Who’s the heaviest? London Zoo’s annual weigh-in of all 14,000 animals

Once a year, London Zoo embarks on the mammoth challenge to weigh all 14,000 of its animals. From silverback gorillas to stick insects, the data gathered provides vital information for the zoo and its scientists, particularly regarding threatened species. 

The ‘Annual Weigh In’ also makes for brilliant photos.

london zoo weigh in
A weight-conscious Meerkat is curious to know his results. (Photo by Mark Case/Getty Images)

There’s no correct way to weigh an animal. While a Humboldt penguin may be easily enticed with a spot of breakfast on a normal-sized scale, this method cannot be used for the other 14,000 species housed at London Zoo. It is, nonetheless, essential that zookeepers record the data once a year, monitoring the health of their animals and providing a valuable resource for other zoos and conservation sites around the world. 

london zoo weigh in
(Photo by Oli Scarff/Getty Images)

All of the data collected is entered into the Zoological Information Management System (ZIMS), a worldwide database. While measurements are taken for every animal, in order to check on their well-being, data from London Zoo is of even greater importance for endangered animals. These include a pair of young Sumatran tiger cubs called Zac and Crispin; Sumatran tigers are the rarest tigers in the world. Critically endangered, it is believed that only 300 could be left in the world due to poaching and habitat destruction in their native Indonesia.

The global breeding program for Sumatran tigers is now coordinated by London Zoo.

london zoo weigh in
A tiger at London Zoo ahead of the Annual Weigh In (Photo by Mark Case/Getty Images)

“Having this data helps to ensure that every animal we care for is healthy, eating well, and growing at the rate they should – a key indicator of health and wellbeing,” Angela Ryan, London Zoo’s Head of Zoological Operations, said. 

london zoo weigh in
15th November 1952: Keeper Herbert Jones weighs Prince, the baby King Penguin at London Zoo. At 13 weeks of age, Prince tipped the scales at 28 pounds, a sign of excellent health. (Photo by Fox Photos/Getty Images)

“For example, a growing waistline can help us to detect and monitor pregnancies, which is vitally important as many of the species we care for are threatened in the wild and part of international conservation breeding programs… By sharing information with other zoos and conservationists around the world, we can all use this knowledge to better care for the species we’re striving to protect.” 

Read More: Is this the elephant’s closest relative?

Another endangered newcomer to the zoo, who got his first taste of the annual weigh in in 2023, is Kiburi, a Western lowland gorilla. Kiburi is part of the zoo’s breeding program for gorillas, and joins the population that currently includes the females Mjukuu and Effie, and youngsters Alika and Gernot.

london zoo weigh in
Giraffes prepare to be weighed. Maggie the giraffe is now the heaviest animal at the zoo. (Photo by Oli Scarff/Getty Images)

Maggie the giraffe is the zoo’s heaviest animal, weighing over 1,600 pounds (750 kilograms).

The lightest animal is the leaf cutter ant, coming in at a minuscule 0.0001 oz – 0.0002 oz (between three and five milligrams).

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