What are sexual behavior patterns?
Sexual behavior patterns represent one of the most important aspects of an organism’s life. These patterns not only provide for the successful perpetuation of the species but also allow the individual to contribute genetically to future generations. Sexual behavior is unlike other physical motives, such as feeding and drinking, which are required for the individual’s survival and which are initiated to some extent by measurable changes in blood sugar and cellular hydration. Engaging in sexual behavior is neither necessary to live nor stimulated by the depletion of a bodily fluid or chemical substance.
Two types of questions, relating to ultimate and proximate causality, must be addressed when sexual behavior patterns are examined. The first question asks why the pattern developed; the second asks how it occurs. For example, many species breed only during particular seasons, and the onset of these periods is often associated with changes in plumage or coloration, or the growth of anatomical structures such as antlers. Why do these changes take place? This question of ultimate causality is really asking about purpose or function; in these examples, one answer could be that the alteration makes the animal more attractive to a potential mate.
The second question, concerning proximate causation, asks how these changes come about or what the more immediate cause is. In this case, the answer could be related to a change in the animal’s hormonal secretions. Thus, the sexual behavior pattern of a given species is determined by many factors, each with ultimate and proximate causes.
One of these factors is sexual selection, a concept originated by Charles Darwin in The Descent of Man and Selection in Relation to Sex (1871) and related to the example mentioned above. There are two kinds of selection, intersexual and intrasexual. In the first, one sex’s ability to secure a mate is related to its anatomical and behavioral traits. Examples that pertain to males include antlers, the peacock’s feathers (and the way the male displays the fully fanned-out feathers for the female), and the songs of some bird species used to “advertise” the male’s availability for mating as well as the fact that he has obtained a territory relatively free of intruders.
Intrasexual selection involves those anatomical and behavioral traits that are used to compete with members of the same sex for access to a member of the other sex. The battle between males to establish dominance that for the winner often leads to the opportunity to mate is a common example. A well-known phenomenon in mice, the Bruce effect, provides a different sort of intrasexual selection example. The presence of an unknown male during the early stages of pregnancy can cause a female to abort, which results in her becoming sexually receptive and hence a potential mate for the strange male. In this case, the ultimate cause is that this enables the male to sire more offspring, while the proximate cause is that his odor alters the female’s hormone secretions in such a way as to terminate pregnancy.
Various mating systems have evolved that also determine the type of sexual behavior pattern. Monogamy represents a sexual relationship between one female and one male, sometimes for life. One advantage is that it precludes the effort necessary to search for a mate during each breeding cycle or season. It may, however, sometimes be more advantageous for a female to enter a good territory already inhabited by a male and one or more other females than to form a monogamous relationship with a male who lives in a dangerous territory or one with fewer resources. These systems are called polygynous, as opposed to those that are polyandrous, in which one female has a sexual relationship with more than one male. Although polyandrous systems are uncommon, polyandry does occur in situations in which the female can lay many eggs in various nests while the different males do most or all of the incubating.
Unlike some species, such as humans, who reproduce throughout the year, most species breed only during one or more restricted times of the year. The ultimate cause could be that hatching or birth occurs at a time when the environmental features are more optimal in terms of temperature, predators, or food availability. Proximate factors have been well studied, and it is known that changes in the amount of light per day or temperature can cause an animal’s endocrine system to become reproductively active. In female mammals, these periods are called estrous cycles; it is only during these cycles that pregnancy can occur. Animals kept in laboratories and maintained with constant and optimal amounts of light and other environmental factors will breed all year long.
The changes in hormonal secretions that precede the onset of a breeding period are critical for several reasons. Physiological processes such as maturation of the egg or ovum, the formation of the hard shell of the egg in birds and reptiles, ovulation, preparation of the uterus for implantation of the ovum in mammals, and development of sperm depend on particular hormones. Hormones are also important because they act directly on regions of the brain to increase an organism’s motivation to reproduce. In addition, by affecting sensory processes, hormones directly or indirectly enable an animal to communicate its reproductive readiness over distances. Examples include pheromones, which are odors that are emitted by many species to attract a sexual partner, some types of singing in birds and croaking in frogs, and the increased swelling and reddening of the genital region in monkeys.
Sexual behavior patterns are extremely varied; only by studying them in detail have scientists uncovered some general principles that apply to various groupings of species. Appreciating the differences between even closely related species prevents making oversimplified generalizations from one species to another.
Many species of birds have relatively prolonged and intricate courtship and mating patterns. The ring dove was extensively studied in the laboratory by Daniel Lehrman and his colleagues in the 1950s and 1960s and by a number of other scientists since then. Although the dove has breeding cycles in nature, it reproduces almost the entire year if kept in the laboratory under constant conditions of fourteen hours of light and ten hours of dark per day, at a temperature of 22 degrees Celsius.
The male dove’s courtship begins with cooing sounds while in a bowing posture. This continues for a period of time until he selects a nest site and then coos from that location. When sufficiently aroused, the female also “nest coos,” which tells the male that it is time to gather material for the nest. Eventually the female ovulates, and the birds mate. She lays two eggs; both parents incubate the eggs, and both participate in feeding the young squabs by regurgitation.
Experiments have shown that androgens, the male sex hormones secreted from the testes, stimulate the male dove’s courtship behavior, which in turn stimulates the female’s ovaries to release the female sex hormones estrogen and progesterone. Hearing her own nest coos affects the female’s physiology by playing a major role in the development of the follicles, the ovarian structures that contain her gametes, or eggs, which will be fertilized by the sperm. These hormones are important for ovulation and for mating behavior. Behavioral participation in the building of the nest produces further hormonal changes, which increase each partner’s motivation to sit on the eggs. Visual and tactile sensory input from the eggs stimulates prolactin from the pituitary gland in both sexes, which functions to keep the parents incubating until the eggs hatch; it also causes the production of crop milk, the partially digested food that is regurgitated for the hatchlings. These behavioral-hormonal interrelationships have been shown to exist in other species, and they point out the importance of particular sexual behavior patterns for successful reproduction.
