Whippomorpha

Whippomorpha or Cetancodonta is a suborder of artiodactyls that contains all living cetaceans (whales, dolphins, and porpoises) and the hippopotamids. This makes it a crown group. Whippomorpha is a suborder within the order Artiodactyla (even-toed ungulates). The placement of Whippomorpha within Artiodactyla is a matter of some contention, as hippopotamuses were previously considered to be more closely related to Suidae (pigs) and Tayassuidae (peccaries). Most contemporary scientific phylogenetic and morphological research studies link hippopotamuses with cetaceans, and genetic evidence has overwhelmingly supported an evolutionary relationship between Hippopotamidae and Cetacea. Modern whippomorphs all share a number of behavioural and physiological traits, such as a dense layer of subcutaneous fat and largely hairless bodies. They exhibit amphibious and aquatic behaviors and possess similar auditory structures.

Whippomorpha is a subgroup of Cetancodontamorpha, which also includes the extinct entelodonts and ''Andrewsarchus''.

Etymology

The Latin name Whippomorpha is derived from ''whippo'', a colloquial term coined in reference to the hypothesis that whales and hippopotamuses are each other's closest relatives, and the suffix ''-morpha'' (from Greek (, "form")). Attempts have been made to rename the suborder Cetancodonta, due to the misleading utilization of the suffix ''-morpha'' for a crown group as well as the risk of confusion with the clade Hippomorpha (which consists of equid perissodactyls). However, Whippomorpha maintains precedence.

Ecology

Distribution

Modern whippomorphs are widely distributed. Cetaceans can be found in almost all of the world's marine habitats, and some species, like the blue whale and humpback whale, have migratory ranges that comprise nearly the entire ocean. These whales typically migrate on a seasonal basis, moving to warmer waters to give birth and raise young before travelling to cooler waters with more optimal feeding grounds. Other cetacean species have smaller ranges that are concentrated around either tropical or subtropical waters. Some cetaceans live exclusively within a single marine body, such as the narwhal, whose range is limited to the Arctic Ocean.

By comparison, modern hippopotamuses are confined entirely to the African continent. Despite once being widespread across Europe, Africa and Asia, hippos are now considered vulnerable and are limited to the lakes, rivers and wetlands of southern Africa.

Behaviour

Both whales and hippos must surface to breathe. This can pose problems for sleeping whippomorphs. Cetaceans overcome this problem by unihemispheric sleep, meaning they rest one side of their brain at a time, allowing them to swim and surface during rest periods. Hippopotamuses surface to breathe every three to five minutes, a process that is partially subconscious, allowing them to do it whilst sleeping. Both whales and hippos exhibit symbiotic relationships with smaller fish, which they use as cleaning stations, allowing the smaller organisms to feed on parasites that enter the creature's mouth.

Hippos are herbivores; normally their diet consists entirely of short grasses that they graze on. Some hippos have been observed consuming animals such as zebra and even other hippo carcasses. A hippo normally spends up to five hours a day grazing. They normally feed only on land, though occasional consumption of aquatic vegetation has been observed. By contrast, cetaceans are all carnivores, feeding on fish and marine invertebrates, with some species feeding on larger mammals and birds (such as seals (pinnipeds) and penguins).

Reproduction

All whippomorphs are placental mammals, meaning that embryos are fed by the placenta, which draws nutrients from the mother's body. They are k-selected organisms, producing a limited number of offspring, but with a high rate of survival.

Hippos reach sexual maturity at six years of age and have a gestation period of approximately eight months. Mating typically occurs in the water. Female hippopotamuses isolate themselves for two weeks prior to giving birth. The birthing process also takes place underwater, meaning calves must swim to the surface in order to breathe for the first time. Hippopotamus calves suckle on land.

Cetaceans generally reach sexual maturity around 10 years of age, and have a gestation period of around 12 months. Cetaceans give birth to well-developed calves, like hippopotamuses. When suckling, the mother splashes milk into the calves' mouth, as they have no lips.

Taxonomy and phylogeny

Whippomorpha is a suborder located within the Order Artiodactyla, and the clade Cetancodontamorpha. It contains the clades Hippopotamoidea (ancestors of hippopotamuses) and Cetaceamorpha (ancestors of whales and dolphins). Whippomorpha is considered a sister clade to Ruminantia (which contains cattle, sheep and deer), as well as the extinct Raoellidae. Hippopotamoidea was formerly included to Suiformes with Suidae (pigs) and Tayassuidae (peccaries).

Most of the evidence supporting the Whippomorpha clade is based on molecular or genetic analysis. Early support for the existence of a Cetacea/Hippopotamidae clade originated from analysis of the molecular composition of a blood-clotting protein γ-fibrinogen taken from whales and hippopotamuses. Later studies obtained findings that indicated almost 11,000 orthologous genes between cetaceans and hippopotamuses, in addition to numerous positive indicators of a shared evolutionary history between cetaceans and hippopotamuses. Furthermore, some genetic sequences have been found in both whales and hippopotamuses that are not present in the genomes of other mammals. This would indicate that these groups share ancestry.

Whippomorpha's placement within Artiodactyla can be represented in the following cladogram:(see e.g. Fig S10)

Evolution

It is unknown whether the last common ancestor of whales and hippos led an aquatic, semiaquatic/amphibious, or terrestrial lifestyle. Therefore, it is a matter of contention whether the aquatic traits of both hippopotamuses and cetaceans are linked or the product of convergent evolution. Recent findings seem to indicate that the latter is more likely.

