On Japanese TV yesterday, they were talking about how, in some species of frog, sometimes a male mounts another male. When that happens, it's up to the "mountee" to "sing the appropriate song" and get the other fella down. I found it interesting that the ability to discriminate between sexes can be so ... limited.
Then while reading,
"Individual Development & Evolution" (Gottlieb) p. 167 writes:
Another remarkable organism--environment coaction occurs routinely in coral reef fish. These fish live in spatially well-defined, social groups in which there are many females and few males. When a male dies or is otherwise removed from the group, one of the females initiates a sex reversal over a period of about two days in which she develops the coloration, behavior, and gonadal physiology and anatomy of a fully functioning male (Shapiro, 1981).
Nemo... say it ain't so!
Nature is cool.