There are still only three giant tortoises at Bristol Zoo but last year Biggy, Twiggy and Helen were joined by an orphan gorilla, a baby lemur, an armadillo, hundreds of tropical butterflies and the smallest deer in the world.

This week, the annual stock take of all the animals in the zoo is taking place, which means a complete head count of every single creature, large and small.

More than 450 different species live at the zoo, from tiny leaf insects and hundreds of fish, to dozens of penguins, monkeys and Jock, the 150kg male lowland gorilla.

Some of these species, such as partula snails, are extinct in the wild but survive in zoos like Bristol.

The census is an important way of recording the rise in populations to monitor and manage animal collections. It is also part of compliance with zoo legislation that requires precise records to be kept of every animal birth, death, arrival and departure.

In the penguin enclosure, the hatching of 11 African penguin chicks last year boosted the zoo colony to 71.

Curator of birds Nigel Simpson said: "Monitoring a growing penguin colony is important as we need to know who is who and, although the birds do have a unique spot pattern, they are not easily distinguished from one another. We get round this by fitting silicone bands on the penguins' flippers. Each band has a unique number so we can easily identify and count our penguins."

John Partridge, senior curator of animals at Bristol Zoo, said the annual census is important as it acts as an audit to check that their records are accurate. He said: "Our collection records are far more than a simple count – we know detailed information on individual animals and groups, which we share with colleagues around the world to help manage our most important assets – the animals."