On August 24, 2023, London Zoo embarked on an extraordinary mission: the annual weigh-in of its 14,000 resident animals. From giant gorillas to tiny tadpoles, this event is more than just a spectacle—it’s a vital part of animal conservation and healthcare.

Why the Weigh-In Matters:

Knowing an animal’s weight is crucial for assessing its health and even determining if it’s pregnant. For example, a sudden weight loss in a gorilla could be a sign of illness, while a weight gain in a tiger could be a sign of pregnancy. The data collected from the weigh-in is also used to track the growth and development of animals, as well as to identify any potential health problems.

The Logistics:

Weighing and measuring 14,000 animals is no small feat. It takes the zoo staff several days to complete this task. The animals are weighed and measured in a variety of ways, depending on their size and species. For example, small animals like tadpoles are weighed on a kitchen scale, while larger animals like gorillas are weighed on a commercial scale.

Innovative Techniques:

Zookeepers employ creative methods to get accurate measurements. Squirrel monkeys are lured onto scales with treats, while a curry-scented measuring stick is used to encourage Sumatran tigers to stretch out. Even tarantulas get totted up, proving that no creature is too small for this annual health check.

Global Impact:

The information gathered from the annual weigh-in is invaluable not just for London Zoo but for animal conservation globally. It is shared with zoos worldwide through a global database, contributing to broader conservation efforts. This data can be used to track the health of populations of endangered species, identify potential threats, and develop conservation strategies.


The annual weigh-in at London Zoo is a remarkable example of how meticulous care and data collection can aid in animal conservation. It’s not just about numbers on a scale; it’s about ensuring the well-being of thousands of animals and contributing to global conservation efforts.