More than 19,000 animals at ZSL London Zoo have had their vital statistics taken for the annual weigh-in.
Penguin chicks, squirrel monkeys, gorillas and tigers were just some of the creatures that had their measurements recorded.
The information shared with zoos means zoologists can compare information about endangered species.
Keepers have been using various tactics to entice the animals to stand up and take part, the zoo said.
Zoological manager, Mark Habben said: "We have to know the vital statistics of every animal at the Zoo - however big or small.
"It helps us to ensure that every animal we look after is healthy, eating well, and growing at the rate it should.
"Weight is a particularly important indicator of health, and we can even detect pregnancies through a growing waist line."