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Evolution of cetaceans

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About 80 of the 87 modern species in the order Cetacea.
A phylogeny showing the relationships among cetacean families.[1]

The evolutionary history of cetaceans is thought to have occurred in India from even-toed ungulates 50 million years ago, over a period of at least 15 million years, however a jawbone discovered in Antarctica may reduce this to 5 million years.[2] Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla, and branched off from other artiodactyls around 50 mya (million years ago). Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, they surface to breathe air, they have bones in their fins, they nurse their young, and their skeletal anatomy is more similar to terrestrial mammals than to fish. These mammalian characteristics all point to their terrestrial origin, as well as the fact that they are mammals. Discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.

The two modern parvorders of cetaceans – Mysticeti (baleen whales) and Odontoceti (toothed whales) – are thought to have separated from each other, and the archaeocetes, around 34 million years ago in a second cetacean radiation, the first belonging to the archaeocetes. The adaptation of echolocation in toothed whales distinguishes them from fully aquatic archaeocetes and early baleen whales. The presence of baleen in baleen whales occurred gradually, with earlier varieties like Janjucetus having very little baleen, and their size is linked to baleen dependence.

Modern-day cetaceans' evolution is largely affected by their local culture and social network. Tool-use for foraging in certain societies affects their diet, and allows them to access more varieties of food. This behavior is passed down from mother-to-child, as in the bottlenose dolphins residing in Shark Bay, where many use sponges to protect their beak from sharp objects and stingray barbs while foraging. Social bonds between dolphins in a pod can effect overall fitness in a habitat, decreasing or increasing the survival chances for any particular individual in that pod, as with strong bonds between older and younger juvenile males (adversely affecting older individuals). Social bonds between different species can sometimes cause interbreeding, which, if any, causes infertile hybrids.

Early ancestors

Cladogram showing the position of cetacea within Artiodactylamorpha
Evolution of cetaceans
Indohyus (Archaeoceti cca 49–48 Ma)
Pakicetus (Archaeoceti cca 49–48 Ma)
Ambulocetus (Archaeoceti cca 49–48 Ma)
Kutchicetus (Archaeoceti cca 48 Ma)
Remingtonocetus (Archaeoceti cca 48 Ma)
Rodhocetus (Archaeoceti cca 45 Ma)
Protocetus (Archaeoceti cca 45 Ma)
Dorudon (Archaeoceti cca 35 Ma)
Squalodon (Odontoceti cca 25 Ma)
Kentriodon (Odontoceti cca 20 Ma)
Aulophyseter (Odontoceti cca 20 Ma)
Brygmophyseter (Odontoceti cca 15 Ma)
Janjucetus (Mysticeti cca 25 Ma)
Cetotherium (Mysticeti cca 18 Ma)

The traditional theory of cetacean evolution was that whales were related to the mesonychids, an extinct order of carnivorous ungulates (hoofed animals) that resembled wolves with hooves and were a sister group of the artiodactyls (even-toed ungulates). This theory arose due to similarities between the unusual triangular teeth of the mesonychids and those of whales. However, more recent molecular phylogeny data indicate that whales are more closely related to the artiodactyls, with hippopotamus as the closest living relative.[3] The strong evidence for a clade combining cetaceans and artiodactyls is further discussed in the article Cetartiodactyla. However, the earliest anthracotheres, the ancestors of hippos, do not appear in the fossil record until the Middle Eocene, millions of years after Pakicetus, the first known whale ancestor, appears during the Early Eocene, implying the two groups diverged well before the Eocene.

The molecular data is supported by the recent discovery of Pakicetus, the earliest proto-whale (see below). The skeletons of Pakicetus show that whales did not derive directly from mesonychids. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids. Proto-whales retained aspects of their mesonychid ancestry (such as the triangular teeth) which modern artiodactyls have lost. An interesting implication is that the earliest ancestors of all hoofed mammals were probably at least partly carnivorous or scavengers, and today's artiodactyls and perissodactyls became herbivores later in their evolution. By contrast, whales retained their carnivorous diet, because prey was more available and they needed higher caloric content in order to live as marine endotherms. Mesonychids also became specialized carnivores, but this was likely a disadvantage because large prey was not yet common. This may be why they were out-competed by better-adapted animals like the creodonts and later Carnivora which filled the gaps left by the dinosaurs.

