More than 450 Chinese alligators in east China's Zhejiang Province have begun deep hibernation a week earlier than usual because of the recent cold weather.

The reptiles also known as Yangtze alligators spend winter in self-dug burrows or dens, with constant temperatures above 12 C, and their mating season occurs shortly after they awake.

But early onsets of winter temperatures can leave the reptiles too tired to dig and enforce their burrows.

However, workers at the Chinese Alligator Village, Changxing County of Zhejiang, fortified the dens before the cold spell began as a precaution.

According to experts, disturbed hibernation may lead to death or affect their ability to reproduce.

"They will spend about five months hibernating in complex caves on banks of their ponds to escape climate extremes," Wang Zhenwei, a technician from the site, said on Tuesday.

"The severe cold froze the ponds, which reminds alligators of the coming hibernation season."

Temperatures dropped to -4 C when the cold front hit Changxing earlier this month, which caused the early hibernation of these alligators, Wang told China Daily.

Staff have also used straw and leaves to cover banks and entrances to the burrows to raise the temperature of surface soil, according to Wang.

Meanwhile, about 20 Chinese alligators in Shanghai Wild Animals Zoo have also dug their burrows to remain warm and survive the winter.

Sun Qiang, director of the animal feeding department of the zoo, said on Tuesday that 10 out of the zoo's 30 Chinese alligators had been placed in a special greenhouse, with its temperature controlled by an infrared temperature controller due to a lack of space for them all to dig burrows.

"All the alligators have the ability to burrow by nature, however we don't have enough space for all of our 30 Chinese alligators to dig holes in our zoo, so we have had to build a greenhouse for some of them to hibernate in," Sun said.

"To ensure the alligators have a nice winter in a man-made environment, the greenhouse must resemble the natural condition as much as possible," Sun said. The animal keepers cut off their food supply from mid-October.

The greenhouse's temperature is kept at around 12 C. The animal keepers have to ventilate and disinfect the greenhouse everyday.

Even though the greenhouse is no less suitable for the animals than their own burrows, Sun still hoped the zoo would be able to expand its alligator exhibition area, so that all the Chinese alligators in the zoo can build their own dens when winter comes next year.

The Chinese alligator has existed for 230 million years, and is known as a "living fossil." The species has been listed as one of the most endangered creatures in the world and is now under the country's grade-one protection scheme.

There were more than 10,000 alligators across the country by the end of 2004.

(China Daily December 15, 2005)