Hormones and behavior (zoology)

Behavioral differences are usually attributed to two causes: differences in experience (learning) and differences in heredity (genes). Hormonal interaction involving an organism, its experience, and the highly specific behaviors that result are well illustrated by Daniel Lehrman’s 1964 study of the ring dove, a small relative of the domestic pigeon.

A male ring dove begins courtship by bowing and cooing to the female when they are placed together in a cage. Toward the end of the first day of courtship, the birds choose a location and start building a nest. The doves court, nest-build, and mate, and the female lays an egg about five o’clock in the afternoon on the seventh to eleventh day. A second egg is laid about nine o’clock the next morning. The male sits for about six hours at midday, and the female occupies the nest the rest of the time. After fourteen days of incubation, the eggs hatch, and the parents feed the squabs crop milk, secreted from the lining of the adult dove’s crop (enlarged gullet). At about two weeks of age, the squabs (fledgling birds) begin pecking at grain, and the adults feed them less and less. With feeding chores diminished, the male begins bowing and courting, the pair starts nest-building, and the cycle repeats itself.

The Hormonal Triggers of Behavior

The simplicity of this description belies the hormonal ferment going on beneath the placid exterior. The courtship ritual causes the production and release of estrogen and progesterone, hormones responsible for setting behavior and for the development of the oviduct. The oviduct develops from eight hundred milligrams, when the doves are placed together, to four thousand milligrams, when the first egg is laid. Birds presented with nests already containing eggs when they are first paired will build nests on top of the eggs. Even if the eggs are returned to the top of the nest by the investigator, the doves will not set until they have engaged in nest-building for five to seven days. On the other hand, if the doves are first injected with progesterone, 90 percent will set within three hours after pairing. Courtship and nest-building play a vital role in creating the hormonal conditions necessary for setting behavior. Development of the crop is, in turn, initiated when setting begins. Setting behavior causes the release of prolactin, which stimulates crop development and feeding behaviors. Unpaired doves injected with prolactin will develop crop milk and will feed squabs when exposed to them, even if they have not mated. While they will feed squabs, however, they will not sit on eggs, because they have not courted, engaged in nest-building, or developed the hormonal balance necessary to support setting behaviors.

This ring dove study indicates that the behavior of one individual can alter the behavior and the hormonal balance of another individual; that an individual’s behavior can alter its own hormonal balance and, hence, its own behavior; and that inanimate aspects of the environment (nest and eggs, for example) can alter an individual’s hormonal balance and, hence, its behavior. The interactions among these three factors are complex. For example, the male’s behavior (courting) changes the female’s behavior (nest-building), which changes her hormonal balance (estrogen and progesterone), which causes her to alter the external environment (lay eggs in the nest), which affects the pair’s behavior (setting), which changes their hormonal balance (prolactin), which stimulates them to feed the squabs.

Fetal Hormones Affecting Adult Behavior

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 littermates, 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.

However, some scientists noted that the literature lacked a well-rounded perspective on testosterone's impact on animal behavior and hypothesized that testosterone's impact was far more nuanced than simply increasing aggression and decreasing prosocial behaviors. For their investigation, they used Mongolian gerbils, animals that form long-lasting pair bonds, and both share in raising their young. The scientists observed increased cuddling behavior and the instinct to protect their offspring with increased testosterone. They determined that testosterone positively impacts the neural activity of oxytocin—a hormone that enforces social bonding. So, while it does have negative implications for behavior, testosterone also encourages contextually specific pro-social behaviors.

Scientists have shown that pregnant rhesus monkeys treated with testosterone produced offspring that showed rougher play and more threatening 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 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 gonads respond by increasing in size and producing additional hormones, which stimulate the bird to begin its long migration.

When the male white-crowned 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 the 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 idea 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.

Experimental Endocrinology

The earliest report of experimental endocrinology, in the mid-nineteenth century, demonstrated that replacements of testicular tissue would maintain comb growth and sexual behavior in castrated roosters. Techniques for determining endocrine function used today include ablation and replacement.

Ablation (removal) of endocrine tissue results in deficiency symptoms. The effects of ablation are not always unambiguous. If the testes and accessory tissues are not completely removed when a horse is castrated, for example, tissue capable of producing testosterone remains, and the consequence is an infertile gelding that behaves like a stallion.

