Megafauna

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This article is about large land animals. For naked-eye-visible bottom-dwelling animals, see Macrobenthos.

The African bush elephant, Earth's largest living land animal

In terrestrial zoology, megafauna (Ancient Greek megas "large" + New Latin fauna "animal") are "giant", "very large" or "large" animals. The most common thresholds used are 44 kilograms (100 lb)^[1]^[2] or 100 kilograms (220 lb).^[2]^[3] This thus includes many species not popularly thought of as overly large, such as white-tailed deer and red kangaroo, and for the lower figure, even humans.

In practice the most common usage encountered in academic and popular writing describes land animals roughly larger than a human which are not (solely) domesticated. The term is especially associated with the Pleistocene megafauna — the giant and very large land animals considered archetypical of the last ice age such as mammoths.^[4] It is also commonly used for the largest extant wild land animals, especially elephants, giraffes, hippopotamuses, rhinoceroses, elk, condors, etc. Megafauna may be subcategorized by their trophic position into megaherbivores (e.g. elk), megacarnivores (e.g. lions), and more rarely, megaomnivores (e.g. bears).

Other common uses are for giant aquatic species, especially whales, any larger wild or domesticated land animals such as larger antelope and cattle, and dinosaurs and other extinct giant reptilians.

The term is also sometimes applied to animals (usually extinct) of great size relative to a more common or surviving type of the animal, for example the 1 m (3 ft) dragonflies of the Carboniferous period.

[edit] Ecological strategy

Megafauna — in the sense of the largest mammals and birds — are generally K-strategists, with great longevity, slow population growth rates, low death rates, and few or no natural predators capable of killing adults. These characteristics, although not exclusive to such megafauna, make them highly vulnerable to human overexploitation.

[edit] Evolution of large body size

One observation that has been made about the evolution of larger body size is that rapid rates of increase that are often seen over relatively short time intervals are not sustainable over much longer time periods. In an examination of mammal body mass changes over time, the maximum increase possible in a given time interval was found to scale with the interval length raised to the 0.25 power.^[5] This is thought to reflect the emergence, during a trend of increasing maximum body size, of a series of anatomical, physiological, environmental, genetic and other constraints that must be overcome by evolutionary innovations before further size increases are possible. A strikingly faster rate of change was found for large decreases in body mass, such as may be associated with the phenomenon of insular dwarfism. When normalized to generation length, the maximum rate of large body mass decreases was found to be over 30 times greater than the maximum rate for large body mass increases.^[5]

[edit] In terrestrial mammals

Subsequent to the K-T extinction event that eliminated the dinosaurs about 65.5 Ma ago, terrestrial mammals underwent a nearly exponential increase in body size as they diversified to occupy the ecological niches left vacant.^[6] Starting from just a few kg before the event, maximum size had reached ~50 kg a few million years later, and ~750 kg by the end of the Paleocene. This trend of increasing body mass appears to level off about 40 Ma ago (in the late Eocene), suggesting that physiological or ecological constraints had been reached, after an increase in body mass of over three orders of magnitude.^[6] However, when considered from the standpoint of rate of size increase per generation, the exponential increase is found to have continued until the appearance of Indricotherium 30 Ma ago. (Since generation time scales with body mass^0.259, increasing generation times with increasing size cause the log mass vs. time plot to curve downward from a linear fit.)^[5]

Megaherbivores eventually attained a body mass of over 10 000 kg. The largest of these, indricotheres and proboscids, have been hindgut fermenters, which are believed to have an advantage over foregut fermenters in terms of being able to accelerate gastrointestinal transit in order to accommodate very large food intakes.^[7] A similar trend emerges when rates of increase of maximum body mass per generation for different mammalian clades are compared (using rates averaged over macroevolutionary time scales). Among terrestrial mammals, the fastest rates of increase occurred in perissodactyls, followed by rodents and proboscids,^[5] all of which are hindgut fermenters. The rate of increase for artiodactyls was about a third that of perissodactyls. The rate for carnivorans was slightly lower yet, while primates, perhaps constrained by their arboreal habits, had the lowest rate among the mammalian groups studied.^[5]

