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American alligator

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American alligator
Temporal range:
Ma[1]
American Alligator.jpg
At the Columbus Zoo and Aquarium in Powell, Ohio

Least Concern (IUCN 3.1)[2]
CITES Appendix II (CITES)[3]
Scientific classification edit
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Crocodilia
Family: Alligatoridae
Subfamily: Alligatorinae
Genus: Alligator
Species:
A. mississippiensis
Binomial name
Alligator mississippiensis
(Daudin, 1802)
Rangemapx.gif
Approximate range of American alligator in the United States
Synonyms[4]

The American alligator (Alligator mississippiensis), sometimes referred to colloquially as a gator or common alligator, is a large

extant species in the genus Alligator, and is larger than the only other living alligator species, the Chinese alligator
.

Adult male American alligators measure 3.4 to 4.6 m (11.2 to 15.1 ft) in length, and can weigh up to 453 kg (999 lb), with unverified sizes of up to 5.85 m (19.2 ft) and weights of 1,000 kg (2,200 lb) making it one of the largest members of the

by its broader snout, with overlapping jaws and darker coloration, and is less tolerant of saltwater but more tolerant of cooler climates than the American crocodile, which is found only in tropical and warm subtropical climates.

American alligators are apex predators and consume fish, amphibians, reptiles, birds, and mammals. Hatchlings feed mostly on invertebrates. They play an important role as ecosystem engineers in wetland ecosystems through the creation of alligator holes, which provide both wet and dry habitats for other organisms. Throughout the year (in particular during the breeding season), American alligators bellow to declare territory, and locate suitable mates.[11] Male American alligators use infrasound to attract females. Eggs are laid in a nest of vegetation, sticks, leaves, and mud in a sheltered spot in or near the water. Young are born with yellow bands around their bodies and are protected by their mother for up to one year.[12]

The conservation status of the American alligator is listed as

state reptile of three states: Florida, Louisiana, and Mississippi
.

Taxonomy

The American alligator was first classified by French zoologist François Marie Daudin as Crocodilus mississipiensis in 1801. In 1807, Georges Cuvier created the genus Alligator;[13] the American alligator and the Chinese alligator are the only extant species in the genus. They are grouped in the family Alligatoridae with the caimans. The superfamily Alligatoroidea includes all crocodilians (fossil and extant) that are more closely related to the American alligator than to either the Nile crocodile (Crocodylus niloticus) or the gharial (Gavialis gangeticus).[14]

Phylogeny

Members of this superfamily first arose in the late Cretaceous, about 100–65 million years ago (Mya). Leidyosuchus of Alberta is the earliest known fossil, from the Campanian era 83 to 72 Mya. Fossil alligatoroids have been found throughout Eurasia, because bridges across both the North Atlantic and the Bering Strait connected North America to Eurasia about 66 to 23 Mya.

Alligators and caimans split in North America during the late Cretaceous, and the caimans reached South America by the Paleogene, before the closure of the Isthmus of Panama during the Neogene period, from about 23 to 2.58 Mya. The Chinese alligator likely descended from a lineage that crossed the Bering land bridge during the Neogene. Fossils identical to the existing American alligator are found throughout the Pleistocene, from 2.5 million to 11.7 thousand years ago.[15] In 2016, a Miocene (about 23 to 5.3 Mya) fossil skull of an alligator was found at Marion County, Florida. Unlike the other extinct alligator species of the same genus, the fossil skull was virtually indistinguishable from that of the modern American alligator. This alligator and the American alligator are now considered to be sister taxa, meaning that the A. mississippiensis lineage has existed in North America for over 8 million years.[16]

The alligator's full

mitochondrial genome was sequenced in the 1