The Cold Blooded News

The Newsletter of the Colorado Herpetological Society

Volume 28, Number 10; October, 2001

Crocodiles Too Scared to Protect Their Unborn as Day of the Trifid Arrives

Reprinted from the newsletter of the St. Louis Herpetological Society, Vol.28, No.7, July 2001.

The Guardian, 5/26/01: With their wide gaping jaws and glistening teeth, crocodiles are considered to be cruel, sinister killers with no known predators except humans. 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 Sporila, 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.

When a crocodile succeeds in burying its eggs in a site infested with trifid weed, the reduced temperature within the sand - because of shading by the plant - causes most of the hatchling crocs to develop into females, or prevents eggs from developing at all. 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. In some instances, however, female reptiles can alter the sex of their offspring according to their requirements. This is true of certain lizards and turtles that choose nesting sites of different temperatures according to sex preference.

In St. Lucia, Nile crocodiles do not have the luxury of selecting nest sites. Because they breed only once a year, shortly after the spring rains, they cannot vary the external temperature by breeding in different seasons. They have to dig their nests in any available space, regardless of the presence of the alien plant invader. Consequently, their nesting sites are cooler than normal and the offspring are mainly female.

Their extinction at Lake St. Lucia would have terrible consequences for other species. Though they reach up to six metres and weigh as much as 10 men, Nile crocodiles feed mainly on carnivorous fish and only occasionally capture large mammals. In the absence of crocodiles, predatory fish would proliferate at the expense of herbivorous fish, the main food of birds. Bird numbers would decline, with consequences for other species.

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