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| Penguins | |
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Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Clade: | Austrodyptornithes |
| Order: | Sphenisciformes Sharpe, 1891 |
| Family: | Spheniscidae Bonaparte, 1831 |
| Modern genera | |
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Aptenodytes | |
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| Breeding range of penguins, all species (aqua); some species have wider seasonal migration ranges | |
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Penguins are a group of aquatic flightless birds from the family Spheniscidae (/sfɪˈnɪsɪdiː, -daɪ/) of the order Sphenisciformes (/sfɪˈnɪsəfɔːrmiːz/).[4] They live almost exclusively in the Southern Hemisphere: only one species, the Galápagos penguin, is found north of the Equator. Highly adapted for life in the ocean water, penguins have countershaded dark and white plumage and flippers for swimming. Most penguins feed on krill, fish, squid and other forms of sea life which they catch with their bills and swallow whole while swimming. A penguin has a spiny tongue and powerful jaws to grip slippery prey.[5]
They spend about half of their lives on land and the other half in the sea. The largest living species is the emperor penguin (Aptenodytes forsteri):[6] on average, adults are about 1.1 m (3 ft 7 in) tall and weigh 35 kg (77 lb). The smallest penguin species is the little blue penguin (Eudyptula minor), also known as the fairy penguin, which stands around 30–33 cm (12–13 in) tall and weighs 1.2–1.3 kg (2.6–2.9 lb).[7] Today, larger penguins generally inhabit colder regions, and smaller penguins inhabit regions with temperate or tropical climates. Some prehistoric penguin species were enormous: as tall or heavy as an adult human. There was a great diversity of species in subantarctic regions, and at least one giant species in a region around 2,000 km south of the equator 35 mya, during the Late Eocene, a climate decidedly warmer than today.[8]
Etymology[edit]
The word penguin first appears in literature at the end of the 16th century as a synonym for the great auk.[9] When European explorers discovered what are today known as penguins in the Southern Hemisphere, they noticed their similar appearance to the great auk of the Northern Hemisphere and named them after this bird, although they are not closely related.[10]
The etymology of the word penguin is still debated. The English word is not apparently of French,[11] Breton[12] or Spanish[13] origin (the latter two are attributed to the French word pingouin), but first appears in English or Dutch.[14]
Some dictionaries suggest a derivation from Welsh pen, 'head' and gwyn, 'white',[15] including the Oxford English Dictionary, the American Heritage Dictionary,[16] the Century Dictionary[16] and Merriam-Webster,[17] on the basis that the name was originally applied to the great auk, either because it was found on White Head Island (Welsh: Pen Gwyn) in Newfoundland, or because it had white circles around its eyes (though the head was black).
An alternative etymology links the word to Latin pinguis, which means 'fat' or 'oil'.[18] Support for this etymology can be found in the alternative Germanic word for penguin, fettgans or 'fat-goose', and the related Dutch word vetgans.
Adult male penguins are sometimes called cocks, females sometimes called hens; a group of penguins on land is a waddle, and a group of penguins in the water is a raft.
Pinguinus[edit]
Since 1871, the Latin word Pinguinus has been used in scientific classification to name the genus of the great auk (Pinguinus impennis, meaning "plump or fat without flight feathers"),[19] which became extinct in the mid-19th century.[9] As confirmed by a 2004 genetic study, the genus Pinguinus belongs in the family of the auks (Alcidae), within the order of the Charadriiformes.[20][21]
The birds currently known as penguins were discovered later and were so named by sailors because of their physical resemblance to the great auk. Despite this resemblance, however, they are not auks, and are not closely related to the great auk.[10][19] They do not belong in the genus Pinguinus, and are not classified in the same family and order as the great auk. They were classified in 1831 by Charles Lucien Bonaparte in several distinct genera within the family Spheniscidae and order Sphenisciformes.
Systematics and evolution[edit]
Taxonomy[edit]
The family name of Spheniscidae was given by Charles Lucien Bonaparte from the genus Spheniscus,[22] the name of that genus comes from the Greek word σφήν sphēn "wedge" used for the shape of an African penguin's swimming flippers.[23]
Some recent sources[3][24] apply the phylogenetic taxon Spheniscidae to what here is referred to as Spheniscinae. Furthermore, they restrict the phylogenetic taxon Sphenisciformes to flightless taxa, and establish the phylogenetic taxon Pansphenisciformes as equivalent to the Linnean taxon Sphenisciformes,[24] i.e., including any flying basal "proto-penguins" to be discovered eventually. Given that neither the relationships of the penguin subfamilies to each other nor the placement of the penguins in the avian phylogeny is presently resolved, this is confusing, so the established Linnean system is followed here.
