On Wednesday, the picture became a little clearer after scientists reported in the journal Nature that they'd found a new sensory organ in the jaw of a fin whale, which they think helps the whale decide when to open and shut its mouth as it lunges toward a meal. The whale then expels the water through its baleen plates and gulps the morsels left behind.
Some whales, such as the right whale, are content to just skim along, but many large baleen whales like humpbacks, blue whales and fin whales have adopted this lunge feeding strategy.
But this techniquehas its disadvantages, according to lead author Nicholas Pyenson of the Smithsonian Institution.
When they're taking a gulp like that, they're departing from hydrodynamic form, which slows the whales down, Pyenson says. It also creates a bow wave in front of the whale that alerts prey to where they are.
Another complicating factor is that the lower jaw of lunge feeding whales is made up of two separate bones that can move independently from one another.
Pyenson and his colleagues were able to obtain samples, including an entire chin from a fin whale, from carcasses harvested in Iceland, where industrial whaling is still legal. Before they began dissecting the sample, they used digital imaging techniques, including CT and MRI scans, to peer inside the tissue.
The real 'aha!' moment came when the researchers began looking at sections of the chin under the microscope, according to Pyenson.
They found small finger-like projections of connective tissue and nerves invading the gelatinous cavity between the whale's two lower jaw bones. The tissue was also strewn with special types of neurons called mechanoreceptors, which respond to distortion or pressure.
Artist's representation showing newly discovered sensory organ. Credit: Carl Buell.
Since there are different kinds of tissues involved and they all seem to be doing one thing, it qualifies as an organ, Pyenson says.
The scientists also found that the same nerves that go into sensory organ are also hooked up to two rows of hairs protruding from the whale's chin, which may indicate that the sensory organ has other uses besides lunge feeding, according to Pyenson.
Whales evolved from extinct land animals that looked a bit like wolves with hooves. It's possible that this newly discovered sensory organ might be repurposed from some kind of system akin to the whiskers on cats, mice and other animals, in which hair follicles are integrated with nerves and used for many purposes, including navigation and detecting movement.
But given that around 50 million years separates the fin whale from its land-dwelling ancestor, it's a bit hard to connect those two dots, Pyenson says.
Correction: The article originally said the land-dwelling ancestor of whales belonged to an order called mesonychids. While these creatures were once thought to be the ancestors of whales, the most recent fossil evidence has dispelled this theory.