After probing and peering at the genitalia of nearly 500 crocodiles in Palo Verde, [Chris] Murray and his colleagues found something odd: The sex ratio was way out of whack, with males outnumbering females four to one among hatchling crocs. What's more, the animals' tissues were tainted with a synthetic steroid, which the researchers suspect was causing them to switch sex.

The hormone, 17α-methyltestosterone (MT), is sometimes prescribed for men with testosterone deficiencies and older women with breast cancer. Bodybuilders have been known to abuse it to bulk up. How could it end up in crocodiles from rural Costa Rica? A possible clue: Fish farms around the park raise tilapia on food laced with the hormone, which transforms females into faster-growing and more profitable males. Murray and his colleagues are now investigating whether MT from the farms has somehow contaminated the crocs.

The finding "has implications for the population and the broader ecological community," says physiologist Matthew Milnes of Mars Hill University in North Carolina, who was not connected to the research. Besides skewing the sex ratio, the hormone could be disrupting the animals' reproduction, a concern because American crocodiles are already listed as vulnerable, and this part of Costa Rica is a stronghold for the species. The contamination could also be altering the crocs' behavior, perhaps making them more belligerent. If so, conflict with humans—which Murray says is already a sore spot in Costa Rica—could increase. The substance could harm turtles, birds, fish, and other aquatic creatures as well. And because fish farms throughout the tropics are feeding chow that contains MT to their tilapia, the hormone may be causing problems elsewhere.

For more than 20 years, researchers have fretted about the effects of endocrine disruptors, molecules that meddle with the body's hormones. Crocodilians—the group that includes crocs and alligators—have furnished some of the most dramatic examples. In the 1990s, for instance, scientists reported that male alligators from Florida's Lake Apopka, which was fouled by a brew of hormone-mimicking chemicals, had shrunken genitalia and reduced testosterone levels.