But in Lake St Lucia, a pristine South African wilderness, they may have met their match. An alien plant species is causing Nile crocodiles to develop predominantly into females - a feminisation that may lead to their extinction in the area. Chromolaena odorata, also known as the trifid weed, was accidentally introduced into South Africa last century in seed-contaminated goods from ships in Durban harbour. This alien invader spread rapidly and finally invaded the World Heritage site of Greater St Lucia wetland park.
Female crocodiles prefer to lay their eggs in open, sandy and sunny areas close to water.
They normally return to the same nesting site year after year, despite the intrusion of the trifid weed into their nesting areas. They dig an egg chamber and carefully bury their eggs in the warm sand. Each female then guards her nest, viciously attacking any intruders, until her young hatch three months later. Alison Leslie and James Spotila, from Drexel University, Philadelphia, and the department of nature conservation, University of Stellenbosch, South Africa, report in the journal Biological Conservation that Nile crocodile mothers abandon previous nesting sites if they encounter the trifid's fibrous roots. The sex of hatchlings of other reptiles such as alligators, turtles and certain lizards, also varies with the external temperature, and this is often due to seasonal changes.