Baleen whales, or Mysticetes, include the largest animals on Earth, yet they sustain their immense body mass by consuming some of the ocean’s smallest creatures. This apparent paradox is solved by a specialized feeding system that allows these marine giants to efficiently harvest vast quantities of tiny prey. The unique biological tool that defines this group is the baleen plate, a filter-feeding apparatus that enables a variety of highly effective capture strategies. Understanding how these whales feed provides a window into the anatomical and behavioral adaptations required to thrive at the very top of the aquatic size spectrum while feeding low on the food chain.
The Small Scale of the Baleen Diet
The diet of baleen whales is characterized by a high volume intake of small, densely aggregated organisms. These whales generally feed on a trophic level far below that of toothed whales, targeting invertebrates and small fish that gather in enormous schools or swarms. Primary food sources include krill, which are small, shrimp-like crustaceans, and copepods, which are even tinier, abundant zooplankton.
Different whale species specialize in these food types based on the fineness of their filtering plates. Some rorqual species, such as humpbacks and minke whales, also consume small schooling fish like herring, capelin, or sand lance. The diet is heavily dependent on seasonal productivity and geographical location, requiring the whales to migrate to nutrient-rich polar feeding grounds during summer months.
The Anatomy of the Filter Plate
The characteristic feature of all Mysticetes is the baleen, a rack of plates that hangs from the upper jaw in place of teeth. These plates are composed of keratin, the same fibrous protein found in human hair and fingernails. The baleen plates grow continuously from the whale’s gums in parallel rows, acting like a giant sieve inside the mouth.
Each plate is a composite structure featuring a central core of keratin tubules encased in a harder outer layer. As the plates are worn down by the action of the tongue and water flow, the inner tubules are exposed, forming a fine, hair-like fringe along the inner edge. This fringe is the actual filtering surface, trapping prey while allowing seawater to be expelled.
The physical properties of the baleen are strengthened by calcification, where minerals are incorporated into the keratin tissue. This mineral reinforcement helps to stiffen the plates, compensating for the fact that the tissue is constantly hydrated. The length and coarseness of these plates vary significantly between species, reflecting their specific feeding strategies. Skim feeders, for example, possess plates reaching up to 11.5 feet long with a very fine mesh, while lunge feeders have shorter, stiffer plates.
Specialized Methods of Capture
The various species of baleen whales have evolved three distinct methods to exploit their small-scale food sources, each adapted to different prey distributions.
Lunge Feeding
The most dynamic of these is lunge feeding, employed by rorquals, including blue, fin, and humpback whales. This technique involves accelerating rapidly into a dense patch of prey and opening the mouth to engulf an enormous volume of water and food. Lunge feeding is facilitated by highly elastic ventral grooves, or throat pleats, that stretch dramatically, allowing the mouth to balloon outward and take in a water mass that can exceed the volume of the whale itself.
The lower jaw is connected by a flexible joint, enabling it to swing open nearly 90 degrees during the engulfment phase. Once the water and prey are collected, the whale closes its mouth and uses its tongue and contracting throat muscles to force the water back out through the baleen plates, trapping the prey inside.
Skim Feeding
Skim feeding is utilized by Right and Bowhead whales. These whales feed continuously by swimming slowly through the water with their mouths partially open in areas rich with copepods and other zooplankton. The long, fine baleen plates filter the organisms from the water as it flows through the mouth, a process that requires minimal speed to be effective. Their highly arched upper jaw is an anatomical adaptation that accommodates the exceptionally long baleen plates necessary for this continuous filtration.
Bottom Feeding
Bottom feeding is a unique strategy primarily used by Gray whales. These whales target benthic invertebrates, such as amphipods and marine worms, that live in the soft sediments of the ocean floor. The whale rolls onto its side, scooping up mud, sediment, and organisms into its mouth. The Gray whale then forces the muddy water out through its coarser baleen plates, which act like a sieve to retain the invertebrates. This action often leaves distinctive feeding pits and trenches on the seafloor, and many individuals show a preference for feeding primarily on one side, leading to asymmetrical wear on their baleen.