Indohyus

Possible relationships between cetaceans and other ungulate groups.[1] [4]

Indohyus was a small chevrotain-like animal that lived about 48 million years ago in what is now Kashmir.[5] It belongs to the artiodactyl family Raoellidae, and is believed to be the closest sister group of Cetacea. Indohyus is identified as an Artiodactyla because it has two trochlea hinge, a trait unique to the artiodactyl family.[4] About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales, most notably the involucrum, a bone growth pattern which is the diagnostic characteristic of any cetacean, and is not found in any other species.[1] It also showed signs of adaptations to aquatic life, including a thick and heavy outer coating and dense limb bones that reduce buoyancy so that they could stay underwater, which are similar to the adaptations found in modern creatures such as the hippopotamus.[3][6] This suggests a similar survival strategy to the African mousedeer or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.[7]

Pakicetidae

The pakicetids were digitigrade hoofed mammals that are thought to be the earliest known whales, with Indohyus being the closest sister group.[5][8] They lived in the early Eocene, around 50 million years ago. Their fossils were first discovered in North Pakistan in 1979, located at a river not far from the shores of the former Tethys Sea.[9] After the initial discovery, more fossils were found, mainly in the late-early Eocene fluvial deposits in northern Pakistan and northwestern India. Based on this discovery, pakicetids most likely lived in an arid environment with ephemeral streams and moderately developed floodplains millions of years ago.[1] By using stable oxygen isotopes analysis, they were shown to drink fresh water.[10] Their diet probably included land animals that approached water for drinking or some freshwater aquatic organisms that lived in the river.[1] The elongated cervical vertebrae and the four, fused sacral vertebrae are consistent with Artiodactyla, making Pakicetidae one of the earliest fossils to be recovered from the period following the Cetacean/Artiodactyla divergence event.[11]

Pakicetids were classified as cetaceans mainly based on the structure of the auditory bulla, which is formed from the ectotympanic bone only. The shape of the ear region in pakicetids is highly unusual and the skull is cetacean-like, although a blowhole is still absent at this stage. The jawbone of pakicetids also lacks the enlarged space (mandibular foramen) that is filled with fat or oil, which is used in receiving underwater sound in modern whales.[12] They have dorsal orbits (eye sockets facing up), which are similar to crocodiles. This eye placement helps submerged predators observe potential prey above the water.[10] According to Thewissen et al., the teeth of pakicetids also resemble the teeth of fossil whales, being less like a dog's incisors, with a serrated triangular shape, similar to a shark's tooth, which is another link to more modern whales.[13] It was initially thought that the ears of pakicetids were adapted for underwater hearing, but, as would be expected from the anatomy of the rest of this creature, the ears of pakicetids are specialized for hearing on land.[14] However, pakicetids were able to listen underwater, by using enhanced bone conduction, rather than depending on tympanic membrane like general land mammals. This method of hearing does not give directional hearing underwater.[12]

Pakicetids have long thin legs, with relatively short hands and feet which suggest that they were poor swimmers.[1] To compensate for that, their bones are unusually thick (osteosclerotic), which is probably an adaptation to make the animal heavier to counteract the buoyancy of the water.[4] According to a morphological analysis by Thewissen et al., pakicetids display no aquatic skeletal adaptation; instead they display adaptations for running and jumping. Hence pakicetids were most likely an aquatic wader.[15]

Ambulocetidae

Ambulocetus natans, which lived about 49 million years ago, was discovered in Pakistan in 1994. It was probably amphibious, and resembled the crocodile in its physical appearance.[15] In the Eocene, ambulocetids inhabited the bays and estuaries of the Tethys Ocean in northern Pakistan. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral molluscs.[1] Although they are found only in marine deposits, their oxygen isotope values indicate that they consumed a range of water with different degree of salinity, with some specimens having no evidence of sea water consumption and others that did not ingest fresh water at the time when their teeth are fossilized. It is clear that ambulocetids tolerated a wide range of salt concentrations. Hence, ambulocetids represent a transition phase of cetacean ancestors between fresh water and marine habitat.[10]

