An annual head count got under way Thursday at the London Zoo, where keepers are cataloguing more than 17,000 creatures -- every invertebrate, bird, fish, mammal, reptile and amphibian.

Staffers armed with clipboards will add to this year's "stocktake" two endangered Sumatran tigers, two Galapagos tortoises and a rare white-naped mangabey (a type of monkey), among others.

The zoo is required, as part of its licence, to log its count into the International Species Information System (ISIS). The data is used to manage breeding programs of endangered animals.

Six different kinds of poison dart frogs and half a dozen types of stick insects make for slow work, so the inventory can take several weeks.

Last year the zoo recorded a total of 17,519 animals represented by 767 species - more than 100 of which are listed as threatened.

The famed zoo, opened in the English capital in 1828, is operated by the Zoological Society of London, which counted Charles Darwin as an early fellow.