Terrestrial mammalian carnivores from several eutherian groups (the mesonychid Andrewsarchus, the creodonts Megistotherium and Sarkastodon, and the carnivorans Amphicyon and Arctodus) all reached a maximum size of about 1000 kg^[6] (Arctotherium apparently actually got a bit larger). The largest known metatherian carnivore, Proborhyaena gigantea, apparently reached 600 kg, also close to this limit.^[8] A similar theoretical maximum size for mammalian carnivores has been predicted based on the metabolic rate of mammals, the energetic cost of obtaining prey, and the maximum estimated rate coefficient of prey intake.^[9] It has also been suggested that maximum size for mammalian carnivores is constrained by the stress the humerus can withstand at top running speed.^[8]

Analysis of the variation of maximum body size over the last 40 Ma suggests that decreasing temperature and increasing continental land area are associated with increasing maximum body size. The former correlation would be consistent with Bergmann's rule,^[10] and might be related to the thermoregulatory advantage of large body mass in cool climates,^[6] better ability of larger organisms to cope with seasonality in food supply,^[10] or other factors;^[10] the latter correlation could be explainable in terms of range and resource limitations.^[6] However, the two parameters are interrelated (due to sea level drops accompanying increased glaciation), making the driver of the trends in maximum size more difficult to identify.^[6]

[edit] In flightless birds

During the Paleocene, because of the small initial size of all mammals, apex predator niches were often occupied by members of other classes, such as terrestrial crocodilians (e.g. Pristichampsus), large snakes (e.g. Titanoboa), varanid lizards, or flightless birds^[6] (e.g. Gastornis in Europe and North America, Paleopsilopterus in South America). In the northern continents, large predatory birds were displaced when large eutherian carnivores evolved. In isolated South America, the phorusrhacids could not be outcompeted by the local metatherian sparassodonts and remained dominant until advanced eutherian predators arrived from North America (as part of the Great American Interchange) during the Pliocene. However, none of the largest predatory (Brontornis), possibly omnivorous (Dromornis^[11]) or herbivorous (Aepyornis) flightless birds of the Cenozoic ever grew to masses much above 500 kg, and thus never attained the size of the largest mammalian carnivores, let alone that of the largest mammalian herbivores. It has been suggested that the increasing thickness of avian eggshells in proportion to egg mass with increasing egg size places an upper limit on the size of birds.^[12] The largest species of Dromornis, D. stirtoni, may have gone extinct after it attained the maximum avian body mass and was then outcompeted by marsupial diprotodonts that evolved to sizes several times larger.^[13]

[edit] Mass extinctions

A well-known mass extinction of megafauna, the Holocene extinction (see also Quaternary extinction event), occurred at the end of the last ice age glacial period (a.k.a. the Würm glaciation) and wiped out many giant ice age animals, such as woolly mammoths, in the Americas and northern Eurasia. Various theories have attributed the wave of extinctions to human hunting, climate change, disease, a putative extraterrestrial impact, or other causes. However, this extinction pulse near the end of the Pleistocene was just one of a series of megafaunal extinction pulses that have occurred during the last 50,000 years over much of the Earth's surface, with Africa and southern Asia being largely spared. (The latter areas did suffer a gradual attrition of megafauna, particularly of the slower-moving species, over the last several million years.^[14]^[15]) Outside the mainland of Afro-Eurasia, these megafaunal extinctions followed a distinctive landmass-by-landmass pattern that closely parallels the spread of humans into previously uninhabited regions of the world, and which shows no correlation with climate.^[16]^[17] Australia was struck first around 46,000 years ago,^[18] followed by Tasmania about 41,000 years ago (after formation of a land bridge to Australia about 43,000 years ago),^[19]^[20]^[21] Japan apparently about 30,000 years ago,^[22] North America 13,000 years ago, South America about 500 years later,^[23]^[24] Cyprus 10,000 years ago,^[25]^[26] the Antilles 6000 years ago,^[27] New Caledonia^[28] and nearby islands^[29] 3000 years ago, Madagascar 2000 years ago,^[30] New Zealand 700 years ago,^[31] the Mascarenes 400 years ago,^[32] and the Commander Islands 250 years ago.^[33] Nearly all of the world's isolated islands could furnish similar examples of extinctions occurring shortly after the arrival of Homo sapiens, though most of these islands, such as the Hawaiian Islands, never had terrestrial megafauna, so their extinct fauna were smaller.^[16]^[17]

Continuing human hunting and environmental disturbance has led to additional megafaunal extinctions in the recent past, and has created a serious danger of further extinctions in the near future (see examples below).

