Lions, meerkats and penguins were on display along with a host of other species as the annual weigh-in at London Zoo began Thursday.
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The annual weigh-in provides an invaluable opportunity for zookeepers to make sure the information on the animals they've recorded throughout the year is up to date.
There are around 560 species and 19,000 individual animals in the zoo, meaning the annual weigh-in takes over seven days to complete.
Among the animals ABC News witnessed being weighed were a meerkat named Dracula, the heaviest of the zoo's mob, who weighed in at 2.2 lbs.
Meanwhile, Oni the okapi -- an endangered species from Central Africa and a relative of the giraffe -- tipped the scales at 554 lbs.
"We weigh our animals, some of them every day, some of them every week, but what we're doing at this weigh-in is making sure our records are up to date," Teague Stubbington, London Zoo's assistant curator of mammals, told ABC News. "Weight is really important, as weight is very closely linked to health in animals. Changes in their weight might mean we have to change their diet."
Most importantly, the annual weigh-in can detect pregnancy in some of London Zoo's endangered populations, which can then be shared with other zoos and conservationist groups worldwide.
One of the main challenges is bringing the animals to the scales, with some more willing than others to accept the treats that entice them to stay still enough to record their weight.
"Weighing animals can be a little bit of a challenge, but what we use for our animals is to give them choice and control in their enclosures and give them positive reinforcement as a training technique," Stubbington said. "We introduce the scales gradually into their environments so they become comfortable, and when they're on the scale, sometimes we'll give them a food reward or an enrichment activity."