How do you weigh a live whale? Very carefully, of course. Jokes aside, scientists considered this conundrum and came up with an innovative method that doesn’t involve the world’s largest set of scales, instead just a simple drone.

Taking aerial images of whales in crystal clear waters, researchers measured the body length, width, and height of southern right whales off the coast of Argentina, allowing them to develop a 3D model that accurately calculates the body mass of these huge creatures.

Baleen whales, which include right whales and blue whales, are the largest animals on the planet, and body mass plays a direct role in their success. It also plays an important role in our understanding of them, their health, their habits, and how best to protect them, but getting that information has proven tricky in the past. After all, southern right whales can reach up to 17 meters (56 feet) and weigh about 90,000 kilograms (200,000 pounds).

Because of their size and watery habitat, the only way to obtain this data has been to weigh stranded or dead whales, which is not ideal as other factors come into play, like the physical distortion of a decaying carcass. 

Instead an international team of researchers filmed 86 individuals off the coast of Península Valdés, Argentina during breeding season, when large groups gather. The clear water allowed them to take hundreds of photos of the whales from all angles as they swam, rolled, and fed their young, thus obtaining length, width, and height measurements.

_{Sound on! Eat your heart out Blue Planet.} 

Collaborating with researchers from the Digital Life Project at the University of Massachusetts, they created a 3D model, using the measurements to calculate the density of the whales, and consequently their body volume and mass. They tested their accuracy on whales whose vital statistics were already known, publishing their methods in the British Ecological Society journal Methods in Ecology and Evolution.  

"The model takes into account not only the structural size of the animal (its body length), but also its body condition (width and height), so seasonal variations in body condition are taken into account," lead author Fredrik Christiansen, an assistant professor at the Aarhus Institute of Advanced Studies in Denmark told IFLScience. "Whales can change their body mass with more than 25 percent over a single breeding season, so this was something that our model had to account for, which is why we use length, width and height to calculate volume, and consequently mass."

Being able to calculate the body mass of whales accurately and in a non-invasive way opens up whole new avenues of what we can learn about them. Long-term studies on known individuals will mean researchers can calculate the rate of body mass increase over time, and work out the energy requirements – how much prey they need to consume – for growth. It will allow them to study how stressors affect the health and fertility of groups and even calculate the right amount of sedative dosage for disentangling whales caught up in fishing gear. It's already being used to track the health and growth of southern right whale calves.

So solving the conundrum of how to weigh a whale without scales has many wide-reaching applications in the study and protection of leviathans. Perhaps in future, it can be applied to other difficult-to-access creatures of the deep?

"For sure," Christiansen said. "For other baleen whale species we can use the exact same approach to calculate body mass, we just need to first estimate the volume-to-mass conversion factor, which is likely to differ between species. For other marine mammals, e.g. seals and sea cows, it should also be possible to use a similar approach. For terrestrial animals, it might be a bit more challenging, but once a good understanding of the body shape has been obtained, one should be able to calculate volume from aerial photographs of animals, and then, if their body density is known, calculate their mass."