Diversity in Sexual Partner Preferences Among Animals: Insights into Same-Sex Preference from Rams and Rats
Sexual preference refers to an animal's tendency to choose either males or females as partners for courtship or mating. Although sexual preference is closely related to sexual attraction, sexual motivation, and the actual performance of sexual behavior, these aspects of sexuality do not necessarily coincide. For example, an animal may actively seek sexual stimulation without showing a clear preference for either sex. Conversely, an animal may prefer partners of a particular sex, yet its actual mating behavior may be influenced by other physiological or environmental factors.
If two male animals mount each other, can we immediately conclude that they prefer same-sex partners? The answer is not so straightforward. Mounting can occur during courtship and copulation, but it may also be associated with play, social interactions, or the establishment of dominance hierarchies. Even when an animal voluntarily approaches a same-sex individual, it may simply be attracted by its odor, seeking social contact, or responding to a lack of alternative partners. One of the central challenges in studying same-sex preference in animals, therefore, is determining how researchers can establish which individuals an animal actually prefers as sexual partners.
These distinctions make many seemingly straightforward experiments surprisingly difficult to interpret. In laboratory rats (Rattus norvegicus), for example, researchers can present odors from sexually active males and sexually receptive females, then measure how long the experimental animal investigates each stimulus. Alternatively, a male and a female can be placed behind separate mesh barriers, allowing researchers to measure how much time the experimental animal spends near each one. Such tests reveal which stimuli attract an animal's interest and primarily reflect sexual attraction and motivation. However, these observations alone are insufficient to establish sexual partner preference.
More rigorous experimental paradigms allow animals to choose simultaneously between male and female partners with which they can physically interact. Researchers then record which partner the experimental animal approaches, how long the interactions last, and whether behaviors associated with sexual activity occur, including courtship, mounting, intromission, and sexually receptive postures. Some experiments offer multiple potential partners, allowing animals to choose freely among several individuals and reducing the limitations of traditional binary choice tests. Nevertheless, important challenges remain. Copulation itself can produce rewarding or aversive experiences that influence subsequent partner selection, while simply remaining close to another individual does not necessarily indicate sexual interest. Assessing sexual preference in animals is therefore considerably more complicated than measuring the time spent near a potential partner.
One of the most compelling naturally occurring models of same-sex preference comes from domestic sheep (Ovis aries). Studies reviewed in the literature indicate that approximately 8% of rams exclusively attempt to copulate with other males, maintaining this preference even when ewes are available as alternative partners. These animals are particularly valuable for research because their same-sex preference develops spontaneously, without experimental manipulation through drugs, surgery, or hormonal treatments.
Comparisons of brain anatomy have revealed that male-oriented rams have a smaller sexually dimorphic nucleus (SDN) within the medial preoptic area of the hypothalamus than rams that prefer ewes. Other studies have also identified differences in DNA methylation and gene expression within the hypothalamus. However, these findings establish associations between brain characteristics and sexual preference without demonstrating causation. It remains unclear whether the smaller SDN directly contributes to male-oriented partner preference, and the possibility that both characteristics arise from common influences during early development cannot be excluded.
Experiments involving rats provide researchers with opportunities to manipulate developmental conditions directly and examine their effects on partner preference in adulthood. During critical periods before and shortly after birth, the development of the male rat brain is strongly influenced by sex steroids. Once testosterone enters the brain, some of it is converted into estradiol by the enzyme aromatase. Estradiol participates in the masculinization and defeminization of the brain, developmental processes that contribute to the typical preference of adult male rats for female partners. When researchers inhibit aromatase during prenatal or early postnatal development, the proportion of adult males exhibiting same-sex preference increases.
However, this effect does not occur in every treated animal. Even when rats undergo the same experimental manipulation, some males develop a preference for other males, while others retain their preference for females. The outcome is therefore probabilistic rather than uniform. Furthermore, some male rats that prefer male partners continue to display typical masculine copulatory behaviors when exposed to sexually receptive females. This suggests that sexual partner preference and the performance of sexual behavior can be partly dissociated, indicating that the neural systems regulating these aspects of sexuality are not entirely identical.
Sexual experience can also modify partner preference in some individuals. Certain male rats treated with aromatase inhibitors show a reduced preference for males after successfully copulating with females. Others maintain a stable preference for males even after being given opportunities to mate with females. Thus, identical developmental interventions can ultimately produce different behavioral outcomes.
Prenatal stress may also influence this process. In experiments involving rats and mice (Mus musculus), maternal exposure to stress during late gestation has been associated with a higher proportion of male offspring exhibiting same-sex preference in adulthood. Some of these males also show reduced masculine sexual behavior. One proposed mechanism is that stress during critical periods of fetal development reduces testosterone synthesis and brain aromatase activity, thereby altering sexual differentiation. However, the evidence supporting this relationship is substantially stronger in laboratory animals than in humans. There is currently insufficient evidence to establish prenatal stress as a direct cause of human sexual orientation.
Some researchers have shifted their attention toward the neural circuits involved in regulating sexual preference. In male rats with same-sex preference, exposure to odors from other males activates the medial preoptic area and the bed nucleus of the stria terminalis in patterns resembling those observed in females. The dopaminergic system of the nucleus accumbens has also been implicated in sexual partner selection. Through optogenetic activation of specific dopaminergic projections, researchers have restored female preference in male mice that had lost their typical preference for females. These findings suggest that sexual preference may be associated with the development and activity of particular brain regions. Nevertheless, it remains uncertain which specific alterations in neural circuitry directly account for same-sex partner preference in animals.
Findings from animal studies also cannot be directly extrapolated to human sexual orientation. Human sexual orientation encompasses enduring patterns of attraction, emotional relationships, personal identity, and sociocultural experiences, whereas animal experiments primarily measure partner choice and sexual behavior. Although some underlying neuroendocrine and developmental mechanisms may be shared across species, these phenomena operate at different levels of complexity.
Current evidence supports a multifactorial and probabilistic framework in which the prenatal hormonal environment, stress, epigenetic processes, maternal reproductive history, developmental experiences, and neural activity in adulthood may all contribute to shaping sexual partner preference. Even under identical experimental conditions, only a subset of individuals may exhibit changes in preference. Sexual preference in animals is diverse: some individuals exclusively prefer partners of one sex, whereas others show interest in both males and females. The strength of these preferences may also vary, forming a broader continuum of partner preferences. Biological diversity is an inherent feature of the natural world, and understanding these phenomena can help explain how such a wide range of sexual behaviors and partner preferences arises in nature.
Author: Shui-Ye You
References:
Fernández-Guasti A et al. (2026). Animal Models for the Study of Same-Sex Sexual Behavior. Current Sexual Health Reports.
Rodríguez-Manzo G and Canseco-Alba A. (2023). The endogenous cannabinoid system modulates male sexual behavior expression. Front. Behav. Neurosci.
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