The mandibular foramen in ambulocetids had increased in size, which indicates that a fat pad was likely to be housed in the lower jaw. In modern whales, this fat pad in the mandibular foramen extends posteriorly to the middle ear. This allows sounds to be received in the lower jaw, and then transmitted through the fat pad to the middle ear. Similar to pakicetids, the orbits of ambulocetids are on the dorsal side of the skull, but they face more laterally than in pakicetids.[10]

Ambulocetids had relatively long limbs with particular strong hind legs, and they retained a tail with no sign of fluke (the horizontal tail fin of modern cetaceans).[9] The hindlimb structure of Ambulocetids shows that their ability to engage in terrestrial locomotor activity was significantly limited compared to that of contemporary terrestrial mammals. The skeletal structures of the knee and ankle indicate that the motion of the hindlimbs were restricted in one plane. This suggests that, on land, propulsion of the hindlimbs was powered by extension of dorsal muscles.[16] Although they could walk on land, as well as swim, it is clear that they were not fast in either environment.[17] It has been speculated that Ambulocetids hunted like crocodiles, lurking in the shallows to snatch unsuspecting riparian prey and fish.[15] They probably swam by pelvic paddling (a way of swimming which mainly utilizes their hind limbs to generate propulsion in water) and caudal undulation (a way of swimming which uses the undulations of the vertebral column to generate force for movements), as otters, seals and whales do.[18] This is an intermediate stage in the evolution of cetacean locomotion, as modern whales swim by caudal oscillation (a way of swimming similar to caudal undulation, but uses energy more efficiently).[10]

Remingtonocetidae

Remingtonocetids lived in middle-Eocene South Asia, about 49 to 43 million years ago.[19] Compared to family Pakicetidae and Ambulocetidae, Remingtonocetidae was a diverse family found in north and central Pakistan, and also western India. Remingtonocetids were also found in shallow marine deposits, but they are obviously more aquatic than ambulocetidae. This can be seen from the recovery of their fossils from a variety of coastal marine environments, including near-shore and lagoonal deposits.[1] It is shown that most remingtonocetids did not ingest fresh water, and had hence lost their dependency on fresh water relatively soon after their origin.[10]

The orbits of remingtonocetids face laterally and are small. This suggests that vision is not an important sense for them. The nasal opening, which will later evolve to become the blowhole in modern cetaceans, is located near the tip of the long snout. The position of the nasal opening had remained unchanged since pakicetids.[10] One of the notable features in remingtonocetids is that the semicircular canals, which are important for balancing in land mammals, had decreased in size.[20] This reduction in size had closely accompanied the cetacean invasion of marine environments. According to Spoor et al., this modification of the semicircular canal system may represent a crucial ‘point of no return’ event in early cetacean evolution, which excluded a prolonged semi-aquatic phase.[20]

Compared to ambulocetids, remingtonocetids had relatively short fore and hind limbs.[10] Based on their skeletal remains, remingtonocetids were probably amphibious whales that are well adapted to swimming, and likely to swim by caudal undulation only.[1]

Protocetidae

The protocetids form a diverse and heterogeneous group known from Asia, Europe, Africa, and North America. They lived in the Eocene, approximately 48 to 35 million years ago. The fossil remains of protocetids were uncovered from coastal and lagoonal facies in South Asia; but unlike previous cetacean families, their fossils uncovered from Africa and North America also include open marine forms.[1] Hence they were probably amphibious, but more aquatic compared to remingtonocetids.[19] Protocetids were the first whales to leave the Indian subcontinent and disperse to all shallow subtropical oceans of the world.[10] There were many genera among the family Protocetidae, and some of these are very well known (e.g., Rodhocetus). Great variations in aquatic adaptations exist among them, with some probably able to support their weight on land, whereas others could not.[1] Their supposed amphibious nature is supported by the discovery of a pregnant Maiacetus,[21] in which the fossilised fetus was positioned for a head-first delivery, suggesting that Maiacetus gave birth on land.

