Hormones and Reproduction

The most extensive research involving hormonal effects on behavior has been conducted on reproductive behavior. Among the most powerful behavior-

Hormones control a variety of behaviors in humans and animals, such as maternal imprinting. (Digital Stock)

influencing hormones are the pituitary gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These two hormones target the reproductive organs of both males and females and stimulate these organs to initiate sexual development and the production of sexual steroid hormones—estrogen and progesterone in females, testosterone in males. These sex hormones are responsible not only for the maturation of the reproductive organs but also for secondary sexual characteristics such as male aggression and female nesting behavior.

Reproductive patterns vary from species to species in occurrence, repetition of occurrence, and behaviors associated with courtship, mating, and caring for young. The achievement of reproductive maturity and reproductive readiness in a given species is subject to that species' circadian rhythm, a phenomenon regulated by hormones released from the hypothalamus, hypophysis, and pineal gland. These three endocrine glands are influenced primarily by the earth's twenty-four-hour rotation period and the twenty-eight-day lunar cycle. Furthermore, genetically programmed hormonal changes at specific times during one's life cycle also play a major role in the occurrence of reproductive behaviors.

In female vertebratesm, LH, FSH, and estrogen are responsible for the maturation of the ovaries, the completion of meiosis (chromosome halving) and the release of eggs for fertilization, and secondary sexual characteristics. The secondary sexual characteristics involve physiological and closely related behavioral changes. In bird species, these changes include the con struction of a nest and receptivity to dominant males during courtship rituals. In mammals, these same hormones are involved in female receptivity to dominant males during courtship. Physiological changes in mammals include the deposition of fat in various body regions, such as the breasts and buttocks, and increased vascularization (more blood vessel growth) in the skin. Females of most mammal and bird species go into heat, or estrus, one or several times per year, based on hormonally regulated changes in reproductive organs. Human females follow a lunar menstrual cycle in which LH, FSH, estrogen, and progesterone oscillate in production rates. These hormonal variations influence female body temperature and behavior accordingly.

Male sexual behavior is controlled predominantly by testosterone produced in the testicles and male androgens produced in the adrenal cortex. These steroid hormones cause muscle buildup, increased hair, and aggressive behavior. As a consequence, such steroids are often used (illegally) by athletes to improve their performance. In a number of mammal and bird species, elevation of sex steroids causes increased coloration, which serves both as an attractant for females and as an antagonistic signal to competitor males. The aggressive behavior that is stimulated by the male sex steroid hormones thus plays a dual role in courtship and mating rituals and in territorial behavior, phenomena which are tightly linked in determining the biological success of the individual.

Pheromones are hormones released from the reproductive organs and skin glands. These hormones target the sense organs of other individuals and affect the behavior of these individuals. Sex pheromones, for example, attract males to females and vice versa. Other pheromones enable a male to mark his territory and to detect the intrusion of competitor males into his territory. Others enable an infant to imprint upon its mother. Such hormones number in the hundreds, but only a few dozen have been studied in detail. Pheromones released by males serve as territorial markers, as is evidenced by most mammalian males spraying urine on objects in their own territory. Exchanges of pheromones between males and females are important stimulants for courtship and mating. In some species, the release of pheromones—or even the sight of a potential mate—will trigger hormo-nally controlled ovulation in the female. Furthermore, in several species, such as elephant seals and lions, the takeover of a harem by a new dominant male, a process that usually involves the killer of the previous male's offspring, stimulates the harem females to ovulate. The diversity of reproductive behaviors that is regulated by hormones seems to be almost as great as the number of species.

Hormones and Stress

The fight-or-flight response is a hormonally controlled situation in which the body must pool all of its available resources within a relatively short time span. The detection of danger by any of the special senses (sight, smell, hearing) triggers the hypothalamus to activate the pituitary gland to release adrenocorticotropic hormone, which causes the adrenal gland to release its highly motivating hormones and neurotransmitters. Many body systems are subsequently affected, especially the heart and circulatory system, the central nervous system, the digestive system, and even the immune system. One reason the fight-or-flight response is of major interest to psychologists is its link to stress.

Stress is overexcitation of the nervous and endocrine systems. It is caused by the body's repeated exposure to danger, excessive physical exertion, or environmental pressures that affect the individual psychologically. Stress is a major problem for humans in a fast-paced technological society. The physiological and behavioral manifestations of stress are very evident. There is considerable evidence that stress is associated with heart disease, cancer, weakened immune systems, asthma, allergies, accelerated aging, susceptibility to infections, learning disorders, behavioral abnormalities, insanity, and violent crime.

The demands that are placed upon individuals in fast-paced, overpopu-lated societies are so great that many people exhibit a near-continuous fight-or-flight response. This response, in which the body prepares for maximum physical exertion in a short time span, is the physiological basis ofstress. It is not intended to be maintained for long periods of time; if it is not relieved, irreparable effects begin to accumulate throughout the body, particularly within the nervous system. Medical psychologists seek to understand the hormonal basis of physiological stress in order to treat stress-prone individuals.

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