Evolution[edit]
Although the evolutionary and biogeographic history of Sphenisciformes is well-researched, many prehistoric forms are not fully described. Some seminal articles about the evolutionary history of penguins have been published since 2005.[3][25][26][27]
The basal penguins lived around the time of the Cretaceous–Paleogene extinction event in the general area of southern New Zealand and Byrd Land, Antarctica.[3] Due to plate tectonics, these areas were at that time less than 1,500 kilometres (930 mi) apart rather than 4,000 kilometres (2,500 mi). The most recent common ancestor of penguins and Procellariiformes can be roughly dated to the Campanian–Maastrichtian boundary, around 70–68 mya.[25][27][28]
Basal fossils[edit]
The oldest known fossil penguin species is Waimanu manneringi, which lived 62 mya in New Zealand.[27] While they were not as well-adapted to aquatic life as modern penguins, Waimanu were flightless, with short wings adapted for deep diving.[27] They swam on the surface using mainly their feet, but the wings were – as opposed to most other diving birds (both living and extinct) – already adapting to underwater locomotion.[29]
Perudyptes from northern Peru was dated to 42 mya. An unnamed fossil from Argentina proves that, by the Bartonian (Middle Eocene), some 39–38 mya,[30] primitive penguins had spread to South America and were in the process of expanding into Atlantic waters.[24]
Palaeeudyptines[edit]
During the Late Eocene and the Early Oligocene (40–30 mya), some lineages of gigantic penguins existed. Nordenskjoeld's giant penguin was the tallest, growing nearly 1.80 meters (5.9 feet) tall. The New Zealand giant penguin was probably the heaviest, weighing 80 kilograms (180 lb) or more. Both were found on New Zealand, the former also in the Antarctic farther eastwards.
Traditionally, most extinct species of penguins, giant or small, had been placed in the paraphyletic subfamily called Palaeeudyptinae. More recently, with new taxa being discovered and placed in the phylogeny if possible, it is becoming accepted that there were at least two major extinct lineages. One or two closely related ones occurred in Patagonia, and at least one other—which is or includes the paleeudyptines as recognized today – occurred on most Antarctic and Subantarctic coasts.
Size plasticity was significant at this initial stage of radiation: on Seymour Island, Antarctica, for example, around 10 known species of penguins ranging in size from medium to large apparently coexisted some 35 mya during the Priabonian (Late Eocene).[31] It is not known whether the palaeeudyptines constitute a monophyletic lineage, or whether gigantism was evolved independently in a restricted Palaeeudyptinae and the Anthropornithinae – whether they were considered valid, or whether there was a wide size range present in the Palaeeudyptinae as delimited (i.e., including Anthropornis nordenskjoeldi).[3] The oldest well-described giant penguin, the 5-foot (1.5 m)-tall Icadyptes salasi, existed as far north as northern Peru about 36 mya.
Gigantic penguins had disappeared by the end of the Paleogene, around 25 mya. Their decline and disappearance coincided with the spread of the Squalodontidae and other primitive, fish-eating toothed whales, which competed with them for food and were ultimately more successful.[25] A new lineage, the Paraptenodytes, which includes smaller and stout-legged forms, had already arisen in southernmost South America by that time. The early Neogene saw the emergence of another morphotype in the same area, the similarly sized but more gracile Palaeospheniscinae, as well as the radiation that gave rise to the current biodiversity of penguins.
Origin and systematics of modern penguins[edit]
Modern penguins constitute two undisputed clades and another two more basal genera with more ambiguous relationships.[26] To help resolve the evolution of this order, 19 high-coverage genomes that, together with two previously published genomes, encompass all extant penguin species have been sequenced.[32] The origin of the Spheniscinae lies probably in the latest Paleogene and, geographically, it must have been much the same as the general area in which the order evolved: the oceans between the Australia-New Zealand region and the Antarctic.[25] Presumably diverging from other penguins around 40 mya,[25] it seems that the Spheniscinae were for quite some time limited to their ancestral area, as the well-researched deposits of the Antarctic Peninsula and Patagonia have not yielded Paleogene fossils of the subfamily. Also, the earliest spheniscine lineages are those with the most southern distribution.
The genus Aptenodytes appears to be the basalmost divergence among living penguins.[3][33] They have bright yellow-orange neck, breast, and bill patches; incubate by placing their eggs on their feet, and when they hatch the chicks are almost naked. This genus has a distribution centred on the Antarctic coasts and barely extends to some Subantarctic islands today.
