The high levels of estrogen and progestin during early pregnancy lay a necessary hormonal base for the reproductive fulfillment state that is characterized by operation of the parental motivational system and heightened aggressiveness, both offense and defense,
The parental motivational mechanism, activated by estrogen and by declining levels of progestin that occur at parturition (brain site 5), ensures that the mother will build a nest, clean and suckle her young, and retrieve them or move them from one nest to another. It may also cause the secretion of prolactin that stimulates the production of pheromones that attract her infants (see I-H). The heightened aggressiveness of the lactating female is due to circulating prolactin that is stimulated by the suckling of the young. The prolactin has two types of action: it increases offense against conspecifics by facilitating the offense motivational mechanism directly (site 1); and it increases defense rather than submission against conspecifics by suppressing the consociate modulator (site 6). This latter effect is particularly advantageous to the lactating female because of the antagonistic tendency of conspecifics to destroy the litters of other individuals (St. John and Corning, 1973; Svare and Gandelrnan, 1976b). For example, both male and female mice will cannibalize the young of strange conspecifics (St. John and Corning: 1973); Hahn et a1, 1977), and under conditions of overcrowding such cannibalism can suppress population growth (Brown, 1953; Southwick, 1955).
There is a contradiction between the hormonal state of the female reproductive fulfillment state and the "anti-gonad system" of the reproductive postponement state. At low levels of social stress the secretion of adrenocortical hormones is suppressed by the action of prolactin (site 15). This may help maintain maternal behavior in the face of mild social disruption. At higher levels of social stress, however, the secretion of adrenocortical honnones may occur and parental behavior may be disrupted (site ,), as has been documented in laboratory mice (Brown, 1953) and rats (Calhoun, 1962b).
There is an evolutionary advantage for a higher threshold of anti-gonad action upon parental behavior than upon reproductive readiness, because in the case of parental behavior, the female has already made a substantial metabolic investment in her infants and she has more to lose.