Hormones produced by different glands can have similar physiological effects. Both the adrenal glands and the testes produce androgens (masculinizing hormones). Sexually experienced male cats do not lose their sex drive if castrated, and researchers do not have a satisfactory answer as to why this occurs. Perhaps the adrenal hormones are sufficient to maintain established feline male sexual behavior but not sufficient to initiate it in inexperienced cats. The ablation of the adrenal glands, however, has severe consequences in terms of electrolyte and blood glucose imbalances that are life-threatening. Replacement of ablated endocrine tissue can reinstate normal function. If a male cat is castrated as a kitten, it will not develop normal male sexual behaviors. If, however, a normal testis is later transplanted to the abdominal cavity (or elsewhere), normal behavior will develop.

In the early 1960s, Janet Harker became convinced that the “biological clock” controlling the daily activity cycle of the cockroach was contained in the subesophageal ganglion, a patch of nervous tissue the size of a pin head resting just below the esophagus. When she ablated this ganglion, the cockroach became arrhythmic. Harker removed the legs from a normal roach and glued the roach on top of the arrhythmic roach, surgically uniting their body cavities so that the same body fluids circulated through both roaches. The arrhythmic roach ran about the cage with an activity rhythm dictated by the rhythm of hormones released into the body fluids by the legless roach on its back.

Most hormone and behavioral studies involve nonhuman species and most involve sexual behaviors. Most behaviors are oriented toward perpetuating one’s species. Only those individuals with behaviors conducive to rearing offspring will provide the genetic basis for behaviors represented in the next generation. Mate selection, shelter-seeking, feeding, maintenance of social position, and a host of other behaviors are critical to the success of one’s progeny. Animals that produce more offspring than they can feed or protect usually rear fewer than those who produce fewer to begin with. Many predator species, like owls, deer, and wolves, may avoid mating in years when prey is scarce. This ultimately maximizes reproduction by reserving energies that can best be spent later. This restraint is mediated by adjusting hormonal levels. Hormones have been called the ultimate arbiters of sexual behavior.

The Endocrine System, the Nervous System, and Behavior

The nervous system is usually thought of as the mediator of behavior, but the endocrine system is also a major player. Arguing that one system is more important would be like deciding whether height, width, or depth is more important in describing a box. It is useful, however, to discuss their differences. The nervous system has a shorter response time. Nerve impulses travel at speeds of up to 120 meters per second. Hormonal effects are much slower but are less transitory. If frightened by a false alarm, an animal may jump and run as a direct consequence of nervous system activity, but even after it recognizes that there is no real threat, it will be “keyed-up.” This is a consequence of hormonal activity: Fright triggers the release of epinephrine (adrenaline) and norepinephrine from the adrenal glands. These hormones cause increased cardiac output, increased blood supply to the brain, heart, and muscles, decreased blood flow to the digestive tract, dilation of airways to breathe more efficiently, and a significant increase in metabolic rate.

This is called the “fight or flight reaction,” and it will affect behavior for several minutes and possibly for hours. These hormones enhance perceptions and elevate the responsiveness of the nervous system. In the final analysis, the understanding of hormones, what determines their ebb and flow, how they are affected by the environment, how hormones interact with one another, and how their levels are controlled by genetic programs of the individual and of the species is essential for the understanding of behavior.

Principal Terms

Ablation: The technique of removing a gland to determine its function and observe what effects its removal will precipitate

Androgens: Masculinizing hormones, such as testosterone, responsible for male secondary (anatomical) sex characteristics and masculine behavior

Behavior: An animal’s movements, choices, and interactions with other animals and its environment

Biological Clock: A timekeeping mechanism that is “endogenous” (a part of the animal) and capable of running independently of “exogenous” timers such as day-night cycles or seasons, although the clock is normally set by them

Endocrine System: A collection of glands that secrete their products into the bloodstream

Estrogen: A feminizing hormone responsible for female secondary (anatomical) sex characteristics and sex-related behaviors

Prolactin: A hormone responsible for secretions of milk from the mammary glands of mammals and from the crops of birds

Receptor Molecule: A molecule on the cell membranes of target tissues that binds to the hormone molecule and initiates the action of the hormone

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