So what was altering the sex of the crocodiles? A synthetic hormone called 17α-methyltestosterone (MT) was found in
the tissues of crocodiles from Palo Verde. MT is routinely fed to tilapia in fish farms close to the park, as it converts
females into males, which are preferred commercially as they grow faster. To test the theory that MT could have a
similar effect on the crocodiles, scientists exposed American Alligator eggs to the hormone and found that higher
concentrations resulted in about 60% of the eggs developing into males.
Although the most plausible explanation may be that the crocodiles in Palo Verde are ingesting escaped tilapia that
have been raised using MT, there is as yet no direct link to the fish farms. Bodybuilders use large amounts of MT to
bulk up, and it has been suggested that pollution from waste upstream in the capital of San Jose may be responsible for
MT contamination.
Despite the fact that low concentrations of androgens like MT have been shown to be harmful to pregnant women,
tilapia fed MT are still considered safe for human consumption. However, it is obvious that MT is remaining active in
the environment and not biodegrading, as previously thought. This has possible widespread implications for many
aquatic organisms, since tilapia are farmed in more than 80 countries, and use of MT as a supplement is commonplace.
If fish farms are positively identified as the source of contamination, the solution may be as simple as applying
appropriate protocols to the use of MT and management of stock.
But there are further question marks concerning the American Crocodiles of Palo Verde, particularly since this region
remains a bastion for the species, which is already listed as vulnerable. Are males contaminated with MT fully
functional and able to reproduce normally? Is the male hormone making them more antagonistic and potentially
dangerous to humans?
The research team is now studying MT exposure in American Alligators, and believes that post mortem examination
of crocodiles euthanized for aggressive behaviour may also yield answers. Lead scientist Chris Murray hopes to
expand the project to South Africa and Indonesia.
Reference:
Leslie, M (2017), The Case of the Macho Crocs, Science Magazine, 357(6354) pp. 859-861.
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