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Browse by Category Blog. Book Award Winners Learn about our latest award-winning titles! Picture Books Get ready for a story time adventure! Colors With bright illustrations and embossing on every page, little ones will love this introduction to colo My First Colors and Shapes Introduce little ones to the first concepts of colors and shapes with stylish artwork from Aino-Maija In many toothed whales, the depression in their skull is due to the formation of a large melon and multiple, asymmetric air bags.
Other cetaceans have fused neck vertebrae and are unable to turn their head at all. The baleen of baleen whales consists of long, fibrous strands of keratin. Located in place of the teeth, it has the appearance of a huge fringe and is used to sieve the water for plankton and krill. The neocortex of many cetaceans is home to elongated spindle neurons that, prior to , were known only in hominids. Brain size was previously considered a major indicator of intelligence. Since most of the brain is used for maintaining bodily functions, greater ratios of brain to body mass may increase the amount of brain mass available for cognitive tasks.
Allometric analysis indicates that mammalian brain size scales at approximately two-thirds or three-quarter exponent of the body mass. The sperm whale Physeter macrocephalus is the largest of all toothed predatory animals and possesses the largest brain. The cetacean skeleton is largely made up of cortical bone , which stabilizes the animal in the water. For this reason, the usual terrestrial compact bones, which are finely woven cancellous bone , are replaced with lighter and more elastic material. In many places, bone elements are replaced by cartilage and even fat, thereby improving their hydrostatic qualities.
The ear and the muzzle contain a bone shape that is exclusive to cetaceans with a high density, resembling porcelain. This conducts sound better than other bones, thus aiding biosonar. The number of vertebrae that make up the spine varies by species, ranging from forty to ninety-three. The cervical spine , found in all mammals, consists of seven vertebrae which, however, are reduced or fused. This fusion provides stability during swimming at the expense of mobility. The fins are carried by the thoracic vertebrae , ranging from nine to seventeen individual vertebrae.
The sternum is cartilaginous. The last two to three pairs of ribs are not connected and hang freely in the body wall. The stable lumbar and tail include the other vertebrae. Below the caudal vertebrae is the chevron bone. The front limbs are paddle-shaped with shortened arms and elongated finger bones, to support movement. They are connected by cartilage. The second and third fingers display a proliferation of the finger members, a so-called hyperphalangy.
The shoulder joint is the only functional joint in all cetaceans except for the Amazon river dolphin. The collarbone is completely absent. Cetaceans have powerful hearts. Blood oxygen is distributed effectively throughout the body. They are warm-blooded, i. Cetaceans have lungs, meaning they breathe air. An individual can last without a breath from a few minutes to over two hours depending on the species. Cetacea are deliberate breathers who must be awake to inhale and exhale.
When stale air, warmed from the lungs, is exhaled, it condenses as it meets colder external air. As with a terrestrial mammal breathing out on a cold day, a small cloud of 'steam' appears. This is called the 'spout' and varies across species in shape, angle and height. Species can be identified at a distance using this characteristic. The structure of the respiratory and circulatory systems is of particular importance for the life of marine mammals.
The oxygen balance is effective. During inhalation, about twice as much oxygen is absorbed by the lung tissue as in a land mammal. As with all mammals, the oxygen is stored in the blood and the lungs, but in cetaceans, it is also stored in various tissues, mainly in the muscles. The muscle pigment, myoglobin , provides an effective bond. The stomach consists of three chambers. The first region is formed by a loose gland and a muscular forestomach missing in beaked whales , which is then followed by the main stomach and the pylorus.
Both are equipped with glands to help digestion. A bowel adjoins the stomachs, whose individual sections can only be distinguished histologically. The liver is large and separate from the gall bladder. The kidneys are long and flattened. The salt concentration in cetacean blood is lower than that in seawater, requiring kidneys to excrete salt. This allows the animals to drink seawater. Cetacean eyes are set on the sides rather than the front of the head. This means only species with pointed 'beaks' such as dolphins have good binocular vision forward and downward.
Tear glands secrete greasy tears, which protect the eyes from the salt in the water. The lens is almost spherical, which is most efficient at focusing the minimal light that reaches deep water. Cetaceans make up for their generally poor vision except dolphins with excellent hearing. At least one species, the tucuxi or Guiana dolphin, is able to use electroreception to sense prey. The external ear has lost the pinna visible ear , but still retains a narrow external auditory meatus. To register sounds, instead, the posterior part of the mandible has a thin lateral wall the pan bone fronting a concavity that houses a fat pad.