Whippomorpha diverged from other cetartiodactyls approximately 59 Mya, whilst whales diverged from hippos approximately 55 Mya. The first branch contained ancestors of Cetacea; semi-aquatic protowhales such as ''Pakicetus'' in the group Archaeoceti, which developed into the exclusively aquatic ancestors of modern cetaceans.

One evolutionarily significant whale ancestor was the raoellid ''Indohyus'', which was a Himalayas-dwelling, digitgrade omnivore roughly the size of a raccoon. It was not an adept swimmer, although it was thought to have spent considerable periods of time wading in shallow water. This would have been assisted by its heavy bones, providing stability. ''Indohyus'' was likely to have a diet at least partially based on aquatic foraging. Evidence for this includes the fact that the tooth enamel of ''Indohyus'' was considerably less worn than would be expected for an animal with an exclusively terrestrial diet. One of the most crucial facets of the discovery of ''Indohyus'' was the presence of a thickened auditory bulla, otherwise known as an involucrum. This discovery was significant as the involucrum was a morphology thought previously to be exclusive to cetaceans, a synapomorphy. This feature irrefutably linked cetaceans to raoellids.

It is thought that early whales such as '' Nalacetus'' and ''Pakicetus'' were restricted to freshwater environments, as modern hippopotamuses are. The later '' Ambulocetus,'' was likely to have lived a much more aquatic lifestyle, with shorter legs and paddle-like hands and feet. It also likely represented a transitional organism from freshwater to seawater, as the isotopic analysis of the bones and teeth of ''Ambulocetus'' indicate that it inhabited estuaries.

The second branch of Whippomorpha is thought to have developed into the family Anthracotheriidae, who were the putative ancestors of modern hippopotamuses. The sediments in which anthracotheriid fossils have been fossilized indicate that they were at least partially amphibious, whilst the jaw structure of fossils of select species, particularly '' Anthracotherium'', seem to indicate that it was an ancestral form of modern hippopotamuses.

These findings somewhat explain the once confusing paleontological age gap that existed as a major piece of evidence against an evolutionary link between Hippopotamidae and Cetacea. Previously, the oldest known cetacean fossils were approximately 50 Myr, while the earliest known hippopotamus fossils were around 15 Myr. The sum of the fossil knowledge indicates that whales and hippopotamuses developed amphibious and aquatic traits independently from one another, but that the features developed by their shared ancestors created pathways to the development of said adaptations. Thus the large difference in time between the discovery of cetacean and hippopotamid fossils is explained by the fact that hippos simply developed their semi-aquatic adaptations at a much later time than their cetacean cousins.

Anatomy

All members of the suborder Whippomorpha share some anatomical similarities. Hippopotamus stomachs are multi-chambered as with all ruminants; however, they do not regurgitate food. Instead, the hippopotamus stomach contains two preliminary chambers, which acts similarly to a compost bin, allowing foodstuffs to ferment before entering the animal's main stomach. All whale species possess similar stomach structures. Additionally, both animals bear single-lobed lungs (similar to other aquatic mammals), which allow to be filled with air more rapidly. This is a critical adaptation for both amphibious and aquatic organisms, as it reduces the frequency of dangerous trips to the water surface, where such organisms are more vulnerable to predation.

Hippos' bodies contain a layer of dense fat, reminiscent of a whales' blubber, and situated between skin and muscle. Hippos and whales both possess thick bones, which aid in rapid descent into water, have minimal hair (to aid in hydrodynamics) and a lack of sweat glands. Webbing is also present between the toes of hippopotamuses; a more land-suitable version of a whale's flippers. Hippos possess unique hind-limb musculature that provides them with powerful propulsion capabilities, rather than fine-tuned control. These features are characteristic of other ungulates.

There is strong resemblance between the dentition of primitive cetaceans and primitive ungulates, which seemingly cements the position of Cetacea within Artiodactyla. In addition, both cetaceans and artiodactyls possess two distinct components in their ears, the involucrum and sigmoid process. Similar features are considered responsible for the ability of cetaceans to hear underwater. The skeletons of prehistoric whales also contain uniquely shaped ankle bones, including a double-pulley system found only in even-toed ungulates and crucially not present in odd-toed ungulates.

Both hippos and whales have an unusually large and strangely shaped larynx, which enables the booming calls of whales underwater and the unique noises produced by hippos to communicate while submerged.

Relationship with humans

Whippomorphs have always had complex cultural and social relationships with humans. Hippopotamuses have a reputation for extreme aggression towards humans. Hippos are incredibly territorial and protective of their young, and are the deadliest mammal in Africa, killing between two and three thousand people each year. Despite this, hippos remain popular zoo animals and a recognizable species in popular culture. Hippos were hunted by ancient humans for food and sport. In Ancient Egypt, hippos were recognized as dangerous inhabitants of the river Nile, and a red hippo was the symbol of the god Set. The biblical Behemoth is thought to be based on or inspired by the hippo.

Hippopotamuses face a number of threats from humans. Common hippopotamuses are classed as vulnerable, and are subject to habitat destruction as a result of agriculture, water management, climate change and development of housing and urban areas. Pygmy hippopotamuses are considered endangered, with less than three thousand individuals in the wild. The few surviving pygmy hippopotamuses occupy a much smaller habitat area in Liberia, Sierra Leone, and the Ivory Coast. They face threats from mining and quarrying, hunting, poaching, and logging

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