Usually there's safety in numbers, but it doesn't always work that way. Fin whales (Balaenoptera physalus) filter-feed on immense shoals of krill, engulfing colossal mouthfuls of water containing up to 144kg of the crustaceans. But then the mighty creatures expel the water by squeezing it out through the racks of baleen lining their mouths. Extraordinarily, hungry fin whales can expel the water in 31s, but Ingrid Ackermann from Stanford University, USA, was puzzled. She explains that 144kg of krill would form a layer 6.3cm deep when spread evenly across the 2.9m2 area of baleen, potentially clogging the gigantic strainer. So how much of an impact might this clogging have on how fast the whales can eject water?
Ackermann and her colleagues, David Cade, Jeremy Goldbogen and Mark Denny from Stanford University publish in Journal of Experimental Biology that krill could pose a serious clogging problem for filter-feeding fin whales, slowing water leaving their mouths to a 0.02m/s dribble unless they can prevent the krill from accumulating on the baleen by maintaining a 15% window clear or keeping the krill afloat in their mouths. Then the whales can expel the water in 31s to keep diving for dinner.
'When fin whales lunge, their lower jaw hinges open and water floods into a pouch stretching from their lower jaw to their belly button', says Ackermann. The team started by calculating that the view from the side of a 20m long fin whale's pouch would expand from 30.5m2 to 79.2m2 as it engulfed a 60m3 mouthful of water.
Then, they calculated the pressures exerted as the recoiling blubber and muscle in the pouch squeezes the water out through the baleen strainer, determining pressures ranging from 4.1 kPa up to almost 18 kPa – the equivalent of a 1.8kg weight pressing on a 10cm2 area. These pressures could empty the whale's mouth in just 31s.
But how much of a difference would a layer of krill make to the whale's ability to expel water?
Fortunately, krill are a popular food in the pet trade, so Ackermann purchased a bag of the frozen crustaceans from a local pet store. Gently packing the bottom of a 1.6m tall tube with layers of krill – ranging from 1.9 to 5.6cm deep – and filling the tube with water, the team was able to exert pressures from 8.9 to 12.4kPa, tracking the speed of the water as it flowed through the krill. They then scaled up the speeds for the size and pressure produced by a full-sized whale.
Instead of flowing through the krill at 0.67m/s – the speed needed for the whale to empty its mouth in 31s – the water speeds plummeted to a sluggish 0.04m/s through the 1.9cm layer of krill, dribbling at just 0.02m/s when the plug was 5.6cm thick.
At those rates it would take the whale 16min to empty its mouth, which is twice as long as an average feeding dive.
'The krill might really be a problem', says Ackermann. So how might the whales get around the issue?
This time the team investigated what would happen if the krill were spread unevenly across the baleen, leaving relatively unclogged windows that water could gush through.
They calculated that a whale could empty its mouth in 31s if 15% of its baleen is kept almost completely clear, or the animals somehow keep the krill suspended in the water in their mouths to keep the baleen from clogging.
So, it is possible for fin whales to empty their mouths fast, if they can reduce krill build up against the baleen.
How fin whales do this in practice remains a mystery, but preventing the baleen from clogging is essential for a fin whale to empty its mouth fast, so that it can continue dining on clouds of krill.
