Asexuality in Non-Human Animals?

It’s obvious that humans are unlike all animals. It’s also obvious that we’re a species of big mammal, down to the minutest details of our anatomy and molecules (Jared Diamond, The Third Chimpanzee, 1).

Male Rat

Male Rat
Credit: braindamaged217 on flickr

While at Emory, I earned a graduate certificate in Mind, Brain, and Culture from the Center for Mind, Brain, and Culture (CMBC). As part of the certificate program, the CMBC helped me to arrange a directed reading with Dr. Kim Wallen on the “behavioral neuroendocrinology of sex” in the spring of 2011. As part of the directed reading, Dr. Wallen asked me to write a paper reviewing any existing scientific research on “asexuality” in non-human animals. It was interesting for me, as a scholar in the humanities and social sciences, to think about how scientific research on a/sexuality in non-human animals might (or might not) have relevance for the study of human a/sexuality.

As I am not likely to do anything else with the paper, I finally decided to post a (somewhat) shortened version of it as a blog. I hope you enjoy!

Introduction

Feminist and queer studies scholars have debated the question: can scientific studies investigating the sexuality of non-human animals teach us about human sexuality? Some, including Anne Fausto-Sterling and Jennifer Terry, critique the use of research on non-human animals to answer questions about human sexuality. Others, including Myra Hird and Elizabeth Wilson, argue that research on the sexuality of non-human animals can challenge assumptions about human sexuality in productive ways. This paper explores this question through examining whether scientific research on asexual phenomena (“asexuality”) in non-human animals can shed light on the phenomenon of asexuality in humans. I begin by reviewing the scientific research examining asexuality in non-human animals. Then, I explore three questions suggested by the scholarly debates outlined above: first, what (if any) insights about human asexuality are provided by the scientific research on “asexuality” in non-human animals? Second, how are cultural assumptions about sexuality shaping the research on “asexuality” in non-human animals? Finally, (how) does the research on “asexuality” in non-human animals challenge assumptions about human (a)sexuality?

Scientific Justifications for Using Animal Models to Understand Human Sexuality

In an article, “What Can Animal Models Tell Us About Human Sexual Response,” the authors argue that good animal models will have predictive validity. For example, according to the authors, many of the drugs that enhance erection in male rats (for example, Viagra) also enhance erections in male humans. Thus, it may be possible to learn something about human male sexual arousal from experiments with male rats (Pfaus, Kippin & Coria-Avila 2003, 13). According to these authors, animal models can be used to study the neuroanatomy, neurochemistry, and pharmacology human of sexual behavior (Pfaus, Kippin & Coria-Avila 2003, 50).

Feminist and Queer Critiques of Using Animal Models to Understand Human Sexuality

Male Guinea Pig

Male Guinea Pig
Credit: Randi Deuro on flickr

Feminist and queer scholarship has demonstrated the importance of critiquing the assumptions about gender and sexuality that are influencing scientific research on non-human animals. In the 1970s, feminists inside and outside of the field of animal studies critiqued many scientists for their inability to see female sexuality as anything other than passive. Beginning in the 1970s, careful observation combined with different expectations about masculine and feminine sexuality allowed animal researchers to observe female animals pursuing and initiating sexual activity (Fausto-Sterling 2000).

Feminist and queer scholars have been especially critical of the research on “homosexuality” in non-human animals. They have argued that:

1. In a world in which homosexuality is still considered by many to be immoral or pathological, research into the biological etiology of homosexuality always carries the risk that any research results could be used to prevent the birth of homosexual individuals (Stein 1999; Terry 2000; Lancaster 2004).

2. Many of the scientists investigating the biological causes of homosexuality apparently believe that if they can demonstrate that homosexuality has a biological cause, they will be able to increase societal acceptance of homosexuality. There is some evidence that this can be an efficacious political strategy, as some survey data suggests that Americans who believe that homosexuality is caused by biological factors are more likely to support gay and lesbian rights (Stein 1999). However, feminist and queer scholars have pointed out that using the supposed naturalness of homosexuality to argue for its acceptance is based on a faulty equation of naturalness with goodness and further reifies the category of the natural.

3. Studies confuse same-sex sexual activity with homosexuality as a sexual identity. Sexual identity is defined by how one consciously understands one’s own sexuality, and does not depend on either desires or behavior. Sexual orientation is a “disposition” based on a person’s “sexual desires and fantasies and the sexual behaviors he or she is disposed to engage in under ideal conditions” (Stein 1999, 45). There is no reason to think that non-human animals develop sexual identities. It is possible that non-human animals may develop sexual orientations (in that non-human animals may be disposed to engage in certain types of sexual activities but not others when they are allowed to exercise sexual agency), however, the fact that non-human animals engage in same-sex sexual activity is not, by itself, evidence that these animals are “homosexual.”

