Brain Morphology and Sexual Partner Preference in Sheep

Variations in sexual attraction occur spontaneously in many animals. In domestic sheep populations as many as 8% of rams exhibit an exclusive sexual partner preference for other males, classifying them as male-oriented, whereas the majority of rams can be classified as female-oriented. We are using the male-oriented rams a model to begin to understand the hormonal, developmental and genetic contributions to sexual partner preferences. Our studies demonstrate that there are differences in brain structure between male-oriented and female-oriented rams, suggesting that sexual partner preferences are neurologically hard-wired. We identified a sexually dimorphic nucleus (oSDN) in the sheep preoptic area that is 2 –3 times larger in female-oriented rams than in male-oriented rams and ewes. One signature of the oSDN is the abundant expression of aromatase mRNA. These results suggest that there is a relationship between steroid hormones and oSDN morphology.


The oSDN is present in the sheep brain prior to birth. It can be identified by day 130 – 140 of gestation, term = 150 days. In many species sexual dimorphisms of the preoptic are organized by testosterone during a critical period for sexual differentiation of the brain. We showed that the volume of the oSDN in genetic females could be enlarged to the size of a genetic male by exposing them to testosterone during the critical period for sexual differentiation. These data demonstrate that testosterone acts during fetal development to organize the development of aromatase-expressing neurons into the male-typical oSDN. By inference these results, suggest that sexual partner preferences may also be organized prenatally.