The above brief discussion of the natural history of asexual and sexual reproduction only indirectly relates to the main subject of this book: asexual humans. The type of reproduction of a species—sexual or asexual—is somewhat different from the phenomenon of asexual beings, including human beings, within an exclusively sexually reproducing species. There is also a distinction between sexuality and the capacity for reproduction. For example, the vast majority of asexual people can still reproduce sexually (i.e., are still part of the sexually reproducing species of humans), even if they are not interested in the sexual mechanisms of it. Finally, in modern humans, sexuality is often divorced from reproduction, so asexuality is (partially) a different phenomenon than asexual reproduction.
More to the main point of this book, then, are there examples of asexual animals within a normally sexually reproducing species? There are. For example, a chorus line of sexual variability exists in farm and lab animals, even though they are often bred—and sometimes genetically altered—to be nearly identical food-producing or lab-friendly machines. As in humans, you can find some that are studs or sexual dynamos. You can also find some having “homosexual” tendencies, in that they prefer sexual relations with the same sex; for example, males not only affiliating with other males but also actively preferring to mount other males rather than females. You can also find animals that have no sexual interest whatsoever in other animals. This pattern of sexual variation among animals is often most clearly observed in males, as it is sometimes harder to determine female sexual tendencies, in part because they, relative to males, are less likely to initiate mating, often having a more subtle sexual response associated with receptivity (see chapter 6 on gender).
Male rodents raised in experimental labs (e.g., mice, gerbils, guinea pigs) often demonstrate wide sexual variability, with behaviors ranging from hypersexual to asexual. These extremes are often called “studs” and “duds,” respectively. Such variability in rodents may have parallels to human sexual variability, including asexuality. Yet there are a number of unknowns. First, some non-mating males may have sexual attraction to other males, and so a lack of mating may not mean asexuality. A second, related unknown is that the conclusions rely on the behavior of the animals, and this is an imperfect measure of underlying sexual attractions, particularly if we are using it as a model of human sexual attractions (see also chapter 2). Third, researchers are unclear as to what factor(s), biological or otherwise, underlie the stud/dud difference. One biological explanation is variations in circulating testosterone, as this hormone is related to sex drive in both animals and humans. However, some animal experiments, including early ones on guinea pigs, indicate that variations in adult-level circulating hormones are probably not the main cause of the difference between duds and studs, because these animals, after being castrated and then readministered a constant level of testosterone, still showed the same behavioral differences (Grunt & Young, 1952; Adkins-Reagan, 2005). Another possibility is that there are differences in certain brain cell structures laid down before birth—called receptors—that make some animals more or less sensitive to testosterone exposure in adulthood. A related possibility is that prenatal hormones organize certain sites of the brain into groupings of cells (nuclei) with specialized functions. This difference in how brain cells are organized/structured prior to birth has implications for human sexual orientation, as a leading biological theory of human sexual orientation proposes that similar mechanisms underlie human variations in sexual attraction. Thus, from a sexual orientation perspective, these “dud” animals may have an asexual orientation because of prenatal factors organizing the brain in a certain way, just as variations in human sexual attractions, including perhaps asexuality, may be affected by such prenatal influences.
Did you know that there are sexual-orientation “tests” in male sheep (rams)? And no, I do not mean measures of genital responding while the rams view pictures of sheep, or even a “drunk test” (see chapter 2). In the United States Sheep Experiment Station in Dubois, Idaho, researchers test the erotic inclinations of their animals by allowing rams to have access to two “stimulus” females (ewes) in estrus or to two “stimulus” rams. These sexual-orientation tests occur in specialized pens, over repeated time periods (e.g., three thirty-minute exposures). The stimulus animals are restrained, so the test rams have easy access to them. The rams’ sexual interest is measured by observing various mating behaviors characteristic of these animals: sniffs, kicks, mounting attempts, and “flehmans” (i.e., head raisings with curling of the upper lip) (Roselli, Larkin, Schrunk, & Stormshak, 2004).
But why would researchers test rams for their erotic inclinations in the first place? They do so to understand and ultimately increase the productivity of rams for sheep farmers, who want, as you might expect, stud rams to produce lots of little lambs.
If the rams are sexually attracted to ewes, they are called FORs (female-oriented rams); if they are attracted to other rams, they are called MORs (male-oriented rams). If they are attracted to neither, they are called NORs (no-oriented rams), or asexuals.
A surprisingly high percentage are MORs, but equally surprising is that a high percentage are evidently NORs (i.e., asexual). For example, here is a quote by researchers Charles Roselli and colleagues in a paper summarizing their results: “Over the past 2 years, 584 rams were tested. Of these 12.5% were asexual, 55.6% mounted and attained ejaculation with ewes, 9.5% mounted other rams, and 22% interacted sexually with both males and females” (Roselli et al., 2004, p. 235).
As in rodents, there does not seem to be strong evidence that sexual variation among the rams is related to circulating testosterone. For example, asexual rams (NORs) do not have lower levels of testosterone than the FORS or the MORs (Perkins, Fitzgerald, & Price, 1992), also suggesting that asexual rams may develop their sexual orientation because of prenatal factors organizing the brain in a certain way.
Rams may be especially relevant to understanding human sexuality; indeed, they provide a better animal comparison to human sexual orientation than male rodents, because rams show similar patterns of sexual behavior to human males. For example, MORs actively pursue other males for sexual liaisons, as do human men sexually interested in other men. Thus, sexual orientation patterns in both species, perhaps including asexuality, may emerge because of similar prenatal mechanisms, such as exposure to prenatal hormones organizing sites of the brain (see chapter 13). Indeed, research suggests that structural differences in a site of the lower brain, the SDN-POA (sexually dimorphic nucleus of the preoptic area) of the hypothalamus, is relevant to ram sexual orientation, just as a similar site is likely relevant to men’s sexual orientation (Roselli, Stormshak, Stellflug, & Resko, 2002; LeVay, 1991). However, the researchers have not as yet examined the brains of NORs to determine if this site is related to asexual rams’ sexual orientation.