Unlike remingtonocetids and ambulocetids, protocetids have large orbits which are oriented laterally. Increasingly lateral-facing eyes might be used to observe underwater prey, and are similar to the eyes of modern cetaceans. Furthermore, the nasal openings are large and are now halfway up the snout. The great variety of teeth suggests diverse feeding modes in protocetids.[19] In both remingtonocetids and protocetids, the size of mandibular foramen had increased.[10] The large mandibular foramen indicates that the mandibular fat pad was present. However the air-filled sinuses that are present in modern cetaceans, which function to isolate the ear acoustically to enable better underwater hearing, are still not present.[12] The external auditory meatus (ear canal) which is absent in modern cetaceans is also present. Hence, the method of sound transmission present in them combines aspects of pakicetids and modern odontocetes (toothed whales).[12] At this intermediate stage of hearing development, the transmission of airborne sound was poor due to the modifications of ear for underwater hearing; while directional underwater hearing was also poor compared to modern cetaceans.[12]

Some protocetids had short, large fore- and hindlimbs that are likely to be used in swimming, but the limbs give a slow and cumbersome locomotion on land.[10] It is possible that some protocetids had flukes. However, it is clear that they are adapted even further to an aquatic life-style. In Rodhocetus, for example, the sacrum (a bone that in land-mammals is a fusion of five vertebrae that connects the pelvis with the rest of the vertebral column) was divided into loose vertebrae. However, the pelvis was still connected to one of the sacral vertebrae. The ungulate ancestry of these early whales is still underlined by characteristics like the presence of hooves at the ends of the toes in Rodhocetus.[22]

The foot structure of Rodhocetus shows that protocetids were predominantly aquatic. Gingerich et al. hypothesized that Rodhocetus locomoted in oceanic environment similarly to Ambulocetids pelvic paddling supplemented by caudal undulation. Terrestrial locomotion of Rodhocetus was very limited due to their hindlimb structure. It is thought that they moved in a way similar to how eared seals move on land.[23]

Basilosauridae and Dorudontinae

Basilosaurids were discovered in 1840 and initially mistaken for a reptile, hence its name. Together with dorudontids, they lived in the late Eocene around 41 to 35 million years ago, and are the oldest known obligate aquatic cetaceans.[14] They were fully recognizable whales which lived entirely in the ocean. This is supported by their fossils usually found in deposits indicative of fully marine environments, lacking any freshwater influx.[1] They were probably distributed throughout the tropical and subtropical seas of the world. Basilosaurids are commonly found in association with dorudontids. In fact, they are closely related to one another.[10] The fossilised stomach contents in one basilosaurid indicates that it ate fish.[1]

Although they look very much like modern whales, basilosaurids and dorudontids lacked the 'melon organ' that allows their descendants to use echolocation as effectively as modern whales. They had small brains; this suggests they were solitary and did not have the complex social structure of some modern cetaceans. The mandibular foramen of basilosaurids and dorudontids now cover the entire depth of the lower jaw as in modern cetaceans. Their orbits face laterally, and the nasal opening had moved even higher up the snout, closer to the position of blowhole in modern cetaceans.[10] Furthermore, their ear structures are functionally modern, with the major innovation being the insertion of air-filled sinuses between ear and skull.[12] Unlike modern cetaceans, basilosaurids retain a large external auditory meatus.[12]

Both basilosaurids and dorudontids have skeletons that are immediately recognizable as cetaceans. A basilosaurid was as big as the larger modern whales, up to 60 ft (18 m) long; dorudontids were smaller, about 15 ft (4.6 m) long. The large size of basilosaurids is due to the extreme elongation of their lumbar vertebrae. They had a tail fluke, but their body proportions suggest that they swam by caudal undulation and that the fluke was not used for propulsion.[1] In contrast, dorudontids had a shorter but powerful vertebral column. They too had a fluke, and unlike basilosaurids, they probably swam similarly to modern cetaceans, by using caudal oscillations.[10] The forelimbs of basilosaurids and dorudontids were probably flipper-shaped, and the external hind limbs were tiny and are certainly not involved in locomotion.[1] Their fingers, on the other hand, still retain the mobile joints of their ambulocetid relatives. The two tiny but well-formed hind legs of basilosaurids which were probably used as claspers when mating; they are a small reminder of the lives of their ancestors. Interestingly, the pelvic bones associated with these hind limbs were now no longer connected to the vertebral column as they were in protocetids. Essentially, any sacral vertebrae can no longer be clearly distinguished from the other vertebrae.

