Transvestite Snakes

Some male red-sided garter snakes secrete female scents from their skin to seduce unsuspecting male competitors and impel them to exhaust their energy trying to mate with what they believe is a female. With their competitors duped and drained, the 'she-male' snake has a much greater opportunity to approach the real females and copulate with them.

How do they do that ?
If a female is emitting the appropriate pheromone, she is quickly surrounded by a large number of males. This group is called a "mating ball" with 10 to 100 males intertwined with a single female. When the female raises her tail, exposing her cloaca (sexual cavity), the best-positioned male inserts one of its two penises. The other males slither off.

When a chemical cross-dresser slithers into the mating ball, heads begin to turn. The disguised male smells just like a female. Flicking their tongues in the air, the normal male snakes detect the pheromones of a female. Sensing another "female" in the sexual arena, many of the normal males turn their attention to the newcomer.

Male garters that exhibit this trait are more than twice as successful at mating with females!!

Male and female garter snakes share almost all of their pheromones, but exhibit them in different ratios. Zoologist Robert Mason found out that squaline, a chemical exhibited in much lesser amounts in females than in males. When squaline was added to a paper towel soaked in female pheromones, half of the male snakes lost interest and slithered away. Conversely, without the squaline, they were all intensely interested.

Apparently, squaline is what sets males apart from females. It also separates males from she-males. The she-males exhibited male and female pheromones, but more importantly they lacked this male-repelling squaline.

Would the red-sided garter population be overrun with she-males ?
NO, because less than one in every hundred males have this trait!

Source: http://www.discoverychannel.ca/animals/snakeshome/snakearchives/transnake/