THE RÔLE OF HORMONES IN THE INITIATION OF MATERNAL BEHAVIOR IN RATS

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Abstract
More than 2900 tests were made with 19 different hormones or hormonal preps. to determine the role of hormones in the exhibition of unlearned maternal behavior in rats. In responding rats whose mammaries had been developed with estrone during 20 days the validity of the criteria of response was attested by the ability of these rats actually to feed and rear young. The maternal drive was initiated in large or in significant numbers of rats of all sex types during inj. (or pellet implantation) with prolactin, progesterone, desoxycorticosterone, intermedin, LH, phenol and thyroxine; testosterone failed only in normal [male][male]. The drive was not activated by inj. with FSH, "growth" hormone, adreno-trophin, Prolan, pregnant mare serum, parathyroid extract nor by a special extract of thyroid tissue. Intact [male][male] were as likely as intact [female][female] to be "normal reactors," but non-reacting [male][male] were more resistant than (the slightly smaller) [female][female] to the maternalizing action of all the effective hormones. PMS, FSH + and estrone had appreciable ability to terminate well-established maternal behavior in most sex types. These substances did not exert this action in rats deprived of their hypophyses, but they were effective in the absence of the thyroid. Hypophysectomy alone precipitated maternal behavior in approx. 50% of otherwise intact rats, but not if a decrease of the ovarian function was prevented by injs. of pregnant mare serum. Non-reactors following hypophysectomy could, in most cases, be made maternal with either prolactin or progesterone. In [male] rats, thyroidec-tomy alone excited maternal behavior in about 25% of such tests. [male] rats not made maternal by this operation were more resistant than normal rats to the positive action of prolactin, progesterone and LH. Progressively larger doses of prolactin[long dash]6, 18, and 30 units[long dash]progressively increased the % of rats of all sex types made maternal by treatment. Recurrent maternal behavior in the reproducing animal (which the rats in these tests were not) probably does not depend upon the several hormones here found to be directly or very indirectly active, but upon that one of the group which is released in increased amt. at the right time; exerts an antigonad action; and then directly or indirectly increases the excitability of the sensorimotor mechanism specifically involved in this instinctive behavior. Though present information is inadequate the hormone which apparently best fits these requirements is prolactin. Genetic relationships and differences between mating drive and maternal drive seem clearer in the light of results of this study.