Both basilosaurids and dorudontids are relatively closely related to modern cetaceans, which belong to parvorders Odontoceti and Mysticeti. However, according to Fordyce and Barnes, the large size and elongated vertebral body of basilosaurids preclude them from being ancestral to extant forms. As for dorudontids, there are some species within the family that do not have elongated vertebral bodies, which might be the immediate ancestors of Odontoceti and Mysticeti.[19]

Evolutionary history

Baleen whales

Artistic impression of two Eobalaenoptera pursued by the giant shark Carcharodon megalodon

All modern mysticetes are large filter-feeding or baleen whales, though the exact means by which baleen is used differs among species (gulp-feeding with balaenopterids, skim-feeding with balaenids, and bottom plowing with eschrichtiids). The first members of some modern groups appeared during the middle Miocene. Filter feeding is very beneficial as it allows modern baleen whales to efficiently gain huge energy resources, which makes the large body size in modern baleen whales possible.[24] These changes may have been a result of worldwide environmental change and physical changes in the oceans. A large-scale change in ocean current and temperature could have initiated the radiation of modern mysticetes, leading to the demise of the archaic forms. Generally it is speculated the four modern mysticete families have separate origins among the cetotheres. Modern baleen whales, Balaenopteridae (rorquals and humpback whale, Megaptera novaengliae), Balaenidae (right whales), Eschrichtiidae (gray whale, Eschrictius robustus), and Neobalaenidae (pygmy right whale, Caperea marginata) all have derived characteristics presently unknown in any cetothere.[25]

Toothed whales

The adaptation of echolocation occurred when toothed whales (Odontoceti) split apart from baleen whales, and distinguishes modern toothed whales from fully aquatic archaeocetes. This happened around 34 million years ago.[26][27][26] Modern toothed whales do not rely on their sense of sight, but rather on their sonar to hunt prey. Echolocation also allowed toothed whales to dive deeper in search of food, with light no longer necessary for navigation, which opened up new food sources.[28][19] Toothed whales echolocate by creating a series of clicks emitted at various frequencies. Sound pulses are emitted through their melon, reflected off objects, and retrieved through the lower jaw. Skulls of Squalodon show evidence for the first hypothesized appearance of echolocation.[29] Squalodon lived from the early to middle Oligocene to the middle Miocene, around 33–14 million years ago. Squalodon featured several commonalities with modern toothed whales: the cranium was well compressed, the rostrum telescoped outward (a characteristic of the modern toothed whales), giving Squalodon an appearance similar to them. However, it is thought unlikely that squalodontids are direct ancestors of living dolphins.[30]

The earliest known ancestor of modern monodontids is Denebola brachycephala from the late Miocene around 9–10 million years ago.[31] A single fossil from Baja California indicates the family once inhabited warmer waters.[32] The fossil record also indicates D. brachycephala's range varied with that of the polar ice packs expanding during ice ages and contracting when the ice retreated.[19][33]

Ancient sperm whales differ from modern sperm whales in tooth count and the shape of the face and jaws. For example, Scaldicetus had a tapered rostrum. Genera from the Oligocene and Miocene, with the exception of Aulophyseter, had teeth in their upper jaws. Acrophyseter also had teeth in both the upper and lower jaws as well as a short rostrum and an upward curving mandible. These anatomical differences suggest that these ancient species may not have necessarily been deep-sea squid hunters like the modern sperm whale, but that some genera mainly ate fish.[19][34] Contrary to modern sperm whales, ancient sperm whales were built to hunt whales. Livyatan melvillei, residing along ancient Peru 12 million years ago, had a short and wide rostrum measuring 10 feet (3.0 m) across. This gave the whale the ability to inflict major damage on large struggling prey. This mouth was specially adapted for eating eobalaenids, and did so much more effectively than other predators such as Megalodon. Other physeteroids such as Zygophyseter and Brygmophyseter, collectively known as killer sperm whales, also preyed upon early baleen whales, as well as seals.[34][35]

