London Zoo's animals have annual weigh in

This is not a job for the faint hearted. Keepers at London Zoo are undertaking the monumental - and annual - task of weighing and measuring all of the zoo's 18,430 animals.

While squirrel monkeys had to be lured to sit still on the scales with a treat, the zoo's penguins were far better behaved. In fact, they even queued up to get weighed.

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The head of zoo management at the attraction said the task shows visitors the sheer number of animals in residence - and how different they all are.

Mark Habben, Head of Zoo Management at London Zoo, said: "Our heaviest animal here is one of our giraffes weighing nearly a tonne... about 860 kilograms, the lightest would have to be something like a leaf cutter ant which is just minuscule, it wouldn't even reach in the grams, it is a tiny, tiny species."

The zoo uses the data it collects to check on the overall health of their animals - and to see whether they might need to go on a diet, or be eating for two.

Habben adds: "It tells us if an animal is going up and we need to change its nutritional diet. It might even be an indication of pregnancy in some species, so again we might need increase food as much as reduce the levels."

London Zoo also conducts a head count of all of its creatures every January - and adds it to a global database of wild animals in captivity.