Baleen whales, also called whalebone whales, are a group of marine mammals distinguished by numerous keratinized plates called baleen hanging from their upper jaw. They belong to the parvorder Mysticeti (hence also called mysticetes) under Cetacea, an infraorder comprising whales, dolphins, and porpoises. Unlike tooth whales (parvorder Odontoceti) with a single nostril, these whales have two nostrils or blowholes on the top of their head and spurt water out in a V-shaped blow.
These animals are obligate filter-feeders that sieve zooplankton and small invertebrates, like krill, copepods, squids, and amphipods, through their baleen plates. They are mostly attacked by killer whales found in waters of higher latitudes.
There are 15 recognized species of baleen whales divided into four extant families: Balaenidae (right whales), Balaenopteridae (rorquals), Cetotheriidae (the pygmy right whale), and Eschrichtiidae (the gray whale). Most members of this group are giant, with the blue whale, a type of rorqual, being the largest animal to have ever existed on this planet, measuring up to 98 ft.
These mammals are extremely vulnerable to commercial whaling for their meat, baleen, and blubber. Their numbers are also increasingly affected by climate change, ocean acidification, and accidents like ship strikes.
The largest species, the blue whale (which also happens to be the largest animal on Earth), has a recorded length of 98 ft and weighs up to 199 t. In contrast, pygmy right whales are among the smallest, measuring barely 20 ft and weighing around 6,600 lb. Slightly larger, minke whales grow to about 25 ft in length.
Fin whales, on the other hand, can exceed 60 ft in length and weigh over 50 t, while sei whales are slightly smaller, reaching around 50 ft.
These whales have an elongated, fusiform body with tapering ends. Their forelimbs are modified into paddle-shaped pectoral flippers (for swimming) but retain the skeletal elements homologous to those of terrestrial mammals, such as the humerus, radius, and ulna. In contrast, the hindlimbs are internal and vestigial, being mere traces of two rod-shaped, reduced pelvic bones.
The tails of baleen whales have two flattened horizontal lobes called flukes. They are pads of connective tissue controlled by the longitudinal muscles of the back and the caudal peduncle (the narrow muscular region connecting the body to the fluke).
Baleen whales are usually black or gray, but some species, like the blue whale, exhibit countershading, with a darker back and lighter belly, helping them camouflage from both above and below. Blue whales, as their name suggests, are bluish-gray, while humpback whales are generally black or gray except on the flippers and the underside of the tail. In contrast, minke whales living in the Northern Hemisphere possess a striking white band across their otherwise black flippers.
While members of the families Balaenidae (right whales) and Eschrichtiidae (the gray whale) lack a dorsal fin, those of the families Cetotheriidae (the pygmy right whale) and Balaenopteridae (rorquals) possess a dorsal fin composed of dense, fibrous connective tissue.
As mammals, baleen whales have three skin layers, namely, epidermis, dermis, and hypodermis, supported by a connective tissue layer.
They have two nostrils, known as blowholes, on the top of their heads. These blowholes are longitudinal slits that converge at the front and diverge toward the rear end, producing a V-shaped blow as air is exhaled.
The blowholes are separated by a septum and covered by a muscular flap, which quickly shuts as the animal dives.
Although these whales possess tooth buds in the embryonic stage, they are lost before birth. Instead, they have stiff plates called baleen that hang from the gums of the upper jaw.
These plates are composed of hard, calcified α-keratin (also found in hair and nails), though the amount of calcification varies across the different species. While sei whales have about 14.5% hydroxyapatite (a mineral form of calcium apatite), resulting in stiffer baleen, minke whales have more flexible baleen with only 1 to 4% of the mineral.
Baleen plates are absent in the mid-jaw, forming two separate comb-like structures. The outer edge of these plates is smooth, whereas the inner edge is frayed. Depending on the species, they range from black to yellow or white in color.
The plates become progressively smaller toward the back of the jaw, with the largest known as main baleen plates and the smallest referred to as accessory plates. The accessory plates end in small, tapering hair-like structures.
The number and length of the baleen plates vary across species.
Although the hearts of baleen whales are functionally similar to those of any other mammal, they are considerably large, measuring up to 1,000 lb (454 kg). They have a resting heart rate of about 60 to 140 beats per minute (bpm) at the surface, which drops to 4 to 15 bpm while diving to conserve oxygen[1].
