Baleen is a specialized biological structure that has enabled some of the largest animals on Earth to thrive by consuming some of the smallest. This apparatus is found exclusively in marine mammals known as baleen whales, serving as an efficient replacement for teeth. Baleen allows these creatures to filter vast quantities of microscopic or small-bodied prey from the ocean water. Understanding the function of this filter system is necessary to appreciate how these whales sustain their enormous size.
The Physical Structure and Composition of Baleen
Baleen consists of hundreds of parallel plates that grow downward from the upper jaw, functioning like a sieve or a comb. Each plate is primarily made of keratin, the same durable, fibrous protein that forms human hair and fingernails. This material provides strength and flexibility, allowing the plates to withstand the constant pressure of filtering seawater.
The plates are arranged in two racks, one on each side of the upper palate, with their length varying among species. The outer edge of each plate, which faces the whale’s cheek, is smooth and solid. The inner edge is frayed into thousands of fine, brush-like bristles called the fringe. These bristles form a dense, tangled mat inside the whale’s mouth, which serves as the filtering surface. The material also contains a small amount of calcium salts, specifically hydroxyapatite, which contributes to the structure’s rigidity.
The dimensions of the baleen plates are directly related to the whale’s preferred diet. Whales that feed on small prey, such as copepods, possess long plates with a fine, silky fringe, as seen in the right whales. Conversely, species that target larger prey like small fish or krill tend to have shorter, coarser baleen plates. Bowhead whales, for example, have some of the longest baleen, with plates reaching up to 4 meters (13 feet) in length.
Baleen Whales: Classification and Context
The presence of baleen defines the parvorder Mysticeti, the group known as baleen whales. This classification distinguishes them from their relatives, the Odontoceti, or toothed whales, which include dolphins, porpoises, and sperm whales. Baleen whales lack functional teeth, relying entirely on their filtration apparatus for feeding.
The Mysticeti suborder includes four main families:
- The Balaenidae (right and bowhead whales)
- The Balaenopteridae (rorquals like blue and humpback whales)
- The Eschrichtiidae (the gray whale)
- The Cetotheriidae (the pygmy right whale)
These animals consume large amounts of small organisms, such as copepods, krill, and small schooling fish, to support their size. The blue whale, the largest animal known to have ever existed, is a baleen whale, demonstrating the efficiency of this specialized feeding strategy.
The size of these animals necessitates a feeding mechanism that can harvest food from dense patches of prey. The switch from hunting individual prey with teeth to bulk filter-feeding tiny organisms around 34 million years ago allowed these whales to evolve into the giants of the ocean. The different evolutionary paths within Mysticeti have resulted in three distinct feeding mechanics, all utilizing the same baleen structure.
The Mechanics of Filter Feeding
The overall function of baleen is to strain food from water, but the specific technique varies based on the whale species and its environment. One of the primary methods is gulp feeding, employed by rorquals, such as the humpback and blue whales.
Gulp feeding begins with the whale accelerating into a dense patch of prey and opening its mouth to engulf a large volume of water. Rorquals possess specialized, accordion-like throat pleats that expand the mouth cavity, allowing the whale to take in water and prey that can weigh more than the whale itself. The lower jawbones are not fused at the chin, enabling them to pivot outward and increase capacity. After engulfment, the whale closes its mouth and uses its tongue to compress the fluid, forcing the water out through the baleen plates. The fringed mat of baleen acts as a sieve, trapping the prey inside the mouth while the water is expelled, allowing the whale to swallow the concentrated food mass.
A second strategy is skim feeding, characteristic of right whales and bowhead whales, which possess the longest and finest baleen. These whales swim slowly and continuously with their mouths open, allowing water to flow through the baleen plates. As the water passes through the fine fringe, small prey, such as copepods and other zooplankton, are filtered out and accumulate on the bristles. The whale periodically scrapes the accumulated food off the baleen fringe with its tongue before swallowing it.
The third technique is bottom feeding, used almost exclusively by the gray whale. This whale swims on its side along the seafloor, using its mouth to scoop up sediment and water. The gray whale then filters out benthic organisms, such as amphipods, from the mud and water using its short, stiff baleen. Regardless of the specific feeding method, the baleen plates consistently perform the function of a high-capacity filter, enabling the efficient transfer of energy from the smallest ocean life to the largest.