4. A number of feminist and queer studies scholars have argued that animal models may not be particularly useful for understanding human sexuality because human sexuality may be qualitatively different from the sexuality of non-human animals. Perhaps most important for feminist and queer studies scholars is the fact that in addition to biological processes, human sexuality involves complex cognition, language, social and cultural expectations, and political and legal institutions.

Scientific Responses to Feminist and Queer Critiques

In response to these feminist and queer critiques of research on the sexuality of non-human animals, some scientists have committed to the following:

1. Ensuring that study finding are not used to eradicate non-normative sexualities.

2. Recognizing the difference between acts and orientations. Vasey (2002) admits that much of the scientific research on “homosexuality” in animals has focused on animals that only exhibit same-sex sexual behavior as a result of brain lesions, exposure to abnormal levels of hormones, or exposure to abnormal rearing environments. In addition, according to Vasey, many of these studies have not really tested the sexual partner preference of these animals. According to Vasey, sexual partner preference testing involves giving an animal the opportunity to interact sexually with both male and female con-specifics, and then observing whether the animal expresses a preference for interacting sexuality with one sex but not the other. Vasey sets himself the task of searching for better animal models for human homosexuality. He looks for animals that “spontaneously exhibit same-sex sexual partner preference as part of their species typical behavioral repertoire” (Vasey 2002, 145). He argues that evidence for such preferences exists for female pukekos, cows, domestic rams, female Uganda kobs, and female Japanese macaques. According to Vasey, studying these animals may provide some insight into human homosexuality.

New Materialist Feminist Approaches to Research on Non-Human Animals

In a departure from traditional feminist and queer approaches to research on non-human animals, scholars who identify as “new materialist feminists” argue that studying the sexuality of non-human animals may lead us to question some of our commonly held assumptions about human gender and sexuality. Myra Hird argues that the diversity we find in the sexual behavior of non-human animals (including sex for pleasure and to build social bonds) can lead us to question certain cultural assumptions about the “purpose” of sex. She writes, “The diversity of sex and sexual behavior amongst (known) species is much greater than human cultural notions typically allow. This diversity confronts cultural ideas about the family, monogamy, fidelity, parental care, heterosexuality, and perhaps most fundamentally, sexual difference” (Hird 2006, 39).

Recap

To summarize, this review suggests three possible approaches: first, the work of some scientists suggests that carefully chosen animal models can provide insight into certain carefully defined aspects of human sexuality. Second, the work of feminist and queer scholars suggests that we must pay attention to how cultural assumptions about gender and sexuality are shaping scientific observations of non-human animals. Finally, the work of new materialist feminist scholars suggests that we may use our observations of non-human animals to question our cultural assumptions about human gender and sexuality.

Scientific Research on “Asexuality” in Non-Human Animals: Defining the Object of Study

Male Gerbil

Male Gerbil
Credit: myshadow768 on flickr

While asexuality can be considered both an identity (people consciously identify as asexual) and an orientation (a dispositional lack of other-directed sexual interest), animal models are unlikely to illuminate much about the identity aspects of asexuality, thus here I will focus on the relevance of animal models for understanding the orientation aspects of asexuality. For the purposes of this inquiry, I define the human phenomenon of asexuality as follows: a lack of other-directed sexual interest (involving both proceptive components – disinterest in seeking out sexual activity with others, and receptive components – disinterest in engaging in sexual activity initiated by others, although asexual individuals may engage in sexual activity for non-sexual reasons). We cannot measure interest in non-human animals by asking them, we can only observe their behavior. Analogous phenomena in non-human animals would thus consist of animals who, as part of their species typical behavioral repertoire, do not engage in sexual activity with either male or female con-specifics when given the ability to do so and when given the ability to control their own sexual activity.

