Whales are the largest animals on the planet and important predators in the marine ecosystem. As a marine biologist I have been lucky enough to see them up close. It’s an amazing experience to watch a whale mother, 14 meters long and weighing nearly 40 tonnes, gently push her “tiny” five-meters baby, already weighing nearly one tonne at birth, up to the surface to breathe.

But how do I actually know that whale weighed 40 tonnes? After all, we can’t exactly capture an animal the size of a bus and simply put it on a scale. And swimming out into the ocean and putting a measuring tape around its body is not a very wise thing to do.

For whale scientists, this is a big problem. The sheer size of these animals is fundamental to their success, as it allows them to store enough energy to undertake long-distance migrations to find food in scattered locations. But though this should be factored in, it is seldom possible to incorporate bodyweight as a variable when studying free-living whales as it is so difficult to measure them.