Last week marked the start of the London Zoo’s annual weigh-in, a near-herculean effort in which keepers weigh and measure each of the zoo’s more than 14,000 animals.

“We record the vital statistics of every animal at the zoo—from the tallest giraffe to the tiniest tadpole,” says Angela Ryan, head of zoological operations, in a statement. “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.”

London Zoo's Annual Weigh In

While keepers repeat these measurements many times throughout the year, this zoo-wide event is a chance to ensure records are accurate and up-to-date. It can also help detect changes in an animal’s health, such as a growing waistline that indicates the beginning of a pregnancy.

“We can see a lot from their behaviors and how they’re responding to each other, but we also need to be responsible and know what’s going on internally,” says senior zookeeper Glynn Hennessy in a video from Reuters.

A keeper measures a group of penguins. Mark Case / Getty Images

The process is also mandated by British law: All zoos must take an annual census in order to keep their licenses, as Smithsonian magazine’s Danny Lewis wrote during the zoo’s 2016 count.

The zoo measures “every mammal, bird, reptile, fish and invertebrate in its care” over the course of several days, per the Associated Press.

Bolivian black-capped squirrel monkey
A keeper coaxes a Bolivian black-capped squirrel monkey onto the scale with a treat. London Zoo

Taking the measurements, however, isn’t always simple. Some animals need to be coaxed to step onto the scales or to stand up to be measured.

Different animals respond to different strategies. This year, keepers used treats to convince the Bolivian black-capped squirrel monkeys to climb onto the scales. To weigh the Humboldt penguin chicks, keepers strategically placed scales so that the birds would step onto them as they lined up for their morning feed. 

Tigers near a height measurement board in their compound Mark Case / Getty Images

Some newcomers are being weighed for the first time. Among them are Zac and Crispin, Sumatran tiger cubs who recently turned one, and Kiburi, a Western lowland gorilla who arrived last November.

The zoo’s heaviest animal is a giraffe named Maggie, who weighs about 750 kilograms (over 1,600 pounds), reports the New York Times’ Claire Moses. Meanwhile, the smallest is the leaf cutter ant, each of which weighs just 5 milligrams. (Rather than weigh each ant, keepers estimate based on the weight of the entire colony.)

Stick bug
During the annual weigh-in, keepers record the vital statistics of all animals, from giraffes to insects. London Zoo

Staffers enter the measurements into the Zoological Information Management System, a database that zoos around the world use to compare numbers. This practice helps zoos make informed decisions about each animal’s care.

“We have critically endangered animals here,” Ryan tells the Associated Press. “It’s absolutely vital that we are managing them [and] managing their health.”

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