Hormones and Reproduction

The most extensive research involving hormonal effects on behavior has been
conducted on reproductive behavior. Among the most powerful behaviorinfluencing 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 twentyeight-
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 th construction 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 hormonally
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.