I am not sure of the reasons for homosexual behaviour in birds, but the
following paper reviews the incidence and reasons of homosexual behaviour in
mammals:
Dagg, A.I. (2008). Homosexual behaviour and male-female mounting in mammals
- a first survey. Mammal Review 14: 155-185.
The abstract for this paper can be viewed at:
http://www3.interscience.wiley.com/journal/119853880/abstract
ABSTRACT
Information from the literature is given on the presence or absence of
homosexual behaviour and female-male mounting in 125 species of mammals,
both captive and wild. Such behaviour occurs in the male and often female
young of many species soon after their birth. It is more common in young,
often in play, than in adults. Adult homosexual behaviour is widespread in
male and female mammals (recorded in 63 and 71 species respectively), but
common in few species. In males it is most likely to be correlated with
dominance and thus to occur in species with hierarchies such as terrestrial
monkeys and members of the sheep and goat tribe. In females it is often
correlated with sexual condition; a female in heat most often mounted
another female, and one in heat was next most likely to be mounted by
another female. Anoestral females rarely mounted other anoestral females.
Females of 43 species mounted males, which often excited them sexually.
Captive mammals tended to mount animals of the same sex more often than did
wild ones when comparative data were available. Domestic animals also
mounted more man did wild ones, with several exceptions. Some phylogenetic
groups of animals displayed similar degrees of homosexual mounting, but
there was often considerable variation between closely related species. Nor
could homosexual mounting be always correlated with the social structure of
a group. The four reasons for, or contexts of, homosexual and female-male
mountings were social play (in 34 species), aggression (19 species), sexual
excitement (36 species), and physical contact-non-play (30 species). This
last category included a state of tension, getting attention, greeting,
grooming, caressing, reassurance and appeasement. There was some overlap
between categories. Homosexual pair-bonds occur in captive mammals and have
been observed throughout the year in non-captive female Japanese monkeys.
Stephen Ambrose
Ryde, NSW
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