Beaked whales consist of over 20 genera.[36][37] Some included ancestors of giant beaked whales, Berardius, such as Microberardius, and Cuvier's beaked whale, Ziphius, had many relatives, such as Caviziphius, Archaeoziphius, and Izikoziphius. They were probably preyed upon by killer sperm whales and sharks such as Megalodon as many other species were. In 2008, a large number of fossil ziphiids were discovered off the coast of South Africa, confirming the remaining ziphiid species might just be a remnant of a higher diversity that has since gone extinct. After studying numerous fossil skulls, researchers discovered the absence of functional maxillary teeth in all South African ziphiids, which is evidence that suction feeding had already developed in several beaked whale lineages during the Miocene. Researchers also found extinct ziphiids with robust skulls, signaling that tusks were used for male-male interactions (speculated with extant beaked whales).[36]

Skeletal evolution

The skeleton of a Bowhead whale with the hind limb and pelvic bone structure circled in red. This bone structure stays internal during the entire life of the species.
Dolphins (aquatic mammals) and ichthyosaurs (extinct marine reptiles) share a number of unique adaptations for fully aquatic lifestyle and are frequently used as extreme examples of convergent evolution

Modern cetaceans have rudimentary hind limbs, such as reduced femurs, fibulas, and tibias, and a pelvic girdle, consisting of an ilium, ischium, and pubis bone. Cetacean hind limbs and bones of the pelvic girdle can be compared to terrestrial mammals. Indohyus has a thickened ectotympanic internal lip of the ear bone. This feature compares directly to that of modern cetaceans. Another similar feature was the composition of the teeth, which contained mostly calcium phosphate which is needed for eating and drinking of aquatic animals.[38] Although they somewhat resembled a wolf, the skeletons of pakicetids showed the eye sockets were much closer to the top of their head than normal, but similar to the structure of the eyes in cetaceans. Their transition from land to water led to rebuilding of the skull and food processing equipment because the eating habits were changing. Ultimately, the change in position of the eyes and limb bones is what led the pakicetids to become waders. The ambulocetids also began to develop long snouts, which is seen in current cetaceans. Their limbs were compared closely to otters because of the swimming that occurred with their hind legs.[39]

Limblessness in cetaceans does not represent a regression of fully formed limbs nor the absence of limb bud initiation, but rather arrest of limb bud development.[40] Initially, limb buds develop normally in cetacean embryos.[3][41] Limb buds progress to the condensation phase of early skeletogenesis, where nerves grow into the limb bud and the apical ectodermal ridge (AER) appears functional.[40] The AER is one of two signaling centers responsible for limb bud outgrowth,[40][41] the other being the zone of polarizing activity (ZPA) located in the posterior mesenchyme of the limb bud.[41] The maintenance of the AER is dependent on signaling from the ZPA, and thus Thewissen proposed that the degeneration of the AER was caused by a dysfunctional ZPA.[41] The ZPA is regulated in part by expression of the Sonic hedgehog (Shh), which in turn is controlled by the upstream expression of the Heart and neural crest derivatives-expressed protein 2 (HAND2) transcription factor.[41] Though HAND2 was found in the forelimb region in Thewissen’s study of Stenella attenuata (pantropical spotted dolphin), it was absent in the hindlimb region. Thus the hindlimb would fail to express Shh and establish a functioning ZPA, which would result in the degeneration of the AER.[40][41] These findings are consistent with the experimental observations of the regression of the hindlimb bud during development, and suggest that the reduction of Shh expression was a gradual evolutionary process.[41] Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs (atavism).[42]

Pakicetus had a pelvic bone most similar to that of terrestrial mammals. In later species, such as Basilosaurids, the pelvic bone that was no longer attached to the vertebrae and the ilium was reduced.[38] Certain genes are believed to be responsible for the changes that occurred to the cetacean pelvic structure. Possible gene candidates for modifications made to the cetacean pelvic girdle include BMP7, PBX1, PBX2, Prrx1, and Prrx2.[43] The pelvic girdle in modern cetaceans were once thought to be vestigial structures that served no purpose at all. The pelvic girdle in male cetaceans is different in size compared to females, and the size is thought to be a result of sexual dimorphism. The pelvic bones of modern male cetaceans are more massive, longer, and larger than those of females. It is thought that, due to the sexual dimorphism displayed, they are involved in supporting male genitalia that remain hidden behind abdominal walls until sexual reproduction occurs.[43][44] [45]

