Sheep & Sexuality: Looking at Sexual Dimorphism in the Hypothalamus

The Sexually Dimorphic Nucleus (SDN), located in the preoptic area of the hypothalamus, was first identified in 1978 in rodents. This area of the brain is the most visible difference between male and female brains, such that it is nearly 2.2 times larger in volume in males than females (Hofman & Swaab, 1989).

In one of my labs, we were asked to identify and measure this structure in a sheep brain and compare our answers to other classmates in order to determine the gender of our animal. Students with female sheep brains had SDN measurements of 3-5 cm and others with males had measurements of 9-11 cm. We asked our instructor what would be concluded if the SDN was somewhere in the middle of those values, and they informed us that the SDN has been explored in a number of studies which found that an individual’s sexuality may be linked to the size of this structure.

I found an article published in 2004 looking specifically at the various sizes of male sheep SDN with consideration to their sexual preference. The researchers found that rams considered “female-oriented” (sexually active with females) had larger SDNs than “male-oriented” (sexually active with male rams (Roselli, Larkin, Resko, Stellflug & Stormshak, 2004). Female sheep have the smallest SDNs. The neurons that form this cell cluster express cytochrome p450 aromatase. It was found that female-oriented rams expressed more aromatase than male-oriented rams, who expressed intermediate levels. It is thought that this structure plays a role in the expression of sexual behaviors and possible estrogen synthesis. The rams classified as male-oriented did not behave like females or have any different social behaviors.

Further studies have been conducted on this topic in rodents and humans. These findings show a scientific association between brain structure and sexuality. The implications of these conclusions could have a positive impact on society’s understanding of sexual orientation. It provides a biological substrate for sexual orientation that could disprove any notion that homosexuality is a mental illness or a choice.

Hofman, M. A., & Swaab, D. F. (1989). The sexually dimorphic nucleus of the preoptic area in the human brain: a comparative morphometric study. Journal of anatomy, 164, 55–72.

Roselli, C. E., Larkin, K., Resko, J. A., Stellflug, J. N., & Stormshak, F. (2004). The Volume of a Sexually Dimorphic Nucleus in the Ovine Medial Preoptic Area/Anterior Hypothalamus Varies with Sexual Partner Preference. Endocrinology, 145(2), 478–483. https://doi.org/10.1210/en.2003-1098