Whales are marine mammals, and a distinct group possesses a remarkable adaptation for feeding: baleen. Instead of traditional teeth, these whales have bristly plates that allow them to filter food directly from ocean water. This anatomical feature is central to their survival and enables them to thrive in marine environments.
Understanding Baleen Plates
Baleen is a plate-like structure found in the mouths of certain whales, suspended from their upper jaw. It is composed of keratin, the same protein that forms human fingernails and hair. These flexible, fibrous plates are arranged in rows, with hundreds lining each side of the upper jaw, hanging downward towards the tongue. The inner edges of the baleen plates feature fine, hairy fringes that interlock to form a dense, sieve-like mat. Baleen appearance varies significantly between species, with plates ranging from 0.5 to 3.5 meters (1.6 to 11.5 feet) in length.
The Mechanics of Filter Feeding
Baleen whales use filter-feeding methods to gather food from the ocean. They take in vast amounts of water containing prey, then expel the water through their baleen plates, trapping food inside.
One common strategy is gulp feeding, utilized by whales like blue, fin, and humpback whales. These rorqual whales accelerate rapidly, opening their triple-hinged jaws to engulf enormous volumes of water and prey. During this process, a specialized “oral plug” moves backward to seal off the upper airways, and the larynx closes, preventing water from entering the lungs. The whale then contracts its throat pleats and uses its tongue to push the water out through the baleen, leaving the concentrated food to be swallowed.
Another method is skim feeding, where whales like the bowhead and right whales swim slowly with their mouths open, allowing water to continuously flow through their baleen plates, trapping prey. Gray whales are bottom feeders, stirring up seafloor sediment to suction tiny creatures like amphipods. They filter these organisms through their baleen while often swimming on their sides.
Diverse Diets of Baleen Whales
The diet of baleen whales primarily consists of small marine organisms. Their main food sources include krill, copepods (a type of small crustacean), and small schooling fish. Blue whales, for example, consume krill, sometimes up to 4 tons per day. The specific structure and spacing of the baleen plates influence the type and size of prey a whale can effectively filter.
Right whales, with their long and fine baleen, specialize in feeding on small zooplankton like copepods. Humpback whales also feed on krill and small fish such as herring, anchovies, and sardines. Gray whales primarily consume invertebrates found in bottom sediments, such as amphipods and marine worms. This diversity allows different baleen whale species to exploit various abundant food sources in their marine habitats.
The Evolutionary Journey of Baleen
The evolution of baleen in whales represents a significant transition from their toothed ancestors. Early whale ancestors possessed teeth, and the shift to baleen occurred over millions of years. Fossil evidence indicates the oldest true baleen fossils are approximately 15 million years old, though skull modifications suggestive of baleen appeared much earlier. Some fossil whales, such as Aetiocetus weltoni, dating back 24-28 million years, show evidence of both teeth and baleen simultaneously, indicating a gradual transition.
This suggests early toothed whales may have used the spaces between their teeth for filtering, with baleen gradually replacing teeth over time. Modern baleen whale embryos even develop teeth that are later reabsorbed before birth, a vestige of their toothed ancestry. Baleen development was likely driven by changes in ocean productivity and the availability of dense patches of small prey, making filter feeding an energy-efficient method for consuming large quantities of food. Being keratin-based, baleen rarely fossilizes, making the study of its exact evolutionary timeline complex.