Evolution of whales, dolphins, and porpoises

Pakicetus attocki skeleton

The evolution of whales, dolphins, and porpoises (cetaceans) is thought to have begun in the Indian subcontinent, from even-toed ungulates 50 million years ago, over at least 15 million years.

Cetaceans are thought to have evolved during the Eocene ( 56 to 33.9 million years ago) or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, the surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurture their young; and, despite their fully aquatic lifestyle, they retain many skeletal features from their earthly ancestors.

Evolution of modern cetaceans ( whales, dolphins, and porpoises)

Baleen whales

All recent baleen (Baleen is a filter-feeding system inside the mouths) whales or mysticetes are filter-feeders which have baleen in place of teeth, though the exact means by which baleen is used differs among species (gulp-feeding within balaenopterids, skim-feeding within balaenids, and bottom plowing within eschrichtiids). The first members of both groups appeared during the middle Miocene. Filter feeding is very beneficial as it allows baleen whales to efficiently gain huge energy resources, which makes the large body size in recent variations possible. The development of filter-feeding 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 contributed to the radiation of modern mysticetes. The earlier varieties of baleen whales, or “archaeomysticetes”, such as Janjucetus and Mammalodon had very little baleen and relied principally on their teeth.

There is also evidence of a genetic component of the evolution of toothless whales. Multiple mutations have been identified in genes related to the creation of enamel in modern baleen whales. These are primarily insertion/deletion mutations that result in premature stop codons. It is hypothesized that these mutations occurred in cetaceans already possessing preliminary baleen structures, leading to the pseudogenization of a “genetic toolkit” for enamel production. Recent research has also indicated that the development of baleen and the loss of enamel-capped teeth both occurred once, and both occurred on the mysticete stem branch.

Toothed whales

The adaptation of echolocation occurred when toothed whales (Odontoceti) split apart from baleen whales and divides modern toothed whales from completely aquatic archaeocetes. This happened around 34 million years ago in second cetacean radiation. 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. Toothed whales echolocate by producing a series of clicks released at various frequencies. Sound pulses are emitted, bounced off objects, and retrieved through the lower jaw. Skulls of Squalodon show evidence for the first hypothesized emergence of echolocation. Squalodon lived from the early to middle Oligocene to the middle Miocene, around 33–14 million years ago. Squalodon highlighted diverse commonalities with modern toothed whales: the cranium was well compressed (to make room for the melon, a part of the nose), the rostrum telescoped outward into a beak, a characteristic of the modern toothed whales that gave Squalodon an appearance similar to them. However, it is thought unlikely that squalodontids are direct ancestors of modern toothed whales.

Dolphins

The first oceanic dolphins such as Kentriodonts, evolved in the late Oligocene and diversified greatly during the mid-Miocene. The first fossil cetaceans near shallow seas (where porpoises inhabit) were found around the North Pacific; species like Semirostrum were found along California (in what were then estuaries). These animals spread to the European coasts and Southern Hemisphere only much later, during the Pliocene. 

Cultural Evolution of cetaceans in the current era

Culture is a group-specific behavior carried by social learning. The tool used to aid with foraging is one example. Whether or not a dolphin uses a tool affects their eating habits, which 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 variations, fitness levels change within the dolphins of a population, which further causes evolution to occur in the long run. Culture and social networks have played a large role in the evolution of modern cetaceans, as assumed 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 are due to culture and social structure.

Based on a 2014 study, the population of Indo-Pacific bottlenose dolphins (Tursiops sp.) around Shark Bay of Western Australia can be divided into spongers and nonspongers. Spongers put sea sponges on their snout as a shielding 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. This foraging action is mainly passed on from mother to child. Therefore, since this is a group behavior being passed down by social learning, this tool use is considered a cultural trait.

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