Early archaeocetes such as Pakicetus had the nasal openings at the end of the snout, but in later species such as Rodhocetus, the openings had begun to drift toward the top of the skull. This is known as nasal drift.[46] The nostrils of modern whales have become modified into blowholes that allow them to break to the surface, inhale, and submerge with convenience. The ears began to move inward as well, and, in the case of Basilosaurus, the middle ears began to receive vibrations from the lower jaw. Today's modern toothed whales use the 'melon organ', a pad of fat, for echolocation.[47]

Modern evolution of cetaceans

Culture

Researchers pushed a pole with a conical sponge attached along the substrate to simulate sponging behavior by dolphins. They videotaped this experiment to learn what prey was available on the seafloor and why a sponge would be beneficial to foraging rather than echolocation.[48]

Culture is group-specific behavior transferred by social learning. One of the best examples for dolphins where culture affects evolution involves tool use and foraging. Generally speaking, whether or not a dolphin uses a tool affects their eating behavior, which therefore causes differences in diet. Also, using a tool allows a new niche and new prey to open up for that particular dolphin. Due to these differences, fitness levels change within the dolphins of a population, which further causes evolution to occur in the long run.[49] Culture and social networks have played a large role in the evolution of modern cetaceans, as shown in studies showing dolphins preferring mates with the same socially learned behaviors and humpback whales using songs between breeding areas. For dolphins particularly, the largest non-genetic effects on their evolution seem to be due to culture, social structure, and environmental factors.[50]

Based on a study done in 2014, the population of Indo-Pacific bottlenose dolphins (Tursiops sp.) around Shark Bay of Western Australia can be divided into spongers and nonspongers. Spongers put sponges on their noses as a protective means against abrasions from sharp objects, stingray barbs, or toxic organisms. The sponges also help the dolphins target fish without swim bladders, since echolocation cannot detect these fish easily against a complex background. Spongers also specifically forage in deep channels, but nonspongers are found foraging in both deep and shallow channels.[51] It has also been discovered that this foraging behavior seems to be passed on from mother to daughter/son most of the time. Therefore, since this is a group behavior being passed down by social learning, this tool use is considered a culture.[49]

In an experiment done in 2014 with these Shark Bay populations, researchers compared data from the East and West Gulf of Shark Bay, deep and shallow channels, and sponger vs. nonsponger dolphins. In order to test differences of diet, they took superficial blubber samples and performed a fatty acid analysis. Overall, the researchers found the fatty acid analyses to differ between West and East Gulf, which is simply due to the two areas having different food sources. However, when comparing data from within the West Gulf, the spongers vs. the nonspongers in the deep channels had very different fatty acid analyses even though the habitat is the same. Meanwhile, nonspongers from deep and shallow channels had similar data. This suggests that sponging was the cause of the different data and not the deep vs. shallow channels. Sponging opened up a new niche for the dolphins and allowed them access to new prey, thereby causing long-term diet changes. By producing different food sources within a population, there is less intrapopulation competition for resources, showing ecological character displacement. As a result, carrying capacity actually increases since the entire population does not depend on one food source. Meanwhile, the fitness levels within the population also change, thus allowing evolution to act on this culture.[51]

Social structure

Social structure forms groups that interact with one another, and this allows for cultural traits to emerge, flow, and evolve. This relationship is especially seen in the bottlenose dolphin populations in southwestern Australia, who have been known to beg for food from fishermen. Research shows that this begging behavior spread through the population due to two types of learning: individual (dolphins spending time around boats) and social (dolphins spending time with other dolphins who express begging behavior).[49]

Culture can, however, impact social structure by causing behavior matching and assortative mating. Individuals with a certain culture are more likely to associate/mate with individuals using the same behaviors rather than a random individual, thus influencing social groups and structure. For example, the sponger dolphins of Shark Bay preferentially stick with other spongers.[49] As another example, some bottlenose dolphins in Moreton Bay, Australia followed prawn trawlers to feed on their debris, while other dolphins in the same population did not. The dolphins preferentially associated with individuals with same behavior even though they all lived in the same habitat. Later on, prawn trawlers were no longer present, and the dolphins integrated into one social network after a couple of years.[49]

