. Lactation was never- 

 theless initiated at the normal time for 

 parturition, without any stimulation of the 

 nipples. Meites and Turner (1942c) noted 

 that lactation begins at the time of parturi- 

 tion in nonsuckled rabbits, although it is 

 not maintained unless the animals are 

 suckled. Similarly, Williams (1945) ob- 

 served milk secretion in the mammary 

 glands of immediately postparturitive, un- 

 suckled mice. The glands are empty of milk 

 within 48 hours, and the alveoli have com- 

 pletely disappeared by the 6th day post- 

 partum. Further, lactation is not initiated 

 by i)resenting foster young during preg- 

 nancy (Masson, 1948; Mayer and Cani- 

 venc, 1951), even when the condition of the 

 nipple indicates that active suckling has 

 taken place. 



The prolactin content of the pituitary 

 gland normally rises just after parturition, 