A number of other mass extinctions occurred earlier in Earth's geologic history, in which some or all of the megafauna of the time also died out. Famously, in the Cretaceous–Tertiary extinction event the dinosaurs and most other giant reptilians were eliminated. However, the earlier mass extinctions were more global and not so selective for megafauna; i.e., many species of other types, including plants, marine invertebrates^[34] and plankton, went extinct as well. Thus, the earlier events must have been caused by more generalized types of disturbances to the biosphere.

[edit] Effect of megafaunal extinctions on methane emissions

Many herbivores produce methane as a byproduct of foregut fermentation in digestion, and release it through belching. Large populations of herbivore megafauna have the potential to contribute greatly to the atmospheric concentration of methane, which is an important greenhouse gas. Today, around 20% of annual methane emissions come from livestock methane release. Recent studies have indicated that the extinction of megafaunal herbivores may have caused a reduction in atmospheric methane. This hypothesis is relatively new.^[35]

Several studies have examined the effect of elimination of megaherbivorous mammals on methane emissions. One study examined the methane emissions from the bison that occupied the Great Plains of North America before contact with European settlers. The study estimated that the removal of the bison caused a decrease of 2.2 Tg/yr. This is a proportionally large change for the time period.^[36]

Another study examined the change of methane concentration in the atmosphere at the end of the Pleistocene epoch after the extinction of megafauna in the Americas. After early humans migrated to the Americas ~13,000 BP, their hunting and other associated ecological impacts led to the extinction of many megafaunal species in the region. Calculations suggest that this extinction decreased methane production by ~9.6 Tg/yr. Ice core records support this hypothesis of rapid methane decrease during the time period. This suggests that the absence of megafaunal methane emissions may have contributed to the abrupt climatic cooling at the onset of the Younger Dryas.^[35]

[edit] Examples

The following are some notable examples of animals often considered as megafauna (in the sense of the "large animal" definition). This list is not intended to be exhaustive:

class Mammalia
- infraclass Metatheria
  - order Diprotodontia
    - The red kangaroo (Macropus rufus) is the largest living Australian mammal and marsupial at a weight of up to 85 kg (190 lb). However, its extinct relative, the giant short-faced kangaroo Procoptodon goliah reached 230 kg (510 lb), while extinct diprotodonts attained the largest size of any marsupial in history, up to an estimated 2,750 kg (6,100 lb). The extinct marsupial lion (Thylacleo carnifex), at up to 160 kg (350 lb) was much larger than any extant carnivorous marsupial.
- infraclass Eutheria
  - superorder Afrotheria
    - order Proboscidea
      - Elephants are the largest living land animals. They and their relatives arose in Africa, but until recently had a nearly worldwide distribution. The African bush elephant (Loxodonta africana) has a shoulder height of up to 4.3 m (14 ft) and weighs up to 13 tons. Among recently extinct proboscideans, mammoths (Mammuthus) were close relatives of elephants, while mastodons (Mammut) were much more distantly related. The Songhua River mammoth (M. sungari) is estimated to have weighed 17 tonnes, making it the largest proboscid and second largest land mammal after indricotherines.
    - order Sirenia
      - The largest sirenian at up to 1500 kg is the West Indian manatee (Trichechus manatus). Steller's sea cow (Hydrodamalis gigas) was probably around five times as massive, but unfortunately was exterminated by humans within 27 years of its discovery off the remote Commander Islands in 1741. In prehistoric times this sea cow also lived along the coasts of northeastern Asia and northwestern North America; it was apparently eliminated from these more accessible locations by aboriginal hunters.
  - superorder Xenarthra
    - order Cingulata
      - The glyptodonts were a group of large, heavily armored ankylosaur-like xenarthrans related to living armadillos. They originated in South America, invaded North America during the Great American Interchange, and went extinct at the end of the Pleistocene epoch.
    - order Pilosa
      - Ground sloths were another group of slow, terrestrial xenarthrans, related to modern tree sloths. They had a similar history, although they reached North America earlier, and spread farther north. The largest genera, Megatherium and Eremotherium, reached sizes comparable to elephants.
  - superorder Euarchontoglires
    - order Primates
      - The largest living primate, at up to 266 kg (590 lb), is the gorilla (Gorilla beringei and Gorilla gorilla, with three of four subspecies being critically endangered). The extinct Malagasy sloth lemur Archaeoindris reached a similar size, while the extinct Gigantopithecus blacki of Southeast Asia is believed to have been several times larger. Some populations of archaic Homo were significantly larger than recent Homo sapiens; for example, Homo heidelbergensis in southern Africa may have commonly reached 7 feet (2.1 m) in height,^[37]^[38]^[39]^[40] while Neanderthals were about 30% more massive.^[41]
    - order Rodentia
      - The extant capybara (Hydrochoerus hydrochaeris) of South America, the largest living rodent, weighs up to 65 kg (140 lb). Several recently extinct North American forms were larger: the capybara Neochoerus pinckneyi (another neotropic migrant) was about 40% heavier; the giant beaver (Castoroides ohioensis) was similar. The extinct blunt-toothed giant hutia (Amblyrhiza inundata) of several Caribbean islands may have been larger still. However, several million years ago South America harbored much more massive rodents. Phoberomys pattersoni, known from a nearly full skeleton, probably reached 700 kg (1,500 lb). Fragmentary remains suggest that Josephoartigasia monesi grew to upwards of 1,000 kg (2,200 lb).
  - superorder Laurasiatheria
    - order Carnivora
      - Big cats include the tiger (Panthera tigris) and lion (Panthera leo). The largest subspecies, at up to 306 kg, is the Siberian tiger (P. tigris altaica), in accord with Bergmann's rule. Members of Panthera are distinguished by morphological features which enable them to roar. Larger extinct felids include the American lion (Panthera leo atrox) and the South American saber-toothed cat Smilodon populator.
      - Bears are large carnivorans of the caniform suborder. The largest living forms are the polar bear (Ursus maritimus), with a body weight of up to 680 kg (1,500 lb), and the similarly sized Kodiak bear (Ursus arctos middendorffi), again consistent with Bergmann's rule. Arctotherium augustans, an extinct short-faced bear from South America, was the largest predatory land mammal ever with an estimated average weight of 1,600 kg (3,500 lb).^[42]
      - Seals, sea lions, and walruses are amphibious marine carnivorans that evolved from bearlike ancestors. The southern elephant seal (Mirounga leonina) of Antarctic and subantarctic waters is the largest carnivoran of all time, with bull males reaching a maximum length of 6–7 m (20–23 ft) and maximum weight of 5,000 kilograms.
    - order Perissodactyla
      - Tapirs are browsing animals, with a short prehensile snout and pig-like form that appears to have changed little in 20 million years. They inhabit tropical forests of Southeast Asia and South and Central America, and include the largest surviving land animals of the latter two regions. There are four species.
      - Rhinoceros
        
