Sexual dimorphism in the context of "Amazon river dolphin"

Pinnipeds (pronounced /ˈpɪnɪˌpɛdz/), commonly known as seals, are a widely distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammals. They comprise the extant families Odobenidae (whose only living member is the walrus), Otariidae (the eared seals: sea lions and fur seals), and Phocidae (the earless seals, or true seals), with 34 extant species and more than 50 extinct species described from fossils. While seals were historically thought to have descended from two ancestral lines, molecular evidence supports them as a monophyletic group (descended from one ancestor). Pinnipeds belong to the suborder Caniformia of the order Carnivora; their closest living relatives are musteloids (weasels, raccoons, skunks and red pandas), having diverged about 50 million years ago.

Seals range in size from the 1 m (3 ft 3 in) and 45 kg (100 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal. Several species exhibit sexual dimorphism. They have streamlined bodies and four limbs that are modified into flippers. Though not as fast in the water as dolphins, seals are more flexible and agile. Otariids primarily use their front limbs to propel themselves through the water, while phocids and walruses primarily use their hind limbs for this purpose. Otariids and walruses have hind limbs that can be pulled under the body and used as legs on land. By comparison, terrestrial locomotion by phocids is more cumbersome. Otariids have visible external ears, while phocids and walruses lack these. Pinnipeds have well-developed senses—their eyesight and hearing are adapted for both air and water, and they have an advanced tactile system in their whiskers or vibrissae. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in cold water, and, other than the walrus, all species are covered in fur.

Human variability, or human variation, is the range of possible values for any characteristic, physical or mental, of human beings.

Frequently debated areas of variability include cognitive ability, personality, physical appearance (body shape, skin color, etc.) and immunology.Variability is partly heritable and partly acquired (nature vs. nurture debate).As the human species exhibits sexual dimorphism, many traits show significant variation not just between populations but also between the sexes.

The Dmanisi hominins, Dmanisi people, or Dmanisi man were a population of Early Pleistocene hominins whose fossils have been recovered at Dmanisi, Georgia. The fossils and stone tools recovered at Dmanisi range in age from 1.85 to 1.77 million years old, making the Dmanisi hominins the earliest well-dated hominin fossils in Eurasia and the best preserved fossils of early Homo from a single site so early in time. Though their precise classification is controversial and disputed, the Dmanisi fossils are highly significant within research on early hominin migrations out of Africa. The Dmanisi hominins are known from over a hundred postcranial fossils and five famous well-preserved skulls, referred to as Dmanisi Skulls 1–5.

The taxonomic status of the Dmanisi hominins is somewhat unclear due to their small brain size, primitive skeletal architecture, and the range of variation exhibited between the skulls. Their initial description classified them as Homo (erectus?) ergaster (an otherwise African taxon), or potentially an early offshoot of later Asian H. erectus. The discovery of a massive jaw, D2600, in 2000 led researchers to hypothesize that more than one hominin taxon had been present at the site and in 2002, the jaw was designated as the type specimen of the new species Homo georgicus. Later analyses by the Dmanisi research team have concluded that all the skulls likely represent the same taxon with significant age-related and sexual dimorphism, though this is not a universally held view. In 2006, the team favoured subsuming the taxon under Homo erectus as H. erectus georgicus or H. e. ergaster georgicus. The nomenclature is still debated.

Baboons are primates comprising the genus Papio, one of the 23 genera of Old World monkeys, in the family Cercopithecidae. There are six species of baboon: the hamadryas baboon, the Guinea baboon, the olive baboon, the yellow baboon, the Kinda baboon and the chacma baboon. Each species is native to one of six areas of Africa and the hamadryas baboon is also native to part of the Arabian Peninsula. Baboons are among the largest non-hominoid primates and have existed for at least two million years.

Baboons vary in size and weight depending on the species. The smallest, the Kinda baboon, is 50 cm (20 in) in length and weighs only 14 kg (31 lb), while the largest, the chacma baboon, is up to 120 cm (47 in) in length and weighs 40 kg (88 lb). All baboons have long, dog-like muzzles, heavy, powerful jaws with sharp canine teeth, close-set eyes, thick fur except on their muzzles, short tails, and nerveless, hairless pads of skin on their protruding buttocks called ischial callosities that provide for sitting comfort. Male hamadryas baboons have large white manes. Baboons exhibit sexual dimorphism in size, colour and/or canine teeth development.

Pterosaurs are an extinct clade of flying reptiles in the order Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 million to 66 million years ago). Pterosaurs are the earliest vertebrates known to have evolved powered flight. Their wings were formed by a membrane of skin, muscle, and other tissues stretching from the ankles to a dramatically lengthened fourth finger.

Traditionally, pterosaurs were divided into two major types. Basal pterosaurs (also called non-pterodactyloid pterosaurs or 'rhamphorhynchoids') were smaller animals, up to two meter wingspan, with fully toothed jaws and, typically, long tails. Their wide wing membranes probably included and connected the hindlimbs. On the ground, they would have had an awkward sprawling posture due to short metacarpals, but the anatomy of their joints and strong claws would have made them effective climbers, and some may have lived in trees. Basal pterosaurs were insectivores, piscivores or predators of small land vertebrates. Later pterosaurs (pterodactyloids) evolved many sizes, shapes, and lifestyles. Pterodactyloids had narrower wings with free hindlimbs, highly reduced tails, and long necks with large heads. On the ground, they walked well on all four limbs due to long metacarpals with an upright posture, standing plantigrade on the hind feet and folding the wing finger upward to walk on the metacarpals with the three smaller fingers of the hand pointing to the rear. They could take off from the ground, and fossil trackways show that at least some species were able to run, wade, and/or swim. Their jaws had horny beaks, and some groups lacked teeth. Some groups developed elaborate head crests with sexual dimorphism. Since 2010 it is understood that many species, the basal Monofenestrata, were intermediate in build, combining an advanced long skull with long tails.

