Influence on Reproductive Behavior

The influence of the endocrine system on behavior has been studied on
many levels. Much of the work has been done on animals; laboratory rats
have been the most frequently used subjects. There is, however, a growing
body of information on hormonal effects on a variety of human behaviors,
including reproductive and developmental behavior, reaction to stress,
learning, and memory. Studies carried out in reproductive and developmental
biology on both animal and human subjects have substantiated the
belief that hormones influence mating behavior, developmental events including
sexual differentiation, and female and male sexuality.
Castration experiments have linked the testes with a male mating behavior
pattern in animals. The sexually active male animal aggressively seeks
and attempts to mount the female, whether she is receptive or not. The castrated
male retains the ability to mount a female but loses the aggressiveness
and does not persistently pursue females. The male may assume the more
submissive female behavior and even engage in homosexual encounters.
Normally, the release of reproductive hormones in the male is noncyclic,
whereas in the female it is cyclic. Castrated animals begin to exhibit the female,
cyclic pattern of hormone release. The hormonal influence is confirmed
by administering androgens (male hormones) to the castrated animals.
Male mating behavior and the noncyclic release of hormones returns.
The presence of male hormones has an effect on the female cycle and
sexual receptivity. Pheromones are substances secreted on the body of one
individual which influence the behavior of another. These chemical messengers
function during mate attraction, territoriality, and episodes of fear.
Their existence and functions are well documented throughout the animal
kingdom, especially among insects and mammals. In experiments using
rats, it was shown that the pheromones act in conjunction with male hormones
in bringing the female to a state of receptivity. The urine of noncastrated
male rats contains androgens. When a male rat is introduced into
a cage of sexually inactive females, the male sends off chemical signals by
way of pheromones and the androgen-containing urine. The result is the accelerated
onset of estrus, or sexual receptivity, on the part of the females.
Castrated males produce pheromones but do not have androgens in the
urine. When castrated males are introduced into a cage of inactive females,
the estrous cycle is not affected.
All female mammals, with the exception of monkeys, apes, and humans,
experience estrus. Under hormonal control, the female is receptive to the
male once or twice a year, when her eggs are available for fertilization. This
period of receptivity is known as the estrous phase, or heat. Research shows
that the particular female hormone which induces estrus is progesterone.