Transsexuality in animals other than humans? In some research papers, transsexuality is correlated with measurable differences in brain structure.  For example:


Zhou et al. (1995) inspected the central subdivision of the bed nucleus of the stria terminalis (BSTc), and found female-sized BSTc's in male-to-female transsexuals.
Berglund et al. (2008) found female-like hypothalamus activation in male-to-female transsexuals in response to smelling odorous steroids.


If there is a neurobiological basis for transsexuality, it seems reasonable to expect to see transsexuality in non-human animals.  This leads me to my question:

Has transsexuality been observed in animals other than humans, and how was it observed?

References:


Zhou et al., A sex difference in the human brain and its relation to transsexuality, Nature 378 (1995), 68-70.
Berglund et al., Male-to-Female Transsexuals Show Sex-Atypical Hypothalamus Activation When Smelling Odorous Steroids, Cereb. Cortex 18 (2008), 1900-1908.
Transexuality is a very social (and human) phenomena that's directly a consequence of how we define it as a society and indirectly a consequence of having distinct male/female ideologies.

Humans can be born as an ambiguous sex.  We have a wide spectrum of hermaphrodites from androgen insensitivity syndrom (i.e. genetically "male" but can be legally "female" and can have varying degrees of both sex organs).  So our definitions of male/female aren't exactly robust in the first place.

Most (probably all) animals do not have language and are unlikely to have social symbols that are meaningful to them.  Most plants can change sex under stress, oysters can, too.  Many organisms don't have a defined sex role (all of them are hermaphrodites).  C. elegans are mostly hermaphrodites but can have the rare male.  Often, the gender changes here are a direct biological response, rather than a social response, whereas with humans, it's more about responding to a social label.

There's some small chance that some other species have developed some sort of social sex expectations (I.e., gender) and have somehow had members challenge it, but it would be really hard to tell without sharing brain architecture, facial expressions, or language with them.