The pad passes anteriorly into the greatly enlarged mandibular foramen to reach in under the teeth and posteriorly to reach the thin lateral wall of the ectotympanic. The ectotympanic offers a reduced attachment area for the tympanic membrane.
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The connection between this auditory complex and the rest of the skull is reduced—to a single, small cartilage in oceanic dolphins. In odontocetes, the complex is surrounded by spongy tissue filled with air spaces, while in mysticetes, it is integrated into the skull as with land mammals. In odontocetes, the tympanic membrane or ligament has the shape of a folded-in umbrella that stretches from the ectotympanic ring and narrows off to the malleus quite unlike the flat, circular membrane found in land mammals.
In mysticetes, it also forms a large protrusion known as the "glove finger" , which stretches into the external meatus and the stapes are larger than in odontocetes. In some small sperm whales , the malleus is fused with the ectotympanic. The ear ossicles are pachyosteosclerotic dense and compact and differently shaped from land mammals other aquatic mammals, such as sirenians and earless seals, have also lost their pinnae. T semicircular canals are much smaller relative to body size than in other mammals.
The auditory bulla is separated from the skull and composed of two compact and dense bones the periotic and tympanic referred to as the tympanoperiotic complex. This complex is located in a cavity in the middle ear, which, in the Mysticeti, is divided by a bony projection and compressed between the exoccipital and squamosal, but in the odontoceti, is large and completely surrounds the bulla hence called "peribullar" , which is, therefore, not connected to the skull except in physeterids.
In the Odontoceti, the cavity is filled with a dense foam in which the bulla hangs suspended in five or more sets of ligaments. The pterygoid and peribullar sinuses that form the cavity tend to be more developed in shallow water and riverine species than in pelagic Mysticeti. In Odontoceti, the composite auditory structure is thought to serve as an acoustic isolator, analogous to the lamellar construction found in the temporal bone in bats.
Cetaceans use sound to communicate , using groans, moans, whistles, clicks or the 'singing' of the humpback whale. Odontoceti are generally capable of echolocation. They can search for, chase and catch fast-swimming prey in total darkness. Most Odontoceti can distinguish between prey and nonprey such as humans or boats ; captive Odontoceti can be trained to distinguish between, for example, balls of different sizes or shapes. Echolocation clicks also contain characteristic details unique to each animal, which may suggest that toothed whales can discern between their own click and that of others.
Mysticeti have exceptionally thin, wide basilar membranes in their cochleae without stiffening agents, making their ears adapted for processing low to infrasonic frequencies. They have four pairs of telocentric chromosomes whose centromeres sit at one of the telomeres , two to four pairs of subtelocentric and one or two large pairs of submetacentric chromosomes. The remaining chromosomes are metacentric—the centromere is approximately in the middle—and are rather small.
Cetaceans are found in all places. River dolphin species live exclusively in fresh water. While many marine species, such as the blue whale , the humpback whale and the killer whale , have a distribution area that includes nearly the entire ocean, some species occur only locally or in broken populations. These include the vaquita, which inhabits a small part of the Gulf of California and Hector's dolphin , which lives in some coastal waters in New Zealand. Both species prefer deeper marine areas and species that live frequently or exclusively in coastal and shallow water areas.
Many species inhabit specific latitudes, often in tropical or subtropical waters, such as Bryde's whale or Risso's dolphin. Others are found only in a specific body of water. The southern right whale dolphin and the hourglass dolphin live only in the Southern Ocean. The narwhal and the beluga live only in the Arctic Ocean. Sowerby's beaked whale and the Clymene dolphin exist only in the Atlantic and the Pacific white-sided dolphin and the northern straight dolphin live only in the North Pacific.
Cosmopolitan species may be found in the Pacific, Atlantic and Indian Oceans. However, northern and southern populations become genetically separated over time. In some species, this separation leads eventually to a divergence of the species, such as produced the southern right whale , North Pacific right whale and North Atlantic right whale.
