The rorqual whale family is composed of the largest animals on Earth with a unique way of eating their prey: they dive into aggregations of small crustaceans and fish at great depths and lunge with their mouths wide open, engulfing kilograms of fish in a single mouthful. The volume of each mouthful can be larger than that of their body and the engulfed water is filtered through baleen plates in a matter of seconds, leaving them to consume the food that is left behind. Over the past couple of decades researchers have found many unique morphological features that allow rorqual whales to perform these feats: enormous heads with large flexible jaws, the ability to invert their tongues and incredibly stretchy ventral (along the front of the body) pouches that extend from the chin down to their belly button. Recently, a new sensory organ involved in this fascinating feeding strategy was discovered by a team of North American researchers led by Robert Shadwick from the University of British Columbia, Canada, and they published their findings in Nature.
One intriguing anatomical feature of rorqual whales is that the bones in their lower jaws (mandibles) are not fused at the chin but are held together by connective tissue instead. This gives their jaws more flexibility and the ability to rotate outward, enabling the whales to gulp even more water than if their jaws were fused. Dissecting whale carcasses, the team discovered a round, jelly-filled cavity in the connective tissue between the mandibles. A closer look at the tissue with microscopes revealed that the cavity was innervated, and had many papillae (finger-like projections) and nerve endings reminiscent of other sensory organs.
To get a better idea of the structure of this organ, they then performed magnetic resonance imaging (MRI) and X-ray computed tomography (CT) scanning on different species of rorqual whales. When they analysed the distribution of bundles of blood vessels and nerves going to the sensory organ, the team discovered an asymmetric distribution – 60% of the nerves and blood vessels originated from the left mandible in fin whales – and they believe that this asymmetry in the innervation is related to the tendency of individual whales to be biased to one side when side gulping or rolling to feed.
Another intriguing anatomical feature is a Y-shaped cartilage structure that emerges out of the chin and protrudes along the bony jaws, providing support for the stretchy ventral pouch. Using the same imaging techniques that they had used previously, the team discovered that the sensory organ was in contact with the Y-shaped cartilage. They propose that mechanoreceptors (sensors responding to mechanical stimuli) in the sensory organ located in the mandibles relay information about the configuration of the jaws to the brain in order to co-ordinate the movements of the jaw bones and the soft tissue of the ventral pouch during lunge feeding.
This discovery raises interesting questions about the evolution of this organ. Based on its absence in toothed whales, the research team proposes that it evolved either before lunge feeding or along with other specializations associated with lunge feeding and that this organ is an important contributor to this fascinating behaviour.