A zoo keeper has the unenviable task of tallying the total number of squirrel monkeys at the London Zoo in central London during the zoo's annual stock take yesterday.

He uses food to keep the mischievous and highly active creatures sufficiently occupied to make the count.

And they are not even the hardest animals to keep track of. Things get even more complicated with invertebrates such as spiders and jellyfish.

The London Zoo is the world's oldest scientific zoo, first opening to the public in 1847. It now welcomes more than a million visitors a year.

The compulsory annual count is required as part of the zoo's licence.

The Mail Online reported that at last count, there were more than 17,000 animals living at the site, comprising more than 750 species, and all have to be accounted for again this year by the 70-strong team - a process that usually takes about a week.