        Rhinoceroses are odd-toed ungulates with horns made of keratin, the same type of protein composing hair. They are among the largest living land mammals after elephants (hippos attain a similar size). Three of five extant species are critically endangered. Their extinct central Asian relatives the indricotherines were the largest terrestrial mammals of all time.
    - order Artiodactyla (or cladistically, Cetartiodactyla)
      - Giraffes (Giraffa camelopardalis) are the tallest living land animals, reaching heights of up to nearly 6 m (20 ft).
      - Bovine ungulates include the largest surviving land animals of Europe and North America. The water buffalo (Bubalis arnee), bison (Bison bison and B. bonasus), and gaur (Bos gaurus) can all grow to weights of over 900 kg (2,000 lb).
      - The semiaquatic hippopotamus (Hippopotamus amphibius) is the heaviest living even-toed ungulate; it and the critically endangered pygmy hippo (Choeropsis liberiensis) are believed to be the closest extant relatives of cetaceans. Hippos are among the megafaunal species most dangerous to humans.^[43]
    - order Cetacea (or cladistically, Cetartiodactyla)
      - Whales, dolphins, and porpoises are marine mammals. The blue whale (Balaenoptera musculus) is the largest baleen whale and the largest animal that has ever lived. The sperm whale (Physeter macrocephalus) is the largest toothed whale, as well as the planet's loudest and brainiest animal (with a brain about five times as massive as a human's). The killer whale (Orcinus orca) is the largest dolphin.
class Aves (phylogenetically, a clade within Coelurosauria, a taxon within the order Saurischia of the clade Sauropsida; see below)
- order Struthioniformes
  - The ratites are an ancient and diverse group of flightless birds that are found on fragments of the former supercontinent Gondwana. The largest living bird, the Ostrich (Struthio camelus) was surpassed by the extinct Aepyornis of Madagascar, the heaviest of the group, and the extinct giant moa (Dinornis) of New Zealand, the tallest, growing to heights of 3.4 m (11 ft). The latter two are examples of island gigantism.
- order Anseriformes
  - Extinct dromornithids of Australia such as Dromornis may have exceeded the largest ratites in size. (Due to its small size for a continent and its isolation, Australia is sometimes viewed as the world's largest island; thus, these species could also be considered insular giants.)
class Reptilia (or cladistically, Sauropsida)
- order Crocodilia
  - Alligators and crocodiles are large semiaquatic reptiles, the largest of which, the saltwater crocodile (Crocodylus porosus), can grow to a weight of 1,360 kg (3,000 lb). Crocodilians' distant ancestors and their kin, the crurotarsans, dominated the world in the late Triassic, until the Triassic–Jurassic extinction event allowed dinosaurs to overtake them. They remained diverse during the later Mesozoic, when crocodyliforms such as Deinosuchus and Sarcosuchus reached lengths of 12 m. Similarly large crocodilians, such as Mourasuchus and Purussaurus, were present as recently as the Miocene in South America.
- order Saurischia
  - Saurischian dinosaurs of the Jurassic and Cretaceous include sauropods, the longest (at up to 40 m or 130 ft) and most massive terrestrial animals known (Argentinosaurus reached 80–100 metric tonnes, or 90–110 tons), as well as theropods, the largest terrestrial carnivores (Spinosaurus grew to 7–9 tonnes).
- order Squamata
  - While the largest extant lizard, the Komodo dragon (Varanus komodoensis), another island giant, can reach 3 m (10 ft) in length, its extinct Australian relative Megalania may have reached more than twice that size. These monitor lizards' marine relatives, the mosasaurs, were apex predators in late Cretaceous seas.
  - The heaviest extant snake is considered to be the green anaconda (Eunectes murinus), while the reticulated python (Python reticulatus), at up to 8.7 m or more, is considered the longest. An extinct Australian Pliocene species of Liasis, the Bluff Downs giant python, reached 10 m, while the Paleocene Titanoboa of South America reached lengths of 12–15 m and an estimated weight of about 1135 kilograms (2500 lb).
- order Testudines
  - The largest turtle is the critically endangered marine leatherback turtle (Dermochelys coriacea), weighing up to 900 kg (2,000 lb). It is distinguished from other sea turtles by its lack of a bony shell. The most massive terrestrial chelonians are the giant tortoises of the Galápagos Islands (Chelonoidis nigra) and Aldabra Atoll (Aldabrachelys gigantea), at up to 300 kg (660 lb). These tortoises are the biggest survivors of an assortment of giant tortoise species that were widely present on continental landmasses^[44]^[45] and additional islands^[44] during the Pleistocene.
class Amphibia
- order Temnospondyli
  - The Permian temnospondyl Prionosuchus, the largest amphibian known, reached 9 m in length and was an aquatic predator resembling a crocodilian. After the appearance of real crocodilians, temnospondyls such as Koolasuchus (5 m long) had retreated to the Antarctic region by the Cretaceous, before going extinct.
class Actinopterygii
- order Tetraodontiformes
  - The largest extant bony fish is the ocean sunfish (Mola mola), whose average adult weight is 1,000 kg (2,200 lb). While phylogenetically a "bony fish", its skeleton is primarily cartilage (which is lighter than bone). It has a disk-shaped body, and propels itself with its long, thin dorsal and anal fins; it feeds primarily on jellyfish. In these three respects (as well as in its size and diving habits), it resembles a leatherback turtle.
- order Acipenseriformes
  - The critically endangered beluga (European sturgeon, Huso huso) at up to 1476 kg (3250 lb) is the largest sturgeon (which are also mostly cartilaginous) and is considered the largest anadromous fish.
- order Siluriformes
  - The critically endangered Mekong giant catfish (Pangasianodon gigas), at up to 293 kg (646 lb), is often viewed as the largest freshwater fish.
class Chondrichthyes
- order Lamniformes
  - The largest living predatory fish, the great white shark (Carcharodon carcharias), reaches weights up to 2,240 kg (4,940 lb). Its extinct relative C. megalodon (the disputed genus being either Carcharodon or Carcharocles) was more than an order of magnitude larger, and is the largest predatory shark or fish of all time; it preyed on whales and other marine mammals.
- order Orectolobiformes
  - The largest extant shark, cartilaginous fish, and fish overall is the whale shark (Rhincodon typus), which reaches weights in excess of 21.5 tonnes (47,000 lb). Like baleen whales, it is a filter feeder and primarily consumes plankton.
- order Rajiformes
  - The manta ray (Manta birostris) is another filter feeder and the largest ray, growing to up to 2300 kg.
class Cephalopoda
- order Teuthida
  - A number of deep ocean creatures exhibit abyssal gigantism. These include the giant squid (Architeuthis) and colossal squid (Mesonychoteuthis hamiltoni); both (although rarely seen) are believed to attain lengths of 12 m (39 ft) or more. The latter is the world's largest invertebrate, and has the largest eyes of any animal. Both are preyed upon by sperm whales.

