Something is changing the sex of Costa Rican crocodiles.

Deze (sterk bewerkte) foto van Rob Blanken is geplaatst in Science in een artikel over American Crocodiles.

An American crocodile in Costa Rica’s Tárcoles River, where a sex-altering pollutant has been detected.

If you want to know whether a crocodile is a male or a female, you have to catch it. Don’t bring your good shoes. “It’s a muddy, wet mess,” says Chris Murray, who spent three dry seasons in and near Palo Verde National Park in Costa Rica, capturing American crocodiles (Crocodylus acutus) and determining their sex with a revolving team of helpers.

Even at night the heat is smothering, and a halo of bugs swirls around headlamps as the team motorboats down waterways or stalks the animals from shore. When Murray and his colleagues spot a croc, often half-submerged, they wade in after it or pursue it in the boat. In a typical catch, his friend Mike Easter uses a noose on a pole to snare the animal, which can be 2 meters long or more. As the croc thrashes and spins, Murray says, “everyone yells a bunch of stuff.” Once it calms down a bit, they cover its eyes with a towel to reduce stress, close its jaw with tape, and lug it to the bank, stumbling through shoe-stealing mud.

With the animal restrained, telling its sex is straightforward, says Murray, who is a physiological ecologist at Tennessee Technological University in Cookeville. “You have to put a finger in its cloaca,” the cavity at the base of the tail. “If there’s a structure there, it’s a boy. If there isn’t a structure, it’s a girl.” Catching baby crocs is easier, but sexing them is trickier, so the researchers bring the animals back to the park’s research station. Both sexes sport a tiny, red nub in the cloaca, but in males it tends to be longer, redder, and more complex, with an extra lobe.

After probing and peering at the genitalia of nearly 500 crocodiles in Palo Verde, 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. etc etc.