They are all homosexual. In fact, the list is much, much longer. Here is what Petter Bøckman has to say about it:
No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at allWe can thus safely conclude that the argument against human same-sex marriage - that homosexuality is not natural - is invalid. If we wish to be informed by nature in this matter, proposition 8 should not have been passed (but of course, those who are against same-sex marriage are either homophobic or religious - they don't really care if it's natural or not).
In fact, I personally find this little story quite touching:
Roy and Silo, two male chinstrap penguins at New York's Central Park Zoo have been inseparable for six years now. They display classic pair-bonding behavior—entwining of necks, mutual preening, flipper flapping, and the rest. They also have sex, while ignoring potential female mates.The question then is why animals are homosexual at all. As reproduction really only does occur between male and female, it would seem evolutionarily disadvantageous to spend any time sexing up to members of your own sex. Lost effort. So why do it?
There are a number of hypotheses trying to explain the phenomenon. It could be to establish social dominance. I have heard this is the case with orangutans, where the dominant male will take the loser after a fight. It could be practice for prom night. Could even be that sperm is deposited on the other male, who then injects it into the next female he gets frisky with. Social cohesion is another possibility (which is ironic, because religion has also been posited to boost social cohesion). This is most probably the case in bonobos: 75 percent of bonobo sex is nonreproductive and that nearly all bonobos are bisexual (source).
These are all adaptive reasons for homosexuality. But there are non-adaptive possibilities too. Generally, it could be a by-product of another trait that is adaptive. It could be a developmental anomaly, such as an enlarged sexually dimorphic nucleus in the ovine medial preoptic area (source).
Last month a paper came out in the Journal of Evolutionary Biology (reference below) in which the authors suggest that homosexual copulations may be a behavioural mechanism that allow males to expel older, potentially low-quality sperm (reader-friendly report in National Geographic). The authors investigated the "dominance", "practice", and "sperm translocation" hypotheses mentioned above in flour beetles (Tribolium castaneum), but found no evidence for the first two, and only little for the third. Instead, based on direct observation of flour beetle sperm ejaculated upon another male, the researchers conclude that the beetles seem to use homosexual copulatory behavior to get rid of inferior spermatophores.
I find this hypothesis intriguing, but I really don't think there is enough evidence to conclude that flour beetle homosexuality serves such a mundane purpose. Without having to invest any time in such research, I would bet that this behavior is maintained in evolution because it feels good, which sex most probably often does for adaptive reasons. And as long as the males that indulge with other males also do it with females, the behavior need not be lost in evolution.
If the males are only so good at telling males and females apart, the fittest males may be those who don't think twice about who they get behind. Imagine this algorithm:
1) Find another beetle.
2) Mate with other beetle.
3) Check gender of other beetle.
4) Goto 1.
Depending on how easy step 3 is, that might be way more adaptive than this one:
1) Find another beetle.
2) Check gender of other beetle.
3) If other beetle is female, copulate.
4) Goto 1.
K. E. LEVAN, T. Y. FEDINA, S. M. LEWIS (2008). Testing multiple hypotheses for the maintenance of male homosexual copulatory behaviour in flour beetles Journal of Evolutionary Biology DOI: 10.1111/j.1420-9101.2008.01616.x