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Dr. Charles Roselli

Dr. Charles Roselli is a scientist at Oregon Health & Science University in the Department of Physiology and Pharmacology. He is known for his groundbreaking research showing a homosexual orientation in rams. Roselli received his Ph.D. from Hahnemann University in 1981.


Social conservatives like to say that homosexuality is unnatural. However, doesn’t it occur in nature?

Bruce Bagemihl has written a book, Biological Exuberance, where he has documented that over 1,500 species of animals show some form of homosexual behavior. None of them studied as rigorously or as well as we’ve studied rams.

How did you get into studying the sexual orientation of rams?

Sexual behavior is of great interest to sheep farmers. They only use one ram to breed 50-100 ewes. And if that one ram is not performing — a shy breeder as they call it — it has economic consequences. So, our research really started as a collaboration with a sheep station in Idaho.

How do you recognize same-gender attracted rams?

A subset of those animals is given what we call a sexual partner preference test. That’s really a choice test. We put them in with two males and two females and let them make a choice of which one they want to interact with, mate with, mount, that kind of thing. That test is repeated again in a second year to make sure that their behavior and their preferences are not changing over time.

What did your research find?

So, what we found when we looked at the brains of these rams, there is a cluster of cells or neurons in a part of the brain called the pre-optic area, which is known to be involved in regulating sexual behaviors. The cluster was larger in female oriented rams than in male oriented rams. It was also larger in female oriented rams than in ewes. Male oriented rams and ewes had a cluster that was about the same size. That says to us that there is an association between the size of this area and the sexual preference of the sheep.

Was this change due to nature or nurture?

What we’ve done is look at fetal rams to see whether the sex difference in the brain area is present before the animals are born. Before they have any interaction with any other animals. Before they’ve been able to behave or show any behaviors. We reasoned that maybe if we looked towards the end of gestation we might see a fully formed brain area that would correspond to the one that we see in the adult. So, we did this and actually our hunch proved to be correct. By 130 days of gestation, of a 150-day gestational period, there is an SDN, which is called the sexually dimorphic nucleus, that’s the brain area I’m talking about, and it’s present in the brain of the fetal animals. So, that means it developed before the animal was born.  That lets us think that it is the SDN that comes first and it then influences the behavior as the animal grows.

So the rams’ homosexual orientation is due to biological factors?

All the evidence that we have point to there being a biological influence on the development of the brain area. And by inference on development of the behavior. There has been some research on rearing. Rams are reared in all male groups, so you might think it’s sort of a boarding school effect. But there has been some research on that. And actually it doesn’t matter if they are reared with other males, reared with females, or reared alone. There is still a proportion of animals that shows same-sex behaviors.

What would you like people to know about your study?

I think what I’d like people to understand is that sexual behavior in many species, and rams being one that I studied are varied. There is not one type of sexual behavior — that being oriented male to female or female to male. That when you really stop and look, test the animals behaviors, there is quite a bit of variation. And it’s not just situational or one time kind of affairs. There can actually be attractions for same sexes that are as enduring as opposite sex attractions. This is a behavior that occurs naturally and it is a variation that occurs naturally.