The Scientist, Volume 8, #6, page 3 (March 21, 1994)
The Science Of Sex: What Is It And Who's Doing It?
By NEERAJA SANKARAN
If sexual activity is a prickly issue for discussion in society and our daily lives, it appears no less so as a research topic in some areas of the scientific and medical communities. The very definition of the word sex seems troublesomely elusive for many researchers when it comes to classifying their investigative efforts and obtaining the funding to support them.
"When I ask my students to define sex, or sexuality," says physiology and anatomy professor Robert Friar of Ferris State University in Big Rapids, Mich., "their response is usually `male-female.'
"But I tell them they are wrong. That is gender, not sex." In his class, Friar describes sexuality as "a diffused sensuality that permeates our whole personality and everything that we do."
"Sex research covers everything from A to Z--anthropology to zoology," says Howard Ruppell, executive director of the Society for the Scientific Study of Sex, headquartered in Mount Vernon, Iowa, and an adjunct professor of social work at the University of Iowa. A functional definition, according to him and Friar, would include not only behavioral studies (typically characterized as sex research), but also all research pertaining to the structure and function of the sexual and reproductive systems and related organs, as well as their effects on other organs and systems.
Not everyone agrees. For instance, C. Dominique Toran- Allerand, a neuroscientist at the Columbia University College of Physicians and Surgeons in New York who studies the effects of estrogen (the female sex hormone) on the brain, emphatically denies that her work could be classified as sex research. "I am studying the effects of a hormone on the development of the brain," she says, adding that the sexual function of estrogen is incidental to her focus. "It would be a great disservice to call this sex research."
At least part of the reason for scientists' reluctance to label themselves as "sex" researchers is the stigma that continues to be attached to the term, says Terri Fisher, an associate professor of psychology at Ohio State University in Mansfield. Funding difficulties are often associated with such a stigma, adds James Weinrich, a population biologist in the department of psychiatry at the University of California, San Diego (see accompanying story).
Nevertheless, the subject of sex abounds in the research community, spanning a broadening scope of disciplines and ranging in levels from genes and cells to gross structures and behavior.
Hormones, Sex, And The Brain
A psychologist at Columbia University's department of psychiatry, Heino Meyer-Bahlburg, for example, is interested in the effects of prenatal sex hormones on human behavior. He is studying patients with endocrine disorders like congenital adrenal hyperplasia, in which the patients were exposed to excessive levels of testosterone, the male hormone, while still in the womb.
"Genetic females--with two X chromosomes--exposed to testosterone prenatally are very tomboyish as children," he says. "As adults, they show higher rates of bisexuality, and some even go on to change their gender to male." Meyer- Bahlburg adds that with currently available medical technology, behavioral studies are still the best gauge of the effects of prenatal hormones.
"It would be interesting to examine the brain structures, particularly nuclei in the limbic system," he says, "but current imaging techniques are just not powerful enough. Biopsies are obviously not an option."
Scientists are not confining hormone/behavior studies to humans alone, as is evident from several articles that appeared in the scientific and popular press recently (N. Angier, New York Times, Nov. 12, 1991, page C1, and Aug. 31, 1993, page C1 [articles on cichlid fish]; N. Angier, New York Times, Nov. 2, 1993 [prairie voles]; see reading list on page 16 for magazine/journal articles).
Carole Carter-Porges of the University of Maryland, College Park, is one of many biologists who are studying the mating habits of prairie voles, a rare breed of animal that has a monogamous lifestyle, with the male of the species actively helping in rearing pups. Recent data from the laboratories of Carter-Porges and others indicate that such behavior is related to vasopressin, a peptide hormone released in males soon after mating. Furthermore, the synthesis of vasopressin appears to be controlled by testosterone.
"Our studies with voles show that there is a definite chemical and biological basis for social bonding and behavior," says Carter-Porges.
In an interesting twist, Stanford University neurobiologist Russell D. Fernald has shown that, among cichlid fish at least, socio-sexual behavior induces changes in the brain and hormone levels, rather than the other way around. The size of the hypothalamus of the male--the region of the brain responsible for the fish's breeding abilities--is directly and profoundly affected by the cichlid's social status. As the fish battle one another for breeding territory, the hypothalmic cells of the dominant male become six to eight times larger in size, but will shrink back when challenged by a more aggressive male. These cells produce the gonadotropin-releasing hormone, GnRH, that normally regulates the sex organs; shortly after the brain cells shrink, testes of the fish follow suit.
