London Zoo has begun chasing all its 16,000 animals, birds and insects onto scales as part of the annual weigh-in.
"Some animals, especially the ones you can't really get close to, you can't see their condition that well, for example something covered in feathers or something covered in fur, you can't always see the condition that they are in so by weighing them, we know where we are with them," said keeper Angela Ryan.
Ryan said one of the trickiest animals to weigh are the meerkats: "You've got lots of them at once and you've got to get them there one at a time and they all want to be there at the same time.
"We try and keep it positive so they've got a little reward for coming on there which makes it a really exciting place to be, so you've just got to make sure you've got them queuing up, so to speak," she added.
One of the easiest animals to weigh is Dirk, the 72-year-old giant tortoise. Standing calmly as he was weighed and measured, he came in at a whopping 170 kilos, with a 51 inch shell and 34 inch height.
All the data collected by the zoo is shared with other zoos and breeding programmes around the world so they can better understand the life-cycles of all the animals.