[edit] Gallery

[edit] Extinct

Jaekelopterus was a Devonian sea scorpion and likely an ambush predator.
Dunkleosteus was a gigantic, 10 m (33 ft) long predatory Devonian placoderm fish.
Sail-backed pelycosaur Dimetrodon and temnospondyl Eryops from North America's Permian.
Pliosaur Liopleurodon (right) harassing the filter feeder fish Leedsichthys during the Jurassic.
Tyrannosaurus was a large theropod dinosaur, a superpredator from North America
Macronarian sauropods; from left, Camarasaurus, Brachiosaurus, Giraffatitan, Euhelopus.
Indricotheres, the land mammals closest to sauropods in size and lifestyle, were rhinos.
The Late Miocene teratorn Argentavis of South America had an 8 m (26 ft) wingspan.
C. megalodon with a whale shark, great white shark and a human for scale.
Deinotherium had downward-curving tusks and ranged widely over Afro-Eurasia.
Titanis walleri, the only terror bird known to have invaded North America, was 2.5 m (8 ft 2 in) tall.
Hippo-sized Diprotodon of Australia, the largest marsupial of all time, went extinct 40,000 years ago.
Megalania, a giant carnivorous goanna of Australia, might have grown to 7 metres long.
Elephant-sized Megatherium, from South America's Pleistocene, was the largest sloth.
Toxodon, one of South America's largest and last notoungulates. It had a relative in Mexico.
American lions exceeded extant lions in size and ranged over two continents until 10,000 BP.
Woolly mammoths vanished after humans invaded their habitat in Eurasia and N. America.^[1]
Haast's Eagle, the largest eagle known, attacking moa (which included the tallest bird known).