Fernald is currently researching the molecular mechanisms of the transformation, and the role of GnRH in the process. "The GnRH molecule is the same in all animals across evolution and even related to the yeast mating factor [the equivalent of a sex hormone in yeasts, which are not animals]," says Fernald. Thus, while he does not yet know if GnRH-producing cells undergo size changes in humans, his findings could have a great impact on the understanding of human sexual behavior and its control.
The relevance of sex research in animals to human situations is exemplified by the discovery of the vomeronasal organ (VNO) in humans. This organ is found in most vertebrate animals, acting as a receptor and detector for certain chemicals (that may have no discernible odor) called pheromones that mediate sexual/mating behaviors. In mammals like the rat, the VNO is a pair of small sacs located by the vomer bone behind the nostrils. Standard anatomy textbooks have stated that this organ disappears in humans during embryonic development, but investigations by two separate groups of researchers during the 1980s refuted this claim.
Like the rat VNO, the human organ consists of a pair of sacs that open into two shallow pits on either side of the nasal septum. The sensory cells that line these sacs are different from olfactory cells present in the nose. Currently, various scientists are trying to find out more about structure and function by correlating human studies with animal models. Charles Wysocki, a researcher at the Monell Chemical Senses Center in Philadelphia who studies the effects of male odors on hormone levels in females, speculates that the VNO may play a role in regulating these effects.
"In laboratory experiments we have seen that the length and timing of the menstrual cycle are markedly influenced by odors from the underarms of males," he says, suggesting a possible link between such responses and the observation that the menstrual cycle of women living around men is apt to be more regular. It is not clear if the hormones are responding to true odors or to chemical stimuli picked up by the VNO. Wysocki is trying to gauge the physiological effects of destroying this organ in animals and extrapolate this work to human models.
Meanwhile, a New York-based company, Erox Corp., has released a new line of perfumes that exploit the possible applications of VNO-binding compounds. In an interview in the Wall Street Journal last year (J. Bishop, April 7, 1993, page B1), David Berliner, a former professor of anatomy who helped found the company, mentioned that the purpose of these compounds, "human pheromones," was to make the wearers feel better about themselves, not to attract the opposite sex, as many would believe.
But while Berliner doesn't ascribe any aphrodisiac qualities to his perfumes, Raymond C. Rosen, a psychiatrist at the University of Medicine and Dentistry of New Jersey in Piscataway, states that the search for the perfect aphrodisiac has been a perennial cultural pursuit throughout history. He is updating this search, investigating the effects of certain "prosexual" drugs like dopamine and oxytocins. Such research, says Rosen, has contributed immensely to an understanding of the mechanisms of sexual response, and provides new treatment options for sexual disorders like impotence and erectile dysfunction.
Working at the cellular and genetic level, Debra Wolgemuth, a developmental biologist at the Center for Reproductive Sciences of the Columbia University College of Physicians and Surgeons, is studying the development of the testes, finding applications for her work that go beyond sexual function.
"Some of the genes that control how these cells divide have been implicated as oncogenic [cancer-causing genes] in other systems, like sarcomas and leukemias," says Wolgemuth.
At Indiana University, R. Stephen Howard, a graduate student in the laboratory of biologist Curtis Lively, has come up with an explanation of why sex exists at all.
This may not be a moot point, as some might think, says Howard, because in terms of the energy needed, asexual reproduction is a more economical way to transmit genes to one's progeny.
Previous attempts to explain the evolution of sex considered one of two theories, one being that the recombination and segregation of genes arising from sexual reproduction provided a means of escaping from harmful mutations that would accumulate in the gene if reproduction were asexual.
"The second theory we call the `Red Queen' model--after the Alice in Wonderland character--has to do with avoiding infections by parasites," says Howard. "There is a genetic basis to host-parasite interactions, and by reproducing sexually, the host can scramble its genetic makeup [through recombination] so that the parasite no longer recognizes the host genes." Evidence for this lies in the observations by Lively that certain species of snails in New Zealand go from asexual to sexual modes when exposed to trematode parasites.
Howard and Lively believe that the combination of both phenomena is actually responsible for the evolution of sex. "Genetic recombination underlies both explanations," says Howard. The two devised a computer program to simulate the effects of mutation accumulation and parasitism, which bears out the strength of their model.