Pygoscelis contains species with a fairly simple black-and-white head pattern; their distribution is intermediate, centred on Antarctic coasts but extending somewhat northwards from there. In external morphology, these apparently still resemble the common ancestor of the Spheniscinae, as Aptenodytes' autapomorphies are, in most cases, fairly pronounced adaptations related to that genus' extreme habitat conditions. As the former genus, Pygoscelis seems to have diverged during the Bartonian,[34] but the range expansion and radiation that led to the present-day diversity probably did not occur until much later; around the Burdigalian stage of the Early Miocene, roughly 20–15 mya.[25]
The genera Spheniscus and Eudyptula contain species with a mostly Subantarctic distribution centred on South America; some, however, range quite far northwards. They all lack carotenoid colouration and the former genus has a conspicuous banded head pattern; they are unique among living penguins by nesting in burrows. This group probably radiated eastwards with the Antarctic Circumpolar Current out of the ancestral range of modern penguins throughout the Chattian (Late Oligocene), starting approximately 28 mya.[25] While the two genera separated during this time, the present-day diversity is the result of a Pliocene radiation, taking place some 4–2 mya.[25]
The Megadyptes–Eudyptes clade occurs at similar latitudes (though not as far north as the Galápagos penguin), has its highest diversity in the New Zealand region, and represents a westward dispersal. They are characterized by hairy yellow ornamental head feathers; their bills are at least partly red. These two genera diverged apparently in the Middle Miocene (Langhian, roughly 15–14 mya), although the living species of Eudyptes are the product of a later radiation, stretching from about the late Tortonian (Late Miocene, 8 mya) to the end of the Pliocene.[25]
Geography[edit]
The geographical and temporal pattern of spheniscine evolution corresponds closely to two episodes of global cooling (disambiguation) documented in the paleoclimatic record.[25] The emergence of the Subantarctic lineage at the end of the Bartonian corresponds with the onset of the slow period of cooling that eventually led to the ice ages some 35 million years later. With habitat on the Antarctic coasts declining, by the Priabonian more hospitable conditions for most penguins existed in the Subantarctic regions rather than in Antarctica itself.[35] Notably, the cold Antarctic Circumpolar Current also started as a continuous circumpolar flow only around 30 mya, on the one hand forcing the Antarctic cooling, and on the other facilitating the eastward expansion of Spheniscus to South America and eventually beyond.[25] Despite this, there is no fossil evidence to support the idea of crown radiation from the Antarctic continent in the Paleogene, although DNA study favors such a radiation.[35]
Later, an interspersed period of slight warming was ended by the Middle Miocene Climate Transition, a sharp drop in global average temperature from 14 to 12 mya, and similar abrupt cooling events followed at 8 mya and 4 mya; by the end of the Tortonian, the Antarctic ice sheet was already much like today in volume and extent. The emergence of most of today's Subantarctic penguin species almost certainly was caused by this sequence of Neogene climate shifts.
Relationship to other bird orders[edit]
Penguin ancestry beyond Waimanu remains unknown and not well-resolved by molecular or morphological analyses. The latter tend to be confounded by the strong adaptive autapomorphies of the Sphenisciformes; a sometimes perceived fairly close relationship between penguins and grebes is almost certainly an error based on both groups' strong diving adaptations, which are homoplasies. On the other hand, different DNA sequence datasets do not agree in detail with each other either.
What seems clear is that penguins belong to a clade of Neoaves (living birds except for paleognaths and fowl) that comprises what is sometimes called "higher waterbirds" to distinguish them from the more ancient waterfowl. This group contains such birds as storks, rails, and the seabirds, with the possible exception of the Charadriiformes.[36]
Inside this group, penguin relationships are far less clear. Depending on the analysis and dataset, a close relationship to Ciconiiformes[27] or to Procellariiformes[25] has been suggested. Some think the penguin-like plotopterids (usually considered relatives of cormorants and anhingas) may actually be a sister group of the penguins and those penguins may have ultimately shared a common ancestor with the Pelecaniformes and consequently would have to be included in that order, or that the plotopterids were not as close to other pelecaniforms as generally assumed, which would necessitate splitting the traditional Pelecaniformes into three.[37]
A 2014 analysis of whole genomes of 48 representative bird species has concluded that penguins are the sister group of Procellariiformes,[38] from which they diverged about 60 million years ago (95% CI, 56.8-62.7).[39]
The distantly related Puffins, which live in the North Pacific and North Atlantic, developed similar characteristics to survive in the Arctic and sub-Arctic environments. Like the penguins, puffins have a white chest, black back and short stubby wings providing excellent swimming ability in icy water. But, unlike penguins, puffins can fly, as flightless birds would not survive alongside land-based predators such as polar bears and foxes; there are no such predators in the Antarctic. Their similarities indicate that similar environments, although at great distances, can result in similar evolutionary developments, i.e. convergent evolution.[40]
Anatomy and physiology[edit]
