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ECOLOGICALLY, ALL living things are related, sometimes to their benefit, other times not. This is exemplified by an article in the September 1, 2017, issue of Science, the American Association for the Advancement of Science’s weekly magazine.

“The Case of the Macho Crocs,” by Mitch Leslie, describes a heavily male-biased sex ratio among Costa Rican crocodiles that’s traceable to a synthetic hormone used by bodybuilders and fish farmers.

An American crocodile, Crocodylus acutus, This and other images from Science, September 1, 2017.

By the way, I used to think that alligators were New World creatures while crocodiles were Old World. (There was some doggerel rhyming “dile” with “Nile.”) Not to diss yet another bit of my youthful knowledge, this isn’t true.

They’re both species of the order Crocodilia. Crocodiles live throughout the tropics in Africa, Asia, the Americas and Australia. There are two extant species of alligators, the American alligator and the Chinese alligator. And, in particular, crocs and gators coexist in south Florida, for example.

There are ways to tell crocs and gators apart, but my advice is to avoid either unless you’re a Crocodilia researcher.

As noted in Science, unlike humans and many other species, “crocodiles and alligators do not have sex chromosomes. Instead, whether an embryo becomes a male or a female depends on the nest temperature during incubation.”

Crocodile populations in the wild typically tend to a 50/50 split of gender. However, research of crocs in Costa Rica’s Palo Verde National Park identified a male/female ratio of about 3:1. What’s more, a followup study of 474 crocs from seven sites there exhibited an even wider split, about 3.5:1, a ratio that held across croc age. Males made up almost 60 percent of hatchlings and 60 percent of the adults as well.

This contradicted what might have been expected based on the region’s climate change: In less than 20 years, the average low temperature in Palo Verde has risen about 2.5 degrees Celsius/4.5 degrees Fahrenheit.

To test the influence of this temperature increase, researchers stashed temperature recorders inside faux eggs and buried them in 25 croc nests. Based on nest temperatures, female hatchlings should have outnumbered males by nearly 2:1.

Males in green; females in blue.

Clearly something must have been overriding the effect of temperature.

A suspected culprit is the growth hormone 17α-methyltestosterone, aka MT, used by bodybuilders—and also by fish farms near the Palo Verde National Park. The farms put MT in the food pellets of their tilapia stock to transform females into faster-growing and more profitable males.

To test this hypothesis, researchers dabbed three different concentrations of MT on American alligator eggs. They then incubated the eggs at temperatures that would ordinarily yield only females. Noted Science, “About 60 percent of the eggs dosed with the two highest MT levels developed into males.” Later, Science continues, “The group reported finding the chemical in samples of blood and egg yolks from the Palo Verde crocs, confirming that they had been exposed to it.”

Tilapia farming has boomed in Central America. This mild-tasting fish is the fourth most popular in the U.S.

Regulatory agencies studying the matter say that tilapia raised on MT-containing chow are safe for people to eat. Yet, L. Earl Gray is a reproductive toxicologist at the U.S. Environmental Protection Agency, and Science quotes him saying, “MT does not occur normally in the environment. It can only occur in the environment from human activity.” What’s more, contrary to earlier assumptions, it’s reported that MT is not always biodegrading.

It’s not clear that the fish farms must take the rap: MT has shown up in blood and yolk samples in other Costa Rican river systems. It’s possible, for example, that MT used legally or illegally by humans could be the entering the environment through sewage systems. In fact, Science notes that not all scientists think the case against MT is closed. More research is called for, with different crocs living elsewhere in the world.

Researchers carefully draw blood from a Costa Rican crocodile.

Those applying are advised to leave their good shoes at home. This is hot, muggy, and muddy work. To study up on crocodiles, check out “Science Duo” and “Creatures Great and Small” here at SimanaitisSays. ds

© Dennis Simanaitis,, 2017

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