Sexual selection is a mechanism of evolution in which members of one sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual selection). These two forms of selection mean that some individuals have greater reproductive success than others within a population, for example because they are more attractive or prefer more attractive partners to produce offspring. Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males.

The concept was first articulated by Charles Darwin who wrote of a "second agency" other than natural selection, in which competition between mate candidates could lead to speciation. The theory was given a mathematical basis by Ronald Fisher in the early 20th century. Sexual selection can lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristics, such as the ornate plumage of birds-of-paradise and peafowl, or the antlers of deer. Depending on the species, these rules can be reversed. This is caused by a positive feedback mechanism known as a Fisherian runaway, where the passing-on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect. Although the sexy son hypothesis indicates that females would prefer male offspring, Fisher's principle explains why the sex ratio is most often 1:1. Sexual selection is widely distributed in the animal kingdom, and is also found in plants and fungi.

Whales are a widely distributed and diverse group of fully aquatic placental marine mammals. As an informal and colloquial grouping, they correspond to large members of the infraorder Cetacea, i.e. all cetaceans apart from dolphins and porpoises. Dolphins and porpoises may be considered whales from a formal, cladistic perspective. Whales, dolphins and porpoises belong to the order Cetartiodactyla, which consists of even-toed ungulates. Their closest non-cetacean living relatives are the hippopotamuses, from which they and other cetaceans diverged about 54 million years ago. The two parvorders of whales, baleen whales (Mysticeti) and toothed whales (Odontoceti), are thought to have had their last common ancestor around 34 million years ago. Mysticetes include four extant (living) families: Balaenopteridae (the rorquals), Balaenidae (right whales), Cetotheriidae (the pygmy right whale), and Eschrichtiidae (the grey whale). Odontocetes include the Monodontidae (belugas and narwhals), Physeteridae (the sperm whale), Kogiidae (the dwarf and pygmy sperm whale), and Ziphiidae (the beaked whales), as well as the six families of dolphins and porpoises which are not considered whales in the informal sense.

Whales are fully aquatic, open-ocean animals: they can feed, mate, give birth, suckle and raise their young at sea. Whales range in size from the 2.6 metres (8.5 ft) and 135 kilograms (298 lb) dwarf sperm whale to the 29.9 metres (98 ft) and 190 tonnes (210 short tons) blue whale, which is the largest known animal that has ever lived. The sperm whale is the largest toothed predator on Earth. Several whale species exhibit sexual dimorphism, in that the females are larger than males.

A dolphin is a common name used for some of the aquatic mammals in the cetacean clade Odontoceti, the toothed whales. Dolphins belong to the families Delphinidae (the oceanic dolphins), along with the river dolphin families Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), Pontoporiidae (the brackish dolphins), and probably extinct Lipotidae (baiji or Chinese river dolphin). There are 40 extant species named as dolphins.

Dolphins range in size from the 1.7-metre-long (5 ft 7 in) and 50-kilogram (110-pound) Maui's dolphin to the 9.5 m (31 ft) and 10-tonne (11-short-ton) orca. Various species of dolphins exhibit sexual dimorphism where the males are larger than females. They have streamlined bodies and two limbs that are modified into flippers. Though not quite as flexible as seals, they are faster; some dolphins can briefly travel at speeds of 29 kilometres per hour (18 mph) or leap about 9 metres (30 ft). Dolphins use their conical teeth to capture fast-moving prey. They have well-developed hearing which is adapted for both air and water; it is so well developed that some can survive even if they are blind. Some species are well adapted for diving to great depths. They have a layer of fat, or blubber, under the skin to keep warm in the cold water.

Porpoises (/ˈpɔːrpəsɪz/) are small dolphin-like cetaceans classified under the family Phocoenidae. Although similar in appearance to dolphins, they are more closely related to narwhals and belugas. There are eight extant species of porpoise, all among the smallest of the toothed whales. Porpoises are distinguished from dolphins by their flattened, spade-shaped teeth distinct from the conical teeth of dolphins, and lack of a pronounced beak, although some dolphins (e.g. Hector's dolphin) also lack a pronounced beak. Porpoises, and other cetaceans, belong to the clade Cetartiodactyla with even-toed ungulates.

Porpoises range in size from the vaquita, at 1.4 metres (4 feet 7 inches) in length and 54 kilograms (119 pounds) in weight, to the Dall's porpoise, at 2.3 m (7 ft 7 in) and 220 kg (490 lb). Several species exhibit sexual dimorphism in that the females are larger than males. They have streamlined bodies and two limbs that are modified into flippers. Porpoises use echolocation as their primary sensory system. Some species are well adapted for diving to great depths. As all cetaceans, they have a layer of fat, or blubber, under the skin to keep them warm in cold water.