Baleen whales, also called whalebone whales, are a group of marine mammals distinguished by numerous keratinized plates (instead of teeth) called baleen hanging from their upper jaw. They belong to the parvorder Mysticeti (hence also called mysticetes) under the infraorder Cetacea, which includes 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. Baleen whales and tooth whales are believed to have diverged around 34 million years ago.
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 in size, 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 feet in length and weigh over 50 t, while sei whales are slightly smaller, reaching around 50 ft.
These whales have a fusiform body, roughly cylindrical, with tapering ends. Their forelimbs are modified into paddle-shaped pectoral flippers (for swimming) but retain the skeletal elements of terrestrial mammals. In contrast, the hindlimbs are non-functional, 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 lacking bone or muscle. These flukes are controlled by the longitudinal muscles of the back and the caudal peduncle.
Rorquals and pygmy right whales possess a dorsal fin composed of dense, fibrous connective tissue. However, bowhead, gray, and right whales lack this fin.
Baleen whales are usually black or gray, but some species exhibit countershading, with a lighter back and darker belly, making them appear invisible when light shines from above.
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.
As mammals, baleen whales have epidermis, dermis, hypodermis, and connective tissue.
They have two nostrils, known as blowholes, located on the top of their heads. These blowholes are longitudinal slits that converge anteriorly and widen towards the end, creating a V-shaped blow. The blowholes are separated by a septum and covered by a muscular flap.
Although these whales possess tooth buds in the embryonic stage, they are lost before birth. Instead, they possess stiff plates called baleen that grow down 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), minke whales only have around 1 to 4% of it.
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 being the main baleen plates and the smallest, the accessory plates. The latter end in small, tapering hair-like structures.
The number and length of these plates vary across species.
Although the hearts of baleen whales are functionally similar to those of any other mammal, they are incredibly large in size, measuring up to 1,000 lb (454 kg). They have a resting heart rate of about 60 to 140 beats per minute (bpm), which drops to 4 to 15 bpm while diving to conserve oxygen.
The ventricular wall is highly muscular and thick, measuring between 7.6 to 12.7 cm (3 to 5 in). 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 the largest among mammals.
They have uniquely elastic lungs, adapted for deep diving, that can extract about 80% of oxygen from the air. These lungs are sacculated and lack lobes, with the left lung being smaller than the right. However, when compared to terrestrial mammals, they have a lower lung volume.
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 helps 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 that is more concentrated than the surrounding seawater.
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 these animals do not need acute vision 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 connected to the eardrum by connective tissue and an earplug. The inner ear bones lie in a bony capsule called the tympanic bulla.
The fluid-filled cochlea contains tiny sensory hairs that transmit signals to the brain upon detecting vibrations, which are then processed for interpretation.
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 in spite of 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.
The 15 recognized species of baleen whales are grouped under 4 families: Balaenidae (right whales), Balaenopteridae (rorquals), Cetotheriidae (the pygmy right whale), and Eschrichtiidae (the gray whale).
While balaenids have large heads and thick blubbers, others, like rorquals and gray whales, are usually more streamlined and have flat heads with long throat pleats. Most balaenids also have callosities (a thickened piece of skin due to repeated friction), unlike most other groups.
Gray whales are distinguished from other baleen whales by their sleet-gray coloration and gray-white scars left on their bodies 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 somewhat worldwide 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 are found in the open ocean, others prefer coastal areas and the productive continental shelf. Some species give birth to their young in secure coastal bays and lagoons.
They are obligate filter-feeders that typically feed on zooplankton and schools (or bait balls) of small fishes. They also consume crustaceans, such as krill and copepods, as well as squids and amphipods.
These whales primarily employ two feeding strategies: lunge-feeding and skim-feeding. Some species, like sei whales, can feed by both mechanisms.
In lunge-feeding (typically observed in rorquals), the animal inflates its mouth and expands their jaw (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 sensory organ found in the middle of the jaw.
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.
They are generally slow swimmers (unlike dolphins), restricted by the lack of neck movement due to fused cervical vertebrae.
Most baleen whales migrate through 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 golden 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, 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.
Since these whales are generally long-lived, it is difficult to keep track of their age in the wild. Moreover, the absence of teeth (which usually helps in the age estimation of many mammals) makes it even more difficult for scientists to approximate their lifespan. However, from the information collected over the years, it is found that the lifespan of these whales varies across the different species.
Blue whales have been found to live for 80 to 90 years, similar to fin whales and humpback whales, which are also estimated to have 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 polygynous, where a single male mates with multiple females. In bowhead, right, and gray whales, lek mating is observed, 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 seasonal, though it varies across the different species of baleen whales. Many of them breed on tropical grounds, where they migrate during the winter.
Females typically come into estrus (the phase of sexual receptivity) once a year, though failed conceptions may induce them into a second estrus in a single year.
After a gestation period of 10 to 14 months, the females give birth to relatively developed, precocial young (calves). In blue whales, the fetus undergoes a considerable increase in size, growing by some 100 kg (220 lb) per day right before their delivery.
In most species (except minke whales), the females undergo a calving period 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.
The young attain sexual maturity in about 4 to 11 years, depending on the species. 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 at around 39 ft.
The primary predator of baleen whales is the orca or the killer whale. Some shark species, like the cookiecutter shark, also prey upon these whales, though dusky sharks have been reported to attack humpback whale calves.
Of the 15 recognized species, the North Atlantic right whale and the Rice’s whale are Critically Endangered (CR) according to the IUCN Red List. The North Pacific right whale, the sei whale, and the blue whale are considered Endangered (EN), whereas the fin whale is Vulnerable (VU), and the Antarctic minke whale is Near Threatened (NT). Only one species, the Omura’s whale, is Data Deficient (DD).
The primary cause for the global decline in the number of baleen whales is large-scale commercial whaling. Historically, they have been hunted for their baleen, meat, and oil. Despite bans imposed by organizations such as the International Whaling Commission (IWC), some countries, including Japan, Iceland, and Norway, continue to hunt whales for scientific research or even cultural practices. Other factors affecting these whales are ship strikes, entanglement in fishing gear, water pollution, and rising oceanic temperatures.
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