With every mouthful, fin whales take in huge quantities of krill and seawater. New research shows that their baleen should actually become clogged in the process – and raises the question of what previously unknown trick these giants of the seas use to continue feeding in a matter of seconds.
Fin whales (Balaenoptera physalus), with a length of up to 27 metres, are among the largest animals on Earth. When hunting krill swarms, they devour up to 150 kilograms of the tiny crustaceans in a single mouthful – along with around 60 cubic metres of seawater. How they manage to expel these vast volumes of water from their mouths in just about half a minute, without the krill clogging their baleen, has been a mystery until now.
A research team led by Ingrid Ackermann from Stanford University has investigated this question. The findings were published in the Journal of Experimental Biology and show that the known mechanism of the baleen alone cannot explain the rapid outflow of water.
During what is known as ‘lung-feeding’, the fin whale opens its mouth wide and takes in a huge gush of water. In the process, the throat pouch of an animal around 20 metres long expands from about 30 to nearly 80 square metres. After closing its mouth, powerful muscles force the water out through the baleen, whilst the krill remains in the mouth. According to calculations, the whale generates a pressure of between 4.1 and just under 18 kilopascals – sufficient to expel the water within around 31 seconds.
In theory, however, the krill should severely hinder this process. If the prey were evenly distributed across the baleen, it would form a layer more than six centimetres thick and almost block the flow of water.
To test this hypothesis, the researchers simulated the process in the laboratory using frozen krill and a filtration system. The results were clear: even a krill layer just under two centimetres thick reduced the water’s flow velocity from the required 0.67 metres per second to a mere 0.04 metres per second. With a layer nearly six centimetres thick, it even dropped to just 0.02 metres per second.
The measurements show that the baleen cannot simply be covered in krill whilst feeding. Instead, there must be as yet unknown flow mechanisms or movements within the mouth that prevent the food from clogging the baleen. Only this makes it possible to empty the mouth quickly.
The study highlights that even in the case of one of the largest animals on our planet, fundamental questions of biomechanics remain unanswered. Further research will now investigate how fin whales keep their natural filters clean whilst foraging.
Heiner Kubny, PolarJournal