Title & Introduction
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Offense, Defense & Patrol/Marking
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Neural Circuitry & Motivational Mechanisms
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How Circulating Hormones May Affect Behavior
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Pregnancy and Lactation
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ACTH, etc.
Page 8
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Reproductive States
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Conclusion
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References
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HORMONES AND REPRODUCTIVE STATES
Considered in a broad ecological context, the various hormonal effects on the neural substrates of agonistic behavior may be understood as part of a more general role of hormones that establish and maintain the reproductive state of the individual. One may suppose that each individual has only a limited number of possible reproductive states and that at anyone time in its life it can be in only one-in other words, that these are not graded conditions, but all-or-none states. In terms of systems theory they are "complexes of stable, rhythmic limit cycles" (Iberall & McCulloch, 1969, p. 290). Circulating hormones playa critical role in the stability, rhythmicity, and limitation of the reproductive states; each state is established and maintained by a particular hormone that is secreted as part of a positive feedback chain of endocrine interactions.
The "courtship reproductive state" is established and maintained by high levels of the gonadal hormones, androgens in the male and estrogens in the female. These hormones facilitate the patrol/marking motivational system (both motivational mechanism and sensory analyzers) and the motivational systems of male and female sexual behavior. In males, androgens facilitate offense against other males but inhibit offense against females, and androgens facilitate patrol/ marking in response to females but inhibit it in response to males. In females, estrogens have the opposite effects; they inhibit offense against males and facilitate patrol/marking in response to males. Thus, males patrol and exclude other males from their territories and they seek out and mate with females, while females patrol during estrus and seek out and mate with males. Courtship is not interrupted or suppressed by offense, because males are inhibited from attacking estrous females and estrous females are inhibited from attacking males. Although these effects sound complex when considered in a behavioral context, they may be mediated by only four sites of hormonal action in the brain, as shown in Figure 1. Of course, there are other effects of gonadal hormones besides those shown; for example, they facilitate the sexual motivational systems and stimulate production of pheromones from scent glands, which are then spread by the scent-marking behaviors activated by the patrol/marking motivational system. The courtship reproductive state may be maintained by a positive feedback chain that may be visualized in Figure 1: Activation of patrol/marking stimulates the release of gonadotropins as a motor pattern, gonadotropins stimulate the production of gonadal hormones, and gonadal hormones, in turn, facilitate both sensory analyzers and the motivational mechanism of patrol/marking.
The "pregnancy reproductive state" is established and maintained by high levels of progestins in the female. As a result, there is an increased level of defense, offense, and nest-building, by which the female prepares and defends the site for the nest that will house the young following paturition. This state might also be called a "luteal reproductive state" since the progestins are secreted by the corpora lutea, which are induced by copulation either directly or indirectly, and if the copulation is not successful, the animal undergoes a pseudopregnancy with many of the same behavioral characteristics as pregnancy. In most species of muroid rodents, the corpora lutea are directly stimulated by copulation; in the microtines, however, they are caused only indirectly by copulation, which causes ovulation after which the corpora lutea develop spontaneously (Dewsbury, 1977).
The "lactation reproductive state" is characterized by high levels of prolactin and other hormones in the female. Possibly as a result of these hormones, there is an increased level of offense and defense by which the female protects her young. Also, as a result of the prolactin, the female produces milk for the young, who in turn stimulate the parental motivational system. As a result, the motor patterns of the parental motivational system are activated, including nest-building, nursing, pup-retrieval, and grooming of the young. There are apparently several positive feedback chains involved in the establishment and maintenance of this reproductive state, but they have not yet been fully understood.
There are other states that may be called the "reproductive postponement states." One such state, triggered by repeated defeat (submission) or predator pressure (defense) is established and maintained by the pituitary-adrenal hormones ACTH and corticosterone. These hormones facilitate submission, inhibit defense, and suppress the gonadal hormone system. As a result, the animal does not engage in patrol/marking (i.e., does not maintain a territory) or engage in sexual behavior or offense. Also, and perhaps most crucial, because of the reduced levels of gonadal hormones, males in this state do not produce the androgen-dependent pheromones that increase their "attackability" (Leshner, 1975). Males in this state are not attacked. Females in this state do not reproduce; thus they do not run the greater risk and waste their metabolic energy on pregnancy and lactation under conditions that are not propitious for successful reproduction. Since pituitary-adrenal activation not only activates the consociate modulator but also is a motor pattern of submission (Figure 1), there is a potential here for a positive feedback chain that may establish and maintain the reproductive postponement state.
Another type of "reproductive postponement state" occurs during winter for muroid rodents that live in temperate climates. Like the state mediated by corticosteroids, it includes suppression of gonadal hormone production, and therefore it suppresses the offense and patrol/marking motivational systems. Its hormonal basis is not yet well understood (Ellis & Turek, 1980; Morin, Fitzgerald, Rusak, & Zucker, 1977), although it has been determined that the adrenal cortex is not essential (Bittman & Goldman, 1979; Ellis & Turek, 1980).
The fitness value of reproductive postponement states, from the standpoint of evolutionary genetics, presumably derives from the enhanced ability of an animal to reproduce at a later period of its life following the end of the postponement state. Documentation of this under natural conditions is difficult, because it requires the long-term observation of large numbers of individually recognizable animals. One such study has been done on the arctic ground squirrel (Carl, 1971). Over a period of two seasons, approximately half of the adult males obtained territories and the other half remained as "floaters," subordinate males who had apparently entered a reproductive postponement state. Of the 22 floater males, 5 were able to obtain territories later on the disappearance of the former territory holder.
There may also be a reproductive state associated with emigration, although if there is such a state its hormonal basis is not yet known. The behavior of emigrating animals, including locomotor activity and defense, is different from that of animals in the other reproductive states described above (Fairbairn, 1978; Myllymaki, Aho, Lind, & Tast, 1962). By analogy to birds, which have a migrational reproductive state associated with specific hormone changes (Meier, 1973), one might look for hormonal differences in emigrating muroid rodents that might underlie the difference between their behavior and that of the animals who remain behind.
The process by which an animal switches from one reproductive state to another may be initiated by behavioral events and mediated by motivational systems such as patrol/marking and submission. The courtship reproductive state may be initiated by social stimuli and mediated by the following process: un-familiar conspecific odors, particularly those of the opposite sex, activate the patrol/marking motivational mechanism, which activates, as one of its motor patterns, the release of gonadotropin, which initiates, in turn, the courtship reproductive state. The reproductive postponement state may also be initiated by another type of social stimuli and mediated by the following process: Severe or repeated defeat by a con specific activates the submission motivational mechanism, which activates, as one of its motor patterns, the release of ACTH and corticosteroids, which in turn initiate the reproductive postponement state. Once a reproductive state is initiated, it maintains itself by positive feedback chains of hormone secretion until behavioral or other types of environmental stimuli (e.g., seasonal changes) initiate a different state.
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