The ventricular wall is highly muscular and thick, measuring between 7.6 and 12.7 cm (3 and 5 in), depending on the species. Additionally, the aorta is around 1.9 cm (0.75 in) thick and allows a large volume of blood to be circulated throughout the bodies of these whales. Their red and white blood corpuscles, each measuring 10 µm (4.1×10−4 in) in diameter, are among the largest among mammals.
They have uniquely elastic, paired lungs, adapted for deep diving, that can extract about 80% of oxygen from the air. These lungs are divided into sac-like compartments, with the left lung smaller than the right. However, relative to their body size, these whales have lower lung volume than terrestrial mammals.
The mouth leads to the tube-like esophagus, which, in turn, is followed by a three-chambered stomach. The first compartment of the stomach, the fore-stomach, leads to the second (the main compartment), which contains hydrochloric acid and protein-digesting enzymes. The third chamber contains fat-digesting enzymes and alkaline secretions that help neutralize the acidic content of the food. The intestine is highly folded and has multiple blood vessels that allow better absorption of nutrients and water.
The kidneys of baleen whales are well-adapted to expel excess salt and produce urine more concentrated than the surrounding seawater[2].
Their brains are relatively small compared to their body mass, with a large, folded cerebrum that accounts for approximately 68% of the brain weight. The cerebrum is responsible for memory and processing sensory information. In contrast, the cerebellum, which makes up about 18% of the brain weight, manages balance and coordination.
Their eyes, relatively small compared to their body size, are located near the end of their mouth. Such placement is ideal since acute vision is less critical for feeding on immobile or slow-moving prey at oceanic depths where sunlight barely reaches. They also have limited color vision due to the lack of S-cone cells in the retina.
Unlike toothed whales with ears adapted for ultrasonic sound frequencies, baleen whales have ears specially adapted for sounds ranging from as low as 7 Hz to as high as 22 kHz.
The auditory meatus is filled with a waxy earplug that blocks the entry of sound waves through the external ear canal. Instead, these waves travel through surrounding bones and tissues to a bony capsule called the tympanic bulla, which then transmits the vibrations to the inner ear.
Although these whales possess an olfactory tract, they lack an olfactory bulb and, hence, have an impaired sense of smell. They also have a poor sense of taste despite possessing salt-receptor taste buds.
To compensate for the lack of olfactory and gustatory sense organs, they have a small vomeronasal organ, which helps to detect pheromones and chemicals in the water.
Baleen whales are named so due to the presence of characteristic keratinized baleen plates instead of teeth. The term ‘baleen’ is derived from the Old French word baleine, which actually means a whale.
The name of the parvorder, Mysticeti, is derived from the Greek words mystax (meaning mustache) and ketos (meaning whale or sea monster). Alternately, this parvorder has also been referred to as Mystacoceti in the past.
The 15 recognized species of baleen whales are grouped under 4 extant families and 6 genera. There are also 5 extinct families of baleen whales.
While balaenids have large, relatively round heads, others, like rorquals and gray whales, have more elongated, tapering heads. Most balaenids have roughened patches of thickened skin called callosities, which they inherit from birth[3].
The gray whale is distinguished from other baleen whales by their sleet-gray coloration and gray-white scars left on its body by parasites.
Baleen whales are believed to have evolved in the Late Eocene Epoch (around 37 to 33 million years ago), with the earliest specimen being that of the species Mystacodon selenensis. Similar to modern toothed whales, these early forms (commonly classified under the informal group Archaeoceti) also had heterodont teeth adapted for suction feeding. Similarly, other ancient species, like Janjucetus hunderi, discovered in Australia, had incisors and canines for stabbing and premolars and molars for tearing prey. Moreover, these early forms were also considerably smaller than the modern baleen whales, with some, like members of the genus Mammalodon, being no more than 10 ft in size.
Interestingly, species like Aetiocetus weltoni had both baleen and teeth, whereas more evolved ones, like those of the genus Cetotherium, completely lacked teeth. These anatomical differences suggest that these whales went through multiple transitional phases in their evolutionary history.
They are spread worldwide across tropical waters to polar regions. While fin whales, minke whales, gray whales, Bryde’s whales, and sei whales have a near-global distribution, other species are regionally specific. For instance, bowhead whales are restricted to the waters around the Arctic ice edges. The Southern right whales are found only in the Southern Hemisphere, while the Northern right whales are found only in the Northern Hemisphere.
