I researched the “90% of observed giraffe sex is homosexual” thing about a year ago. My conclusion was that this is a very tendentious interpretation that obscures what little we know about giraffe sex.
Pratt and Anderson studied among other things two different behaviors. First, there is “necking”, when two male giraffes, usually both young, have a ritual battle with each other using their necks. The battle is almost always nonviolent, the “blows” are slow and gentle. This might be a way of determining social status within the group, but they’re not entirely sure (there’s a different method of asserting dominance that’s much more unambiguous). In about 10% of the necking incidents they observed, the battle ended by one of the giraffes trying to mount the other. It was always a larger giraffe that tried to mount the smaller. They observed 16 cases of such behavior over three years. Attempts to mount do not seem to be actual sex; there are no thrusts described as there are when mating (see below). Despite their initial assumptions, they found no evidence of dominance/submission (in body posture, subsequent behavior, etc.) associated with these mounting attempts. They also saw 4 attempts of female giraffes trying to mount younger and smaller male giraffes, again with no evidence of dominance/submission.
Mating, on the other hand, is a very different sort of activity, and a very rare one due to several factors. The giraffe cow goes into estrus (“heat”) about one day every two weeks, but the status of being in estrus is not detectable visually. The giraffe bull will not attempt to mount the cow unless he knows she’s in estrus, but the only way to know is to taste the cow’s urine using flehmen response. Typically there’s a small herd of 10-15 cows and calves, and a bull will approach and try to test every cow to find one that is in estrus. Testing is done by nuzzling the cow’s rump with his head, and then the cow must choose to urinate at that moment, so that the bull can taste some urine with the flehmen response. Cows have the freedom to choose whom to urinate for, and exercise this freedom freely to select a desirable mate. Assuming the cow chooses to urinate and the bull tastes that she’s in estrus, the bull may choose to go into courtship mode. In courtship mode, he will accompany her for 1-2 days, they will walk together and eat together, and occasionally he will try to stand behind her and attempt to mount. Nearly always the cow frustrates his attempt by simply walking forward. Very rarely, “she may stand for him; he mounts, makes three or four vigorous thrusts the last of which brings his head and neck into a position so nearly upright that he seems to be about to fall over backwards, and in a moment they have both returned to browsing”.
In three years of studies, Pratt and Anderson saw about 300 urine-testing attempts, about 40 courtship scenarios, and only one successful mating. At the same time, the giraffe population grew by 22 calves in one year out of these three, so it’s absolutely guaranteed that they saw only a tiny fraction of mountings.
To sum up: mating happens very rarely and always during a courtship period. Mating attempts are much more numerous but are nearly always thwarted. Male-to-male mounting attempts happen as part of a ritual sparring battle, do not involve courtship, do not (seemingly?) involve a sex act, and their social significance is obscure. But if you combine 1 observed successful mating with 16 observed male-to-male mounting attempts, and you throw away everything else, you get “94% of observed sex acts between giraffes are homosexual”. This calculation is of course not part of the original articles.
Quoted from: https://goatse.cx/2014/09/15/ozy-a-response-to-spandrell/#comment-145528