Scientists at the University of British Columbia and the Smithsonian Institution have discovered a sensory organ in rorqual whales that coordinates its signature lunge-feeding behaviour - and may help explain their enormous size.
Rorquals 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.
The study, to be featured on the cover of the journal Nature, details the discovery of an organ at the tip of the whale's chin, lodged in the ligamentous tissue that connects their two jaws.
NB: Illustrations of lunge-feeding and the location of the sensory organ, photos of the researchers at work at the Icelandic whaling station and a comprehensive Frequently Asked Questions are available at http://nmnh.typepad.com/pyenson_lab/nature-rorquals-organ.html.
Samples were collected from recently deceased fin and minke whale carcasses captured as part of Icelandic commercial whaling operations. Commercial whaling in Iceland resumed in 2006 and quotas are determined annually by its government.
Scanning of the whale's chin revealed a grape fruit-sized sensory organ, located between the tips of the jaws, and supplied by neurovascular tissue.
The research team was assisted by technicians at FPInnovations, the owner of Canada's only X-ray computed tomography (XRCT) machine large enough to accommodate the massive specimens. Used to scan giant logs, the XRCT machine provides a three dimensional map of the internal structure of whale tissues.
"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," says lead author Nick Pyenson, a paleobiologist at the Smithsonian Institution, who conducted the study while a postdoctoral fellow at UBC. "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 metres 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 Jeremy Goldbogen showed that a fin whale captures 10 kilograms of krill in each gulp in order to sustain its average 50-ton body mass. Goldbogen, who conducted both studies while a PhD student at UBC, is now a scientist with the Cascadia Research Collective in Olympia, Washington.
"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," says co-author and UBC Zoology Prof. Bob Shadwick.
"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," Shadwick adds.
"This also demonstrates how poorly we understand the basic functions of these top predators of the ocean and underlines the importance for biodiversity conservation."
The study was supported by grants from the Natural Sciences and Engineering Research Council of Canada and the Smithsonian Institution. FPInnovations' XRCT machine was a joint project with the University of Northern British Columbia and funded by the Canada Foundation for Innovation and the BC Knowledge Development Fund.