At London Zoo, it's weigh-in day.

Every animal from the smallest snail to the tallest giraffe has its weight and measurements carefully recorded.

The annual event is a chance to update vital health information for more than 400 species living here.

Weighing is not just about numbers on a scale, but it's is a way to track growth, monitor pregnancies, and spot any unexpected changes.

The results are shared with conservationists around the world through a global database.

On Penguin Beach, Humboldt penguins are coaxed onto the scales with a reward of fish.

Robin the penguin weighs 4.9 kilos with his details carefully logged by keepers.

Daniel Simmonds, Zoological Operations Manager at London Zoo, says: “So today is indeed the annual weigh-in at London Zoo. We’ve got a big task ahead of us. We’ve go over 10,000 animals here at the zoo across more than 400 species. Now, the reality is we know all of the weights anyway because we weigh the animals very often. But today’s a slightly more formalised part of that. We’ll be doing the same process that we do on most days, except today, everything will be logged formally onto an international database called ZIMS, the Zoological Information Management System. That we share globally with other zoos, with other conservation organisations, and we learn so much from that.”

Weighing animals on this scale is no small task.

There are 79 penguins in the colony and keepers work year round to teach them to step onto the scales voluntarily.

Jessica Allison Ray, penguin zookeeper at London Zoo, says: “The biggest challenge is the number of penguins we have. So we actually have 79 penguins right now, so it’s quite a lot. And we ideally don’t want to be catching them to weigh them. We want to training them to voluntarily make the choice to come onto the scales on their own. We can reinforce them with fish for doing that, so that they have all the control in the situation. So that takes time. And every year, we teach more and more penguins. So hopefully, at some point in the future, we’ll have all the penguins on the beach trained to voluntarily weigh, but it is a challenge and it takes a lot of time and effort.”

Elsewhere in the zoo Spike the ring tailed lemur is tempted with food onto the scales, tipping them at 2.8 kilos.

Regular checks like these provide early warnings if an animal’s weight changes unexpectedly.

Simmonds continues: “Every single animal’s diet is weighed to the gram every single day by a large team of dedicated keepers. So we don’t expect to see any changes and we just confirm that through the weighing. However where there can be fluctuations are for example with pregnant animals that we expect and want to be gaining weight, juveniles who are moving through that life cycle process and getting larger we expect obviously those increases. Another really good reason for why we do the weigh-ins so regularly, not just today, to make sure that we can manage the general health and well-being of all our animals here at the Zoo."

Not all the participants are small.

Gizmo and Kiwi, a pair of capybaras, step up for their turn.

At 48 kilos Gizmo outweighs his companion Kiwi, who is 38 kilos.

As the largest rodents in the world they are an unusual sight in the capital.

The giants of the zoo are also included.

Priscilla the Galapagos tortoise is tempted forward with cabbage, slowly making her way onto the scales.

These ancient reptiles can grow heavier than 200 kilos and live well over a century.

Simmonds adds: “From a conservation perspective, the Zoological Society of London owns and operates London Zoo and Whipsnade Zoo and the work that we do during the weigh-in is absolutely integral to the conservation. So, for example, we’ll be able to learn today how much a Sumatran tiger weighs. That data is then fed back to our teams in the field, whether they be field workers from a conservation perspective or the veterinary staff, who sadly won’t actually get to see some arching tigers that much because the numbers are dwindling so much and the habitat is under so much stress. So that link between zoos and conservation is very much there and it’s very important.”

Even the tiniest residents are checked.

A Giant Malaysian Katydid, a large insect, weighs just 24 grams.

A magnolia land snail is also gently lowered onto the scales.

The work here aims to accurately record every one of the 10,000 animals here, from towering giraffes to Darwin’s frogs that measure only two centimetres long.

For keepers, it is also a chance to celebrate the animals in their care.

Ray says: “It feels really good. I love getting to talk about our animals and show off our animals and invite people to come and see our amazing animals as well. But for me as the penguin keeper, I’m really proud of my birds for, you know, not just getting on the scales after working on the training for a few months, but also to do it when there’s a lot of people, there’s lot of scary things happening. But it’s the best part in the end is to get to have really nice pictures of us with our animals as your new profile picture, anything like that. But I love sharing our animals, it’s best part of the job.”

The weigh-in may only happen once a year but the results are used every day to help protect animals both at the zoo and in the wild.

AP video by Mustakim Hasnath.