PARENTAL BEHAVIOR 



1327 



suckling stimulation on the prohiftin con- 

 tent of the pituitary gland. Meites and 

 Turner (1942a) divided parturitive rats 

 into two groups. The mothers composing 

 the control group were allowed to suckle 

 their young normally, whereas the young 

 of the experimental group were taken away 

 at parturition. At 7 days postpartum, the 

 pituitary glands of all the animals were re- 

 moved and assayed for prolactin content 

 by a pigeon-crop method. The average 

 amount of prolactin per pituitary gland in 

 the control (suckled) group was 10.75 

 Reece-Turner units, whereas in the experi- 

 mental (nonsuckled) group, it was 5.10. In 

 a more recent study, the same authors 

 (Meites and Turner, 1948) repeated this 

 experiment, and extended it to rabbits, with 

 similar results. In the latter species, the 

 prolactin content of the pituitary gland 

 rose to a peak about 5 days postpartum 

 in nonsuckled as well as in suckled animals, 

 but the peak was much higher (87 per cent 

 more prolactin per pituitary gland) in the 

 suckled animals and the prolactin content 

 fell to the prepartum level much more 

 rapidly in the nonsuckled animals (Meites 

 and Turner, 1942c, 1948; Meites, 1954). 

 Rabbits nursing larger numbers of young 

 (5 to 11) had no more prolactin in their 

 pituitary glands than did those nursing only 

 two young (Meites, Bergman and Turner, 

 1941). 



As might be inferred from the observa- 

 tions on lactation described earlier, suckling 

 stimulation, although necessary for the 

 maintenance of prolactin secretion during 

 lactation, does not seem to be necessary for 

 the induction of the immediate postpartum 

 rise in pituitary prolactin content. Reece 

 and Turner (1937b) found that the post- 

 partum increase in pituitary prolactin con- 

 tent, measured 51 hours after parturition, 

 was the same in animals that had been nor- 

 mally suckled and in animals whose young 

 had dropped through a wide wire-mesh 

 screen as thev w^ere born. 



Although suckling stimulation thus un- 

 doubtedly contributes to the maintenance 

 of a high level of prolactin in the pituitary 

 during the lactation period, the immediate 

 effect of a single suckling episode is to de- 

 plete the ])rolactin content of the gland. 

 Recce and Turner (1936, 1937a, b) re- 



moA'cd young rats from their mothers at 36 

 hours postpartum. At 48 hours postpartum, 

 the young of some of the mothers were per- 

 mitted to remain with their mothers for 3 

 hours, during which they suckled. The pi- 

 tuitary glands of these mothers assayed at 

 5.20 Reece-Turner units of prolactin, com- 

 pared with 9.20 units in mothers treated 

 exactly the same way, except that they were 

 not allowed the 3-hour suckling period with 

 their young. Clearly, the suckling stimulus 

 causes the release of prolactin from the pi- 

 tuitary gland, and the depletion of its pro- 

 lactin content. Grosvenor and Turner 

 (1957) found similar results in animals 

 tested 14 days postpartum. These mother 

 rats were isolated from their young for 10 

 hours. At the end of this isolation period, 

 the control animals were killed for autopsy, 

 whereas the experimental animals were 

 suckled for exactly 30 minutes. The results 

 indicate that the prolactin content of the 

 pituitary gland was reduced about H by 

 this suckling stimulation. 



Suckling stimulation seems to affect the 

 cytologic appearance of the pituitary gland. 

 Gonadectomy in the female rat induces an 

 increase in the number of basophilic cells 

 in the pituitary gland, in many of which 

 large nucleoli form (see chapter by Purves). 

 Desclin (1936, 1947) removed the ovaries 

 of female rats at parturition. Half of his 

 animals were allowed to keep and nurse 

 their young, whereas the young were re- 

 moved from the other half. The suckled 

 animals did not develop castration signs in 

 their pituitary glands. Dawson (1946) 

 found that cytologic changes in the pitui- 

 tary gland of the cat which normally occur 

 during lactation, do not occur after par- 

 turition unless the mother cat is allowed to 

 suckle her young (see chapter by Purves). 



( h I On the mammary gland. Most of the 

 studies to which we have referred seem to 

 contain the implication that the principal 

 effect of suckling stimulation with respect 

 to the stimulation of lactation is the stimu- 

 lation of prolactin secretion by the pitui- 

 tary gland. It is, of course, quite clear that 

 prolactin has a substantial influence on milk 

 secretion. The involution of the mammary 

 glands which follows forced weaning in mice 

 can be delayed by prolactin injection 

 (Hooker and Williams, 1941), and local 