Social networks can still affect and cause evolution on their own by impending fitness differences on individuals.[52] In a longitudinal study performed on bottlenose dolphins, the early social bonds formed by male calves and their resulting survival rates were analyzed. According to the data, male calves had a lower survival rate if they had stronger bonds with juvenile males. However, when other age and sex classes were tested, their survival rate did not significantly change.[53] This suggests that juvenile males impose a social stress on their younger counterparts. In fact, it has been documented that juvenile males commonly perform acts of aggression, dominance, and intimidation against the male calves.[53]

In another experiment of bottlenose dolphins in the East Gulf of Shark Bay, fitness of female dolphins was measured by the success of their own calves. The research revealed differences in fitness within the population, and two possible explanations due to social bonds were suggested: the differences are due to social learning (whether or not the mother passed on her knowledge of reproductive ability to the calves), or due to the strong association between mother dolphins in the population; by sticking in a group, an individual mother does not need to be as vigilant all the time for predators.[52]

Genetic studies conducted on Clymene dolphins (Stenella clymene) focused on their natural histories, and it was revealed was that the origin of the species was actually an outcome of natural mixing between two different species.[54][55] Hybridization between spinner dolphins (Stenella longirostris) and striped dolphins (Stenella coeruleoalba) in the North Atlantic was caused by constant habitat sharing of the two species. Relationships between these three species had been wondered according to notable resemblances between anatomies of the Clymene and the spinner dolphins, resulting in regarding the former as subspecies of the latter until 1981,[56] and the possibility of the Clymene dolphin as a hybrid between the spinner and the striped dolphins have come to question based on anatomical and behavioral similarities between these two species.[57]

Environmental factors

Genome sequences done in 2013 revealed that the Yangtze River dolphin, or "baiji" (Lipotes vexillifer), lacks single nucleotide polymorphisms in the baiji genome. After reconstructing the history of the baiji genome for this dolphin species, researchers found that the major decrease in genetic diversity occurred most likely due to a bottleneck event during the last deglaciation event. During this time period, sea levels were rising while global temperatures were decreasing. Other historical climate events can be correlated and matched with the genome history of the Yangtze River dolphin as well. This shows how global and local climate change can drastically affect a genome, leading to changes in fitness, survival, and evolution of a species.[58]

The European population of common dolphins (Delphinus delphis) in the Mediterranean have differentiated into two types: eastern and western. According to a study in 2012, this seems to be due to a recent bottleneck as well, which drastically decreased the size of the eastern Mediterranean population. Also, the lack of population structure between the western and eastern regions seems contradictory of the distinct population structures between other regions of dolphins, proving again of their differentiation.[59] Even though the dolphins in the Mediterranean area had no physical barrier between their regions, they still differentiated into two types due to ecology and biology. Therefore, the differences between the eastern and western dolphins most likely stems from highly specialized niche choice rather than just physical barriers. Through this, environment plays a large role in the differentiation and evolution of this dolphin species.[60]

The divergence and speciation within bottlenose dolphins has been largely due to climate and environmental changes over history. According to research, the divisions within the genus correlate with periods of rapid climate change. For example, the changing temperatures could cause the coast landscape to change, niches to empty up, and opportunities for separation to appear.[61] In the Northeast Atlantic, specifically, genetic evidence suggests that the bottlenose dolphins have differentiated into coastal and pelagic types. Divergence seems most likely due to a founding event where a large group separated. Following this event, the separate groups adapted accordingly and formed their own niche specializations and social structures. These differences caused the two groups to diverge and to remain separated.[62]

Two endemic, distinctive types of pilot whale, Tappanaga (or Shiogondou) the larger, northern type and Magondou the smaller, southern type, can be found along the Japanese archipelago where distributions of these two types mostly do not overlap by the oceanic front border around the easternmost point of Honshu. Some indicate that the local extinction of long-finned pilot whales in the North Pacific in the 12th century could have triggered the appearance of Tappanaga, causing short-finned pilot whales to colonize the colder ranges of the long-finned variant.[63]

See also

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For a review of whale evolution, see Uhen, M. D. (2010). "The Origin(s) of Whales". Annual Review of Earth and Planetary Sciences. 38: 189–219. Bibcode:2010AREPS..38..189U. doi:10.1146/annurev-earth-040809-152453.

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