Noah may have been able to persuade the animals to go in two by two but zoo keepers were faced with a much more disorderly bunch as they tried to carry out a census.

Meerkats were among the most awkward of customers, with a pair at London Zoo giving birth to a litter only hours before the census started.

While keepers were delighted at the new additions, they were unable to make an accurate count because the meerkat parents had dug a deep burrow that put the unknown number of kittens beyond the reach of human eyes.

Cold weather further complicated the census because a wide range of animals were determined to remain curled up in hidden sleeping areas.

Nevertheless, over the next few weeks keepers in zoos around Britain will have to count all the animals in their care, whether giraffes, snails, frogs or fish.

There are, however, occasional dispensations for keepers facing seemingly impossible counts, such as ants for which they are allowed to record the number of colonies rather than the number of individuals.

David Field, the zoological director of London Zoo, said: “The annual count is a mammoth task, but something that must be done each year as part of the terms of our zoo licence.

“The cold weather made it extra tough this year, as many of the animals are staying in the nest boxes or dens keeping warm.”

He added: “Of course, with the bigger animals it’s a lot easier. With the smaller animals, animals like shoals of fish and flocks of birds, we have to have census techniques where a couple of keepers are actually doing the census at exactly the same time so we can average the results.”

The annual census is carried out to make sure there is a record of every birth, death, arrival and departure that took place over the last year. Other zoos taking part include Bristol, Edinburgh, Belfast, Blackpool and Chester.

Once the keepers are satisfied they have an accurate account of all their charges the census is submitted to the British and Irish Association of Zoos and Aquariums (Biaza).

Data generated by the exercise can help to identify where zoos are most successful and where there may be animal welfare concerns.

The inventories can also be passed on to the International Species Information System (Isis), which holds information about the inhabitants of zoos and wildlife parks around the world.

Such information is invaluable to conservationists in charge of captive breeding programmes because it helps them work out where rare animals should be sent to have the best chance of mating.

When deciding which animals should be paired up for breeding programmes, keepers need details of their parentage to ensure as wide a genetic pool as possible.

Dr Miranda Stevenson, director of Biaza, said: “The annual stock take is a big job so January is a busy time of year for many zoos across the UK.

“It’s important that zoos ensure their figures are correct so they can best manage conservation. Some of the species in captivity, such as Socorro doves, are now totally extinct in the wild. Therefore when zoos submit data to these central databases, it means that we can run detailed and scientifically-based breeding programmes to safeguard these valuable and threatened species.”

Among the new arrivals in 2008 at London Zoo, which a year ago boasted 15,104 creatures — including 612 cockroaches, 331 Spiny anemonefish and 57 Madagascar golden frogs — was the first warthog born there in more than 100 years. At Bristol Zoo the arrivals included 11 African penguin chicks.

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