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Thirty-two species are found in European waters, including twenty-five toothed and seven baleen species. Conscious breathing cetaceans sleep but cannot afford to be unconscious for long, because they may drown. While knowledge of sleep in wild cetaceans is limited, toothed cetaceans in captivity have been recorded to exhibit unihemispheric slow-wave sleep USWS , which means they sleep with one side of their brain at a time, so that they may swim, breathe consciously and avoid both predators and social contact during their period of rest.
A study found that sperm whales sleep in vertical postures just under the surface in passive shallow 'drift-dives', generally during the day, during which whales do not respond to passing vessels unless they are in contact, leading to the suggestion that whales possibly sleep during such dives.
While diving, the animals reduce their oxygen consumption by lowering the heart activity and blood circulation; individual organs receive no oxygen during this time. Some rorquals can dive for up to 40 minutes, sperm whales between 60 and 90 minutes and bottlenose whales for two hours. Most whales are social animals, although a few species live in pairs or are solitary.
A group, known as a pod, usually consists of ten to fifty animals, but on occasion, such as mass availability of food or during mating season, groups may encompass more than one thousand individuals. Inter-species socialization can occur. Pods have a fixed hierarchy, with the priority positions determined by biting, pushing or ramming.
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The behavior in the group is aggressive only in situations of stress such as lack of food, but usually it is peaceful. Contact swimming, mutual fondling and nudging are common. The playful behavior of the animals, which is manifested in air jumps, somersaults, surfing, or fin hitting, occurs more often than not in smaller cetaceans, such as dolphins and porpoises. Males in some baleen species communicate via whale song , sequences of high pitched sounds.
These "songs" can be heard for hundreds of kilometers. Each population generally shares a distinct song, which evolves over time. Sometimes, an individual can be identified by its distinctive vocals, such as the hertz whale that sings at a higher frequency than other whales. Some individuals are capable of generating over distinct sounds. Pod groups also hunt, often with other species.
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Many species of dolphins accompany large tunas on hunting expeditions, following large schools of fish. The killer whale hunts in pods and targets belugas and even larger whales. Humpback whales, among others, form in collaboration bubble carpets to herd krill or plankton into bait balls before lunging at them.
Cetacea are known to teach, learn, cooperate, scheme and grieve. Smaller cetaceans, such as dolphins and porpoises, engage in complex play behavior, including such things as producing stable underwater toroidal air-core vortex rings or " bubble rings ". The two main methods of bubble ring production are rapid puffing of air into the water and allowing it to rise to the surface, forming a ring, or swimming repeatedly in a circle and then stopping to inject air into the helical vortex currents thus formed. They also appear to enjoy biting the vortex rings, so that they burst into many separate bubbles and then rise quickly to the surface.
Whales produce bubble nets to aid in herding prey. Larger whales are also thought to engage in play. The southern right whale elevates its tail fluke above the water, remaining in the same position for a considerable time. This is known as "sailing". It appears to be a form of play and is most commonly seen off the coast of Argentina and South Africa.
Self-awareness appears to be a sign of abstract thinking. Self-awareness, although not well-defined, is believed to be a precursor to more advanced processes such as metacognitive reasoning thinking about thinking that humans exploit. Cetaceans appear to possess self-awareness. Researchers then explore whether the animal shows signs of self-recognition. Critics claim that the results of these tests are susceptible to the Clever Hans effect.
This test is much less definitive than when used for primates. Primates can touch the mark or the mirror, while cetaceans cannot, making their alleged self-recognition behavior less certain. Skeptics argue that behaviors said to identify self-awareness resemble existing social behaviors, so researchers could be misinterpreting self-awareness for social responses. Advocates counter that the behaviors are different from normal responses to another individual. Cetaceans show less definitive behavior of self-awareness, because they have no pointing ability. In , Marten and Psarakos used video to test dolphin self-awareness.
They concluded that their evidence suggested self-awareness rather than social behavior. While this particular study has not been replicated, dolphins later "passed" the mirror test. Most cetaceans sexually mature at seven to 10 years.
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An exception to this is the La Plata dolphin , which is sexually mature at two years, but lives only to about The sperm whale reaches sexual maturity within about 20 years and a lifespan between 50 and years. For most species, reproduction is seasonal. Ovulation coincides with male fertility. This cycle is usually coupled with seasonal movements that can be observed in many species.
