The case of the macho crocs

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Science  01 Sep 2017:
Vol. 357, Issue 6354, pp. 859-861
DOI: 10.1126/science.357.6354.859

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Researchers have found a heavily male-biased sex ratio among crocodiles in Palo Verde National Park in Costa Rica. Among hatchlings, males outnumber females by about four to one. However, warming temperatures in that part of Costa Rica should be tilting the sex ratio toward females, the scientists showed. They have also discovered that the animals are contaminated with a synthetic hormone known as 17α-methyltestosterone, or MT, which may be shifting the sex ratio toward males. MT has some medical uses and is sometimes abused by bodybuilders, but the researchers are investigating whether the hormone comes from fish farms around the park, which use MT-containing feed to turn female tilapia into males. The scientists are now trying to determine whether MT alters the animals' behavior and whether it is having effects in other locations where crocodiles and fish farms coexist.