Crocodiles' hormones stay perfectly balanced even after repeated exposure to pesticides, according to researchers in Japan. 

A team of researchers, lead by Yasuyuki Shibata at the national institute for environmental studies in Tsukuta, studied crocodiles living in an isolated river next to an obsolete cotton farm in Western Australia.   The researchers found very high levels of organochlorine pesticides stored in the crocodiles' bodies, but the toxins caused the crocodiles no ill effects. 

Over 800 tonnes of organochlorine pesticides were sprayed at this isolated cotton farm between 1964 and 1974.  The pollutants drained into the nearby Ord River, home to freshwater crocodiles.  As freshwater crocodiles don't travel far from home, they provide an excellent opportunity to study the accumulation and long term effects of pesticides, said Shibata

The researchers measured the concentrations of many different organochloride pesticides in the liver and body fat of the crocodiles.  The crocodiles' ages were predicted from their snout length, and concentrations of pollutants correlated with the estimated age.  The pesticide concentration in the older crocodiles are some of the highest ever reported for wildlife, said Shibata.

Organochloride pesticides are known to affect sex hormone systems in animals, causing problems with sexual organ development and reproduction.  The sex hormone levels in the crocodiles' testes and ovaries were examined, and the resilient crocodiles showed no signs of hormone imbalance.   

Shibata plans to study the environmental fate of these chemicals, and investigate how other wildlife in this isolated and unique area is affected by the pollutants. 

Nina Athey-Pollard