Hormones produced by developing organisms have marked effects on adult behaviors. In mammals, the fetal testis becomes active and produces testosterone, then becomes inactive before birth and remains so until puberty. The changes resulting from this brief surge of testosterone are remarkable. They include the anatomical features that distinguish male and female genitalia, changes in neural anatomy, and the sensitivity of nervous (and other) tissues to adult hormones.
Rat and mouse fetuses have several litter-mates, which develop side by side in a common uterus, like peas in a pod. Male mice that develop between two male embryos are more aggressive as adults than males developing between females. Similarly, females that develop between two male embryos are more aggressive as adults than females that develop between other females. In another study, females from litters that were predominantly male showed more masculine behavior (such as the mounting of other females) than females from predominantly female litters. The explanation is that testosterone produced by the male embryos' testes is absorbed into the bloodstream of sibling embryos, altering their nervous systems and hence their behaviors. In cattle, testosterone produced by a bull calf twin affects the development of his heifer twin to the extent that she is usually sterile.
Scientists have shown that pregnant rhesus monkeys treated with testosterone produced offspring that showed rougher play and more threat behavior than usual. Male rhesus monkeys experience a decrease in blood testosterone levels within six hours after losing a fight to another male and are more submissive. These studies indicate that hormones play an important role in determining male-female behavioral differences.
Many biological phenomena are repeated or change intensity over and over again throughout the life of an individual. Examples include sleep-wake cycles, menstrual cycles, and the migration cycles of birds; these are repeated approximately once a day, once a month, and once a year, respectively. The regularity of these cycles led biologists to propose a "biological clock." The golden-mantled ground squirrel avoids freezing temperatures by going into hibernation once a year. Even if these squirrels are kept in constant conditions of light and temperature to deprive them of seasonal cues, they will enter hibernation once a year. These and other data lead researchers to believe that the clock resides within the animal. Although it can be reset by environmental cues, it can also run independently of them.
Hormones, Seasons, and Mating Behavior
A white-crowned sparrow, nesting in central Alaska, experiences dramatic seasonal changes and migrates to the southern United States or Mexico (more than three thousand kilometers) to avoid freezing. Central Alaska's short summer demands that the sparrow fly north as early in the spring as is safe and that it be prepared for mating and rearing chicks when it arrives. During the winter, the gonads atrophy to 1 percent or less of their breeding season weight. The bird's ability to sense the approach of spring depends on its sensing the increase in daylight. During the short winter days, the sparrow is content to stay in Arizona or Mexico, but as day length increases to fourteen or fifteen hours, the bird's hypothalamus releases hormones that stimulate the pituitary to release prolactin and gonadotropic hormones. The go-nads respond by increasing in size and producing additional hormones, which stimulate the bird to begin its long migration.
When the male white-crown arrives at his breeding grounds in central Alaska, he chooses a nesting territory, attacks any male territorial intruders, and attempts to attract a mate with his constant singing. Each female chooses a mate and helps him defend the nesting site. In the next few days she feeds to gain nutrients for egg production, and her estrogen levels rise rapidly, stimulating her to solicit mating. Once the eggs are laid, the gonads of both birds begin to atrophy, estrogen and testosterone levels decline, and prolactin levels increase and stimulate feeding of the young. As the gonads atrophy, the birds become less aggressive and the male stops singing.
As the young become independent, both parents enter a "sexual refractory period," during which the gonads will not respond to artificially increased day length as they would in the spring.
The birds feed voraciously, increasing body fat, which serves as fuel for the long trip south. In the next year, by early spring, the birds will have passed through the refractory period and be primed to respond to the increasing day length with a fresh hormonal flurry which will set them off on the long journey north.
Recognizing the existence of a refractory period is important. It underscores the ideas that while birds do respond to environmental conditions (day length), there is a given set of events through which the physiological machinery passes and that specific time parameters are dictated by the biological clock. White-crowned sparrows can be expected to show hormonal changes and migratory restlessness during springtime even if they had been caged and maintained in constant conditions. It is to the bird's advantage, however, to experience and recognize the seasonal changes in day length, because biological clocks tend to run a bit fast or slow. The actual measuring of day lengths allows the bird to reset that clock and arrive in Alaska at the most advantageous time for rearing a family of sparrows.
Studies of a closely related bird, the white-throated sparrow, indicate that the changes of behavior and physiology are primarily the result of two hormones: corticosterone from the adrenal cortex and prolactin from the anterior pituitary. Both hormones have daily peaks of secretion, but the timing of these daily peaks (relative to each other) changes with the seasons. If injections of these hormones are given with timing differences characteristic of specific seasons, the physiological and behavioral changes seen in the birds are characteristic of the seasons that the injections mimic.
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