Another example of the role of behavior patterns in the survival of the species comes from experiments on rats by Norman Adler. A female rat comes into “heat” or estrus on only one day during the latter portion of her four-day estrous cycle. Her period of heat begins several hours before ovulation and ends several hours afterward. It is only during this time that she will mate and can become pregnant. During the first few days of the estrous cycle, the female secretes hormones that cause growth of the follicles, ovulation, and sexual behavior. If her eggs or ova are fertilized, her estrous cycling stops until after delivery of the litter. As in the case of the dove, a female rat will continue to have estrous cycles all year long under constant environmental conditions in the laboratory, unless she becomes pregnant.
Under those constant conditions, the male continues to secrete androgens and is almost always ready to mate. Placing a sexually receptive female and sexually active male together in a cage results in a predictable sequence of behaviors. The male will investigate the female and, on the basis of certain odors attributable to her estrogen and progesterone, will find her “attractive.” In response to the male’s interest in her and her attraction to him, she engages in proceptive behaviors—sexually stimulating activities that maintain the pair’s interaction. In the rat, these behaviors include a “hopping and darting” form of locomotion and ear quivering. The male will mount the female, and if sufficiently motivated, she will show receptivity by adopting the lordosis posture (characterized by immobility, arched back, and raised genital region). On many of these mounts, the male will be able to intromit his penis into her vagina; after an average of ten to fifteen intromissions, he will ejaculate. A number of minutes will elapse and the sequence will begin again; it will be repeated several times in a single sexual session.
In one experiment, males were allowed to intromit a varying number of times with a first female; then, before ejaculating, they were each placed with a second female. In this way, various females received different numbers of intromissions prior to an ejaculation. The significant finding was that the female needs a number of intromissions plus an ejaculation to become pregnant. If she receives only one or two intromissions prior to an ejaculation, her likelihood of becoming pregnant is greatly reduced. The stimulation she receives from these intromissions is necessary to alter her hormonal secretions in preparation for pregnancy. Additionally, males who intromit fewer than six times prior to an ejaculation release fewer sperm, hence reducing the probability that their partners will become pregnant. This result is related to the fact that subdominant male rats have fewer intromissions and reduced fertility, but only when a more dominant male is nearby.
Scientists study primate species both because they are interesting in their own right and because the researchers wish to gain some understanding of human behavior. The rhesus monkey, a commonly studied primate, is polygynous and native to India; it has a breeding season that begins in the fall and lasts about five months. Instead of an estrous cycle, it has a menstrual cycle that is almost identical to that of human females.
Mating behavior is not controlled as exclusively by hormones as it is in lower species, but the frequency of copulation is greatest around the time of ovulation. Attractivity of the female is enhanced by estrogen, but unlike in the rat, it is reduced by progesterone, the hormone that is at its highest level after ovulation in the second half of the menstrual cycle. Experiments have shown that for optimal mating behavior to occur, androgen is necessary for the male, and both estrogen and androgen are required in the female. Female monkeys, like female humans, normally secrete androgen, although at much lower levels than males do (just as male monkeys and humans secrete female sex hormones). Studies on human females have shown that levels of androgen during the menstrual cycle correlate with increased sexual motivation and gratification.
Darwin was influential in convincing scientists and nonscientists alike that humans and other animals are products of evolution and that they share common ancestors. Further, Darwin and his successors have argued that behavior, like anatomy, has changed as a result of natural selection, the process whereby traits that allow an organism to produce more offspring will be inherited by subsequent generations.
In part because of Darwin’s emphasis on the similarity between animals and humans, William James in the late nineteenth century and William McDougall in the early twentieth century proposed the instinct theory, that much of human behavior is based on instincts. Instincts are behaviors that are characterized by their lack of dependence on learning, fairly rigid performance, and presence in all members of at least one sex of a species.
The question of instincts is a key issue in the long-standing controversy in psychology between “nature” and “nurture,” or the relative role of inborn versus environmental or learned factors in behavior. Over the years, some behaviors that were thought to be pure instincts have been shown to be affected by learning or experience, and other behaviors have been shown to be more inborn than originally thought. Advances in the field of genetics has enabled further study into which behaviors, including sexual behaviors, may be more inborn than others. Furthermore, simply calling a behavior an instinct does little to shed light on either its ultimate or its proximate causes.
Partly as a result of the debate over instincts, the study of animal and human behavior has taken two somewhat separate paths. On one side are primarily psychologists, psychobiologists, and neuroscientists who investigate the more proximate causes of sexual behavior patterns in the laboratory under controlled conditions. Their progress has helped to gather information on the nervous system, the endocrine system, the interaction between the two, and their relationship to environmental factors such as light, temperature, and the presence of potential mates.
Evolutionary biologists, animal behaviorists, sociobiologists, and ethologists tend to study sexual and other behaviors under natural conditions. Ethologists Konrad Lorenz and Nikolaas Tinbergen focused on more instinctive, species-specific behaviors emphasizing ultimate causation.
It is often difficult for a laboratory scientist to devote much attention to evolutionary concerns, and it is equally difficult for the animal behaviorist to focus on the nervous and endocrine systems. Information from one approach often complements the other, however, and a complete understanding of the effect of all relevant factors is necessary for the study of sexual behavior patterns.
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