While some baleen whales, like the blue whale, are pelagic and inhabit open oceans, others, like the Southern right whale, prefer coastal waters. The humpback whale typically occupies the continental shelves. The gray whale[4] and the Southern right whale[5] give birth to their young in secure lagoons and coastal bays.
They are filter-feeders that consume a variety of prey, depending on the species.
These whales primarily employ two feeding strategies: lunge-feeding and skim-feeding. Of all species, only the sei whale can feed by both mechanisms.
In lunge-feeding (typically observed in rorquals), the animal inflates its mouth and expands their jaw (sometimes to hold water with a volume greater than the volume of the whale itself), allowing a greater volume of water to be stored in the mouth. Eventually, they swing open their jaw at about a 90° angle and pounce on the prey, which is typically a large bait ball of fish. However, to prevent stretching the mouth too far, lunge-feeders quickly decelerate under the influence of a pressure-sensing mechanoreceptor organ found in the middle of the jaw[6].
In skim-feeding (typically observed in bowhead whales, right whales, gray whales, and pygmy right whales), they swim with an open mouth, allowing prey to enter along with the water current. They resort to this feeding mechanism if the prey is plentiful, slow, and optimal in size to filter through the baleen.
Baleen whales swim by continuous movement of their flippers in a wing-like manner, similar to penguins and sea turtles. The flippers help steer the animal forward while the flukes move up and down vertically, propelling it underwater. Sometimes, they also leap out of water to travel faster.
Most species are generally slow swimmers, restricted by the lack of neck movement due to fused cervical vertebrae.
Most baleen whales migrate over long distances annually. During spring and summer, they stay in higher latitudes, rich in plankton, for feeding, while in winter, they travel to tropical waters to give birth, escaping the cold and avoiding predators like killer whales.
The gray whale, having the longest recorded migration among mammals, travels around 23,000 km (14,000 mi) from the Sea of Okhotsk to the Baja Peninsula. Similarly, California blue whales exhibit seasonal shifting patterns, typically shifting from central California in summer and fall to the Gulf of California in winter, then to the central Baja California Pacific coast in spring.
These mammals are found to vocalize extensively, especially during the breeding season.
These whales are generally long-lived and lack visible age markers, making it difficult to study their age in the wild. The bowhead whale is estimated to live as many as 200 years in the wild, making it the longest-living mammal known to date[8].
Blue whales have been estimated to live for 80 to 90 years, similar to fin whales and humpback whales, which also have estimated lifespans of about 90 years. Sei whales, on the other hand, typically live up to 60 years, while gray whales have a lifespan ranging from 55 to 70 years.
Baleen whales are typically polygynous, where a single male mates with multiple females in their lifetime. Bowhead, right, and gray whales exhibit lek-like polygynous behavior, where several males compete ferociously for access to a single female. The female typically chooses to mate with the most aggressive and powerful male.
Breeding is generally seasonal, though the timing varies across the different species of baleen whales. Many of them migrate to tropical and subtropical grounds during winter before breeding.
Females typically come into estrus (the phase of sexual receptivity) once a year. However, in humpback whales, a second estrus may occur within the same year due to failed conception.
After a gestation period of 10 to 14 months, the females give birth to relatively developed, precocial young (calves). In the last 2 months of gestation, the blue whale fetus gains approximately 100 kg (220 lb) per day[9].
In most species, except minke whales, the females undergo a calving period (interbirth interval) of 2 to 3 years, during which they recover energy reserves after nursing their calves. These calves grow rapidly during the first year after birth, but their growth eventually stalls for several years.
Depending on the species, the young attain sexual maturity between 4 and 11 years. Blue whales become sexually mature on attaining a length of about 66 to 79 ft, while minke whales become sexually mature at around 23 to 26 ft. Bryde’s whales generally attain sexual maturity when they grow about 39 ft long.
Due to their large size, baleen whales have very few predators. The orca, or killer whale, is the primary predator that targets juveniles or sick individuals of gray whales, humpback whales, and minke whales. Occasionally, some sharks, like dusky sharks and great white sharks, may opportunistically attack weak individuals or scavenge on carcasses.
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