The keepers face the daunting task of totting up every mammal, bird, reptile, fish and invertebrate — around 17,000 creatures across 750 different species — during the week-long count.
Last year saw the arrival of many new animals at the central London zoo, from the world’s first zoo-bred Lake Oku clawed frogs to a litter of 11 endangered African hunting dogs.
Edward the two-toed sloth was born in July while December saw the birth of a western lowland gorilla, all of which will have to be included in the 2016 census.
While for some keepers the task is as simple as “one, two, three”, imaginative tactics are used by others to ensure every creature is accounted for.
Aquarium keepers take a photo of the tanks and use still images to avoid counting the same fish twice, while at Penguin Beach keepers wait for the birds to line up at feeding time to do their headcount.
The annual count is a compulsory requirement for the zoo to maintain its licence, with all of the information collected then shared with zoos around the world via the International Species Information System.
It is then used to manage the worldwide breeding programmes for endangered animals.