A few months ago, we acquired about a dozen and a half of guppies. They were fry, or immature at the time we got them, and one of the issues we have, now that they are grown, is that we have too many males for the tank. As a result of this the female guppies are forever harassed, as are the less dominant male guppies. We also now have a number of fry in the tank, which will either further complicate, or perhaps alleviate part of the problem.
Anyone familiar with guppies know just how prolific of breeders they are. I commented on facebook, half in jest (I know mortality rates in the wild are high) that I wondered how streams with natural guppy populations don't quickly become plugged with them.
I was taking a long look at the tank just now and I think one of the females may have developed a strategy to avoid some of the harassment from the boys in the tank. In general guppy females are larger than the males, but I've got one male who is the largest fish in the community. I thought I had two large males, but I now believe that one is a female disguising herself as a male.
This female is more colourful than other females, but not as colourful as the males. She doesn't seem to be bothered much by the males (although there has been some bothering as she appears "pregnant" to me - guppies are livebearers and the females when gravid get boxy and large). She displays like a male from time to time. Most interesting is what she does with her anal fin when there are males near by.
In male guppies the anal fin is modified into what is called the gonopodium. Its structure allows it to transfer sperm to the females vent. Unlike most fish fertilization takes place internally, and the eggs develop inside the female. The gonopodium generally lays along the fish's body, but the fish can swing it around in order to fertilize the female. It doesn't look like a typical anal fin (although it did before the fish matured) and you can think of it as being somewhat analogous to a penis.
Now the very interesting thing about this female is that she will collapse her anal fin up against her body when there are males around, further looking like a male guppy, completing her disguise. I have no idea if this is a common strategy, or that if given her colouring and behaviour she has characteristics of both sexes. But is certainly a fascinating bit of natural history, all in the warm confines of our dining room.