It is common to hear that homosexuality is frequent and "natural" in some animals, particularly in rams. Which leads to the question, If it's natural in animals it must be natural in humans?
Such claims are made about domestic sheep and the Rocky Mountain Sheep.
A typical claim is by J. A. Fitzgerald, 1 who is cited as saying that 16% of domestic rams never mate with females. Of that 16%, 6% are said to be asexual and 10% prefer rams to ewes. The corresponding behaviour is said to be rare among ewes. The figure of 10% homosexuality in domestic rams would have delighted Kinsey 50 years ago. However, like Kinsey's percentages, the ram homosexual percentages are equally inflated (see below). It is also sometimes reported that the circulating sex hormones in these "homosexual rams" are lower than in typical male sheep. This is taken to mean that homosexual behaviour is innate, the assumption being that the hormone levels are dictated by the genes and not influenced by the environment.
Rocky Mountain rams
Let's take the Rocky Mountain Sheep first. Their usual social pattern is a flock of ewes dominated sexually and organisationally by a very limited number of males who have achieved their position through ferocious and bloody combats in which they use their large, curled horns. The losers in these fights form a fringe group together with other bachelor males who are not yet mature enough to challenge for the leadership. In this group there are frequent encounters which seem homosexual in which males show most of the usual sexual behaviours, but in the presence of other males, and will quite often mount them.
However all is not as it seems. During the breeding season these fringe groups disperse and disappear (Fisher and Mathews.2 They have all joined the annual competition for the dominant heterosexual positions. This means at most they are bisexual. Heterosexual sexual expression is dominant for the time being and homosexual expression is abandoned in its favour. In this species homosexual expression in rams is for the losers - rather reminiscent of "situational homosexuality" among men in prisons.
A very similar process was observed following one controlled culling experiment (Shackleton).3 In one group of Rocky Mountain Sheep it was necessary for conservation reasons to shoot most of the dominant males. Following that, the candidate fringe males matured very fast and filled the vacant spots, exercising their usually frustrated heterosexual instincts. Researchers noted that they did this successfully - they were not poorly performing heterosexuals. For a season there was a lack of homosexual activity, because there was practically no fringe group.
This shows (a) exclusive homosexual activity is quite rare (b) it is highly dependent on social environment (c) it is considered by researchers to be an expression of dominance, real or attempted (d) change is possible.
However, Rocky Mountain Sheep are a wild population, and investigation is harder to do than amongst domestic sheep. Domestic sheep have been investigated quite thoroughly, particularly in Montana.
Amongst these sheep, Resko et al. (1996),4 showed that levels of a number of male sex hormones circulating in the blood in heterosexual rams were about double those in homosexual rams.
However it is well known that these hormone levels change in response to experience and the case for saying they are innate has yet to be made. That is, it is very likely that the loss of battles for dominant position lowers the hormone levels.
Domestic "homosexual" rams were identified by putting them through a kind of "rape rack", 2 rams and 2 ewes being confined facing inwards in four separate sides of an octagonal rack with their hind-quarters exposed (Price et al).5 The test ram was allowed to choose one of these four animals for mounting, over a set time limit (often about 10 minutes, which seems too short). If an animal chose a male to mount each time in 5 tests he was classified as homosexual. Large numbers of males had to be tested to find these homosexual rams.
There are other explanations of the rams' homosexual mounting behaviour. Geist6 reported that rams took it as an insult if other rams mounted them. How would an unsuccessful male who had lost dominance encounters until now react to the presence of a trapped male? He could easily (a) retaliate (b) be simply and naturally treating dominance as first priority (c) if more intelligent than usual, realise that it would be more strategic to mount the male and establish dominance - once that was achieved he would have all the females!
A ram with a defective sense of smell would also have no clear sexual preference. This is not as unlilkely as it sounds: one animal in 32 could be affected this way (the same chances as tossing a coin 5 times and getting heads each time). He might by sheer chance choose the male rather than the female several times in succession.
So I have reservations whether the test for "homosexuality" is very good.
Even if it is, the percentage of homosexual rams is not 10% although it tended to be in earlier papers with less rigorous testing. In the paper by Resko et al, 4 they started with 400-500 rams and tested them for homosexuality. Finally after all the tests only 6 classified as homosexual. That is just over 1%.
There was an ejaculation only once in five trials (but in a second group of trials there were about twice as many). This seems like rather apathetic sexual behaviour. (In fact Alexander et al.7 report data on rams which statistically fail to differentiate low sexual activity from homosexuality.) Similar is a paper by Stellflug and Berardinelli (U.S. Department of Agriculture)8 in which they found only one homosexual ram in a test group of 84 rams. Why use rams to argue for a biological homosexuality? These homosexual rams were basically sex-deprived, apathetic, and on the losing fringe of the animal kingdom.
1. The tests for homosexuality are dubious
2. Bisexuality is common - exclusive homosexuality seems rare
3. The documented rate is about 1%
4. Environment has a large effect.
1. Fizgerald, JA. (1997) Sexual orientation in Ellis L, Ebertz L, (eds). (1997) Sexual orientation: Toward biological understanding. Praeger, Westport, CT,
2. Fisher,A; Matthews,L (Date not cited). www.cabi-publishing.org/bookishop/readingroom/0851003974/ch8.pdf
3. Shackleton,DM (1991): Social maturation and productivity in bighorn sheep: are young males incompetent? Appl. Anim. Behav. Sci. 29, 173-184.
4. Resko,JA; Perkins,A; Roselli,CE; Fitzgerald,JA; Choate,JVA; Stormshak,F (1996): Endocrine correlates of partner preference behavior in rams. Biol. Reprod. 55, 120-126.
5. Price,EO; Katz,LS; Wallach,SJR; Zenchak,JJ (1988): The relationship of male-male mounting to the sexual preferences of young rams. Appl. Anim. Behav. Sci. 20, 347-355.
6. Geist,V (1975): Mountain sheep and man in the northern wilds. Cornell University Press, Ithaca. 248 pages.
7. Alexander,BM; Stellflug,JN; Rose,JD; Fitzgerald,JA; Moss,GE (1999): Behavior and endocrine correlates related to exposure of heterosexual, low performing and male-oriented domestic rams to rams and ewes in estrus. J. Anim. Sci. 77, 1869-1874.
8. Stellflug,JN; Berardinelli,JG (2002): Ram mating behavior after long-term selection for reproductive rate in rambouillet ewes. www.nal.usda.gov/ttic/tektran/data/000013/83/0000138334.html.