Most toothed whales have no fixed bonds. In many species, females choose several partners during a season. Baleen whales are largely monogamous within each reproductive period. Gestation ranges from 9 to 16 months. Duration is not necessarily a function of size. Porpoises and blue whales gestate for about 11 months. During gestation, the embryo is fed by a special nutritive tissue, the placenta. Cetaceans usually bear one calf.
In the case of twins, one usually dies, because the mother cannot produce sufficient milk for both. The fetus is positioned for a tail-first delivery, so that the risk of drowning during delivery is minimal. After birth, the mother carries the infant to the surface for its first breath. At birth they are about one-third of their adult length and tend to be independently active, comparable to terrestrial mammals. Like other placental mammals, cetaceans give birth to well-developed calves and nurse them with milk from their mammary glands. When suckling, the mother actively splashes milk into the mouth of the calf, using the muscles of her mammary glands, as the calf has no lips.
In many small cetaceans, suckling lasts for about four months.
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In large species, it lasts for over a year and involves a strong bond between mother and offspring. The mother is solely responsible for brooding. In some species, so-called "aunts" occasionally suckle the young. Among cetaceans, whales are distinguished by an unusual longevity compared to other higher mammals. Some species, such as the bowhead whale Balaena mysticetus , can reach over years. Based on the annual rings of the bony otic capsule , the age of the oldest known specimen is a male determined to be years at the time of death. Upon death, whale carcasses fall to the deep ocean and provide a substantial habitat for marine life.
Evidence of whale falls in present-day and fossil records shows that deep-sea whale falls support a rich assemblage of creatures, with a global diversity of species, comparable to other neritic biodiversity hotspots, such as cold seeps and hydrothermal vents. Deterioration of whale carcasses happens through three stages.
Initially, organisms such as sharks and hagfish scavenge the soft tissues at a rapid rate over a period of months and as long as two years. This is followed by the colonization of bones and surrounding sediments which contain organic matter by enrichment opportunists, such as crustaceans and polychaetes , throughout a period of years.
Finally, sulfophilic bacteria reduce the bones releasing hydrogen sulfide enabling the growth of chemoautotrophic organisms, which in turn, support organisms such as mussels, clams, limpets and sea snails. This stage may last for decades and supports a rich assemblage of species, averaging per site. Brucellosis affects almost all mammals. It is distributed worldwide, while fishing and pollution have caused porpoise population density pockets, which risks further infection and disease spreading.
Brucella ceti , most prevalent in dolphins, has been shown to cause chronic disease , increasing the chance of failed birth and miscarriages , male infertility , neurobrucellosis, cardiopathies , bone and skin lesions , strandings and death. Until , no case had ever been reported in porpoises, but isolated populations have an increased risk and consequentially a high mortality rate. Molecular biology and immunology show that cetaceans are phylogenetically closely related with the even-toed ungulates Artiodactyla. Whales direct lineage began in the early Eocene , more than 50 million years ago, with early artiodactyls.
Most molecular biological evidence suggests that hippos are the closest living relatives. Common anatomical features include similarities in the morphology of the posterior molars , and the bony ring on the temporal bone bulla and the involucre, a skull feature that was previously associated only with cetaceans. Early cetaceans, archaeocetes , show double castors, which only occur in even-toed ungulates. Corresponding findings are from Tethys Sea deposits in northern India and Pakistan. The Tethys Sea was a shallow sea between the Asian continent and northward-bound Indian plate.
Raoellidae Indohyus , Khirharia The direct ancestors of today's cetaceans are probably found within the Dorudontidae whose most famous member, Dorudon atrox , lived at the same time as Basilosaurus. Both groups had already developed the typical anatomical features of today's whales, such as hearing.
Life in the water for a formerly terrestrial creature required significant adjustments such as the fixed bulla, which replaces the mammalian eardrum , as well as sound-conducting elements for submerged directional hearing. Their wrists were stiffened and probably contributed to the typical build of flippers. The hind legs existed, however, but were significantly reduced in size and with a vestigial pelvis connection. The fossil record traces the gradual transition from terrestrial to aquatic life.
The regression of the hind limbs allowed greater flexibility of the spine. This made it possible for whales to move around with the vertical tail hitting the water. The front legs transformed into flippers, costing them their mobility on land. One of the oldest members of ancient cetaceans Archaeoceti is Pakicetus from the Middle Eocene. This is an animal the size of a wolf, whose skeleton is known only partially. It had functioning legs and lived near the shore.
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