In order for researchers to keep track of animal populations, it is necessary not only to know how many there are but also how healthy they are. This often requires tissue or blood samples, but for large animals such as whales, collecting samples is very difficult and it can interfere with their natural behavior. Humpback whales go through especially extreme body condition changes throughout the year. They migrate from their polar feeding areas to warmer breeding grounds, during which time adults spend months without eating.
Researchers measure changes in body conditions of migrating whales by checking the lipid content of their blubber layer – a layer of reserve energy that keeps them warm and allows them to go without eating for long periods of time. They can also use drones equipped with small cameras to measure things such as body length and volume from aerial images. However, in order to find out if the different measurement methodologies are reliable, they need to compare them.
So, a group of researchers in the coast of south-west Australia collected drone images and blubber samples from the same individuals of Humpback whales at the beginning and end of the breeding season to find out if the external (body shape) and internal (blubber fat, measured by tissue biopsy) measurements correlate.
They found that the body volume of the whales decreased through the breeding season. This was expected since the adults are fasting and females are nursing their calves during this time. However, they found no big changes in the fat composition of the blubber layer during the same period. These results differ from other species of whales and it signals that for humpback whales, monitoring their body sizes is a more accurate and less invasive metric than tissue samples for checking if the populations are being able to feed well and reproduce.