Scientific Research on “Asexuality” in Non-Human Animals: Research Questions

1. Do asexual phenomena exist in non-human animals?

2. Are these phenomena related to circulating testosterone levels?

3. Can these phenomena be altered through altering testosterone levels?

Scientific Research on “Asexuality” in Non-Human Animals: Review of Existing Research

Male Rats: Since the 1960s, animal researchers have been aware that some male rats do not engage in sexual activity when provided with the opportunity to do so. These rats were called “noncopulators” by researchers (Whalen, Beach & Kuehn 1961). Elsewhere, these rats have been described as “sexually sluggish” (e.g. Portillo et al 2006). In a 2003 article, Portillo and Paredes review past research on non-copulating (NC) male rats. They report that approximately 3% of male rats do not express sexual interest in either male or female rats. They also report that non-copulating male rats have circulating levels of testosterone that fall within the species-typical range (Portillo and Paredes 2003). In their own experiments, they found that estradiol benzoate (EB) induced similar levels of female-typical sexual behavior in non-copulating male rats as in other male rats. On the basis of their review and on the basis of their own research, they conclude that the “deficits in sexual behavior in NC male rats are not due to low levels of T” (Portillo and Paredes 2003, 159), and they suggest that perhaps “NC male rats have a functional alteration in the MPOA” (Portillo and Paredes 1003, 160).

Male Guinea Pigs: Researchers have found that male guinea pigs also vary in their levels of sexual interest. Harding and Feder classified male guinea pigs as “high drive,” “medium drive,” or “low drive” animals. In an experiment, Harding and Feder found that high drive and low drive guinea pigs did not differ in their resting levels of testosterone, however, after exposure to an estrous female, high drive animals showed slight increases in testosterone levels while low drive animals showed slight decreases in testosterone. As a result, Harding and Feder conclude that “the manner in which individual animals perceive the testing situation may affect their endocrine function, both gonadal and extra-gonadal, and possibly their neural activity as well” (Harding and Feder 1976, 1205). In this study, the low drive animals were not tested with male guinea pigs, so it is possible that they may have been sexually interested in male con-specifics. I do not know if studies have been conducted to test if, when given access to both male and female con-specifics, there are still male guinea pigs who express low levels of sexual interest.

Male Gerbils: Male gerbils also vary in their sexual and reproductive behavior. In a 1992 article, researchers classify males as “studs” or “duds” based on their sexual and reproductive performance (Clark, Tucker & Galef 1992); in a 2000 article, the same researchers use the label “asexual” to describe male gerbils who express low levels of sexual and reproductive activity (Clark and Galef 2000). Unlike in male rats and male guinea pigs, these researchers have found that differences in sexual and reproductive behavior are correlated with circulating levels of androgens (Clark, Tucker & Galef 1992). According to these researchers, the uterine position of male gerbils (whether they are located next to male or female siblings within the uterus) is correlated with adult levels of circulating testosterone, adult copulatory and reproductive behavior, and adult parenting behavior (Clark and Galef 2000). As adults, “2F” males (males located between two female siblings in utero) on average express lower levels of circulating testosterone, lower levels of sexual interest and behavior, and higher levels of parenting behavior (Clark and Galef 2000). Clark and Galef propose an evolutionary explanation for the differences in male sexual, reproductive, and parenting behavior, writing, “for asexual males to enjoy reproductive success comparable to that of sexually active competitors, males’ probability of direct reproduction would have to be relatively low and their probably of increasing their inclusive fitness by helping to rear kin would have to be relatively high” (Clark and Galef 2000, 804). Again, it is not clear whether these gerbils were tested for their levels of sexual interest with both female and male con-specifics.

Male Rabbit

Male Rabbit
Credit: AlishaV on flickr

Male Rabbits: Male rabbits demonstrate individual variations in levels of sexual activity. Agmo classified male rabbits as expressing either high levels or low levels of sexual activity prior to castration. He castrated the animals and then tested the responsiveness of these two groups to testosterone propionate (TP). Unlike in male rats and male guinea pigs, he found that, for most measures of sexual activity, administering testosterone produced similar levels of sexual behavior in the two groups. However, the difference in mount frequency remained even with administration of high levels of testosterone. In addition, the rank order of the animals’ sexual activity did not vary with the administration of testosterone. According to Agmo, his results indicate that a low level of sexual interest in male rabbits is correlated with circulating androgen levels, however he argues that blood testosterone concentration alone cannot explain the individual variation between male rabbits (Agmo 1976).

Male Rams: Compared to other animals, there has been more scientific interest in individual differences in the sexual and reproductive behavior of domestic farm animals, as these differences have important economic consequences (see, for example, Katz 2008). In a review of male behavior, Katz describes male farm animals who express low levels of sexual behavior as displaying “sub-standard sexual performance” (he also uses the term “duds” to refer to these animals). According to Katz, a significant proportion of bulls, rams, and male goats display “sub-standard sexual performance.” In addition, according to Katz, attempts to reliably correlate sub-standard sexual performance with sex steroid levels have not been successful (Katz 2008). Here I focus on the case of rams, as the etiology of sexual variation among rams has been the most extensively investigated. Researchers have recognized significant variation in the sexual interests of rams. In a number of studies, researchers classified rams as female-oriented (FOR), male-oriented (MOR), or asexual (NOR). According to these researchers, rams were classified as asexual if “they demonstrated consistently low levels of courtship with no clear evidence of a preference for either a male or female stimulus animal and no mounting or ejaculatory behavior” (Roselli et al 2002, 264). According to these researchers, the rate of asexuality among ram populations varies from 12.5%-18.5% (Roselli and Stormshak 2009). After a number of experiments, these researchers concluded that differences in circulating androgen levels do not explain the expression of “low libido” in rams (Roselli et al 2002). The researchers hypothesize that “neural substrates” mediate the differences between FORS, MORs, and NORs and suggest the following as possible neural substrates that may differ between different groups of rams: aromatase activity, estrogen receptors, behaviorally induced neuronal fos responses, and neuronal soma sizes (Roselli et al 2002).

Male Rhesus Monkeys: Male rhesus monkeys display significant individual variation in their sexual behavior. Phoenix and Chambers conducted research with five male rhesus monkeys who were selected for their low levels of “sexual performance.” These males had ejaculated in less than 50% of tests in the past, but displayed no observable behavioral pathology. All but one were wild-born. Phoenix and Chambers found that the circulating testosterone levels of these males were within the species-typical range and treatment with exogenous TP did not change sexual non-responsiveness (and, in fact, reduced some measures of sexual behavior) (Phoenix and Chambers 1988). However, these males were tested for their sexual interest only with females, not with males.

Summary of Scientific Research on Asexuality in Non-Human Animals

Scientific Research on “Asexuality” in Non-Human Animals: Answers to Research Questions

1. Yes, asexual phenomena exist in non-human animals

2. In some animals it was correlated with circulating testosterone levels; in most it was not.

3. In most animals, administration of testosterone did not increase sexual interest/activity.

Discussion

Ram

Ram
Credit: Lauren Tucker Photography on flickr

Scientific Insights Gained from this Review: Given the lack of research on asexual phenomena in female animals, we can only relate the scientific research on non-human animals to asexuality in men. What the animal research suggests, when combined with the research on the relationship between testosterone and sexual interest in clinical and non-clinical human populations, is that asexuality in men is not likely to be well explained by differences between asexual men and sexual men in circulating testosterone levels. In addition, given that the administration of exogenous testosterone to non-copulating male animals often did not alter sexual non-responsiveness, without further research, I would recommend against the prescription of testosterone to men who have experienced a relatively stable, lifetime lack of other-directed sexual interest, even if some men are distressed by this lack.

A Critique of the Social and Cultural Assumptions about Sexuality Reflected in this Research: The scientific research on asexual phenomena in non-human animals reflects a number of social and cultural assumptions about sexuality. For one thing, in the use of terms like “dud,” “sexually sluggish,” and “sub-standard sexual performance,” researchers reflect their own valuation of sexual virility in male animals and their associated devaluation of male animals who express low levels of sexual behavior. While this differential valuation of highly sexually active males is understandable in the context of domestic farm animals, I would argue that it also reflects the fact that in our society we view sexual virility as a sign of masculinity, which itself is socially valued. As these terms (i.e. dud, sexually sluggish) do not provide any added clarity to the discussion, I would recommend that researchers opt to use other terms already in use (such as “non-copulating”) which do not have the same kind of pejorative connotations.

In addition, it is notable that most, if not all, of the research I could find dealt with male animals. There are a number of possible explanations for this. First, it is possible that asexual phenomena simply do not occur (or only occur very rarely) in non-human female animals. Second, as noted earlier in the paper, it is only relatively recently that scientists have begun to systematically study the sexual preferences of female animals in testing situations that allow female animals to express sexual choice. Perhaps as more systematic research occurs, we will learn about asexual phenomena in non-human female animals. Finally, it is possible that this phenomenon exists and has been observed by researchers, but has not been focused on in the scientific literature because it has not seemed particularly interesting or in need of explanation. Unlike in the case of male sexual disinterest, because female sexual disinterest is often considered “normal,” asexual phenomena in female animals may not have appeared to require explanation. Perhaps, somewhat ironically, as researchers shift their attention to the expression of active sexuality in female animals, they will come to see asexual phenomena among female animals as interesting and as in need of explanation.

Finally, it is notable that there has not been more scientific interest in this topic. This reflects, I believe, a general lack of interest in individual variation within populations. Researchers often focus on average levels of sexual interest and activity across particular populations, which may obscure asexual phenomena among non-human animals. A focus on individual variation within populations would lead to a more complex picture of animal sexuality.

Male Rhesus Monkey

Male Rhesus Monkey
Credit: CNPRC UC Davis

Using Animal Research to Question Assumptions about Human Sexuality?: Does the research on asexual phenomena in non-human animals lead us to question any assumptions about human sexuality? Hird argues that the diversity we find in the sexual behavior of non-human animals (including sex for pleasure and to build social bonds) can lead us to question certain cultural assumptions about the “purpose” of sex. She writes, “The diversity of sex and sexual behavior amongst (known) species is much greater than human cultural notions typically allow. This diversity confronts cultural ideas about the family, monogamy, fidelity, parental care, heterosexuality, and perhaps most fundamentally, sexual difference” (Hird 2006, 39).

I will not claim that evidence for the existence of asexual phenomena among non-human animals provides evidence for the “naturalness” (and therefore goodness) of asexuality among humans. Rather, I will make the somewhat different, although related, claim that evidence for the existence of asexual phenomena in non-human animals provides support for the more general claim that diversity and variation are the norm, not the exception, in the natural world of which humans are a part. Thus, this research provides support for the claim that we should expect to encounter diversity and variation in human sexual expression, and that, in fact, we should probably be surprised when we do not encounter asexual phenomena among humans, instead of when we do.

References

Ågmo, A. (1976). Individual differences in the response to androgen in male rabbits. Physiology & Behavior, 17(4), 587-589.

Clark, M. M., & Galef, B. G. (2000). Why some male Mongolian gerbils may help at the nest: testosterone, asexuality and alloparenting. Animal Behaviour, 59(4), 801-806.

Clark, M. M., Tucker, L., & Galef, J. (1992). Stud males and dud males: intra-uterine position effects on the reproductive success of male gerbils. Animal Behaviour, 43(2), 215-221.

Diamond, J. M. (2006). The third chimpanzee: The evolution and future of the human animal. HarperCollins.

Fausto-Sterling, A. (2000). Beyond Difference: Feminism and Evolutionary Psychology. Alas Poor Darwin. Hilary Rose and Steven Rose, editors. Crown/Random House.

Harding, C.F., & Feder, H.H. (1976). Relation Between Individual Differences in Sexual Behavior and Plasma Testosterone Levels in the Guinea Pig. Endocrinology, 98(5), 1198-1205.

Hird, M. J. (2006). Animal Transex. Australian Feminist Studies, 21(49), 35-50.

Katz, L. S. (2008). Variation in male sexual behavior. Animal Reproductive Science, 105(1-2), 64-71.

Lancaster, R. N. (2003). The Trouble with Nature: Sex in Science and Popular Culture, 1st ed. University of California Press.

Pfaus, J. G., Kippin, T. E., & Coria-Avila, G. (2003). What can animal models tell us about human sexual response? Annual Review of Sex Research, 14, 1-63.

Phoenix, C. H., & Chambers, K. C. (1988). Testosterone therapy in young and old rhesus males that display low levels of sexual activity. Physiology & Behavior, 43(4), 479-484.

Portillo, W., & Paredes, R. G. (2003). Sexual and olfactory preference in noncopulating male rats. Physiology & Behavior, 80(1), 155-162.

Portillo, W., Díaz, N. F., Retana-Márquez, S., & Paredes, R. G. (2006). Olfactory, partner preference and Fos expression in the vomeronasal projection pathway of sexually sluggish male rats. Physiology & Behavior, 88(4-5), 389-397.

Roselli, C. E., Stormshak, F., Stellflug, J. N., & Resko, J. A. (2002). Relationship of Serum Testosterone Concentrations to Mate Preferences in Rams. Biology of Reproduction, 67, 263-268.

Roselli, C. E., & Stormshak, F. (2009). The neurobiology of sexual partner preferences in rams. Hormones and Behavior, 55(5), 611-620.

Stein, E. (1999). The Mismeasure of Desire: The Science Theory and Ethics of Sexual Orientation. Oxford University Press, Oxford and New York.

Terry, J. (2000). “Unnatural Acts” in Nature: The Scientific Fascination with Queer Animals. GLQ, 6(2), 151-193.

Vasey, P. L. (2002). Same-Sex Sexual Partner Preference in Hormonally and Neurologically Unmanipulated Animals. Annual Review of Sex Research, 13, 141.

Whalen, R. E., Beach, F. A., & Kuehn, R. E. (1961). Effects of Exogenous Androgen on Sexually Responsive and Unresponsive Male Rats. Endocrinology, 69(2), 373-380.

Special thanks to Dr. Kim Wallen, Yerkes Researcher, Samuel Candler Dobbs Professor of Psychology and Behavioral Neuroendocrinology, Emory University.

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