Researchers at the University of British Columbia and the Smithsonian Institution have discovered a new sensory organ in rorqual whales that coordinates its signature lunge-feeding behavior – and may help explain their enormous size.
Rorqual whales are a subgroup of baleen whales – including blue, fin, minke and humpback whales. They are characterized by a special, accordion-like blubber layer that goes from the snout to the navel. The blubber expands up to several times its resting length to allow the whales to engulf large quantities of prey-laden water, which is then expelled through the baleen to filter krill and fish.
A new study published in Nature reports the discovery of an organ at the tip of the whale’s chin, lodged in the ligamentous tissue that connects their two jaws.
Using X-ray computed tomography machine, the research team produced a three dimensional map of the internal structure of whale tissues. Samples were collected from recently deceased fin and minke whale carcasses captured as part of Icelandic commercial whaling operations.
Results of the whale’s chin scanning revealed a grape fruit-sized sensory organ, located between the tips of the jaws, and supplied by neurovascular tissue.
The organ, composed of connective tissue with papillae that contain nerves, is suspended in a gel-like material. Vascular and nervous tissue from an ancestral front tooth socket still remains in today’s whales and connect to the organ. Evidence indicates that the sensory organ responds to jaw rotation when the whale opens and closes its mouth and when the whale’s throat pleats expand as it takes in water.
“We think this sensory organ sends information to the brain in order to coordinate the complex mechanism of lunge-feeding, which involves rotating the jaws, inverting the tongue and expanding the throat pleats and blubber layer,” said lead author Dr Nick Pyenson, a paleobiologist at the Smithsonian Institution. “It probably helps rorquals feel prey density when initiating a lunge.”
A fin whale, the second longest whale on the planet, can engulf as much as 80 cubic meters of water and prey – equal or greater than the size of the whale itself – in each gulp in less than six seconds. A previous study by co-author Dr Jeremy Goldbogen of the Cascadia Research Collective in Olympia, Washington, showed that a fin whale captures 10 kilograms of krill in each gulp in order to sustain its average 50-ton body mass.
“In terms of evolution, the innovation of this sensory organ has a fundamental role in one of the most extreme feeding methods of aquatic creatures,” said co-author Bob Shadwick, a professor of zoology at the University of British Columbia.
“Because the physical features required to carry out lunge feeding evolved before the extremely large body sizes observed in today’s rorquals, it’s likely that this sensory organ – and its role in coordinating successful lunging – is responsible for rorquals claiming the largest-animals-on-earth status,” Prof Shadwick added.
“This also demonstrates how poorly we understand the basic functions of these top predators of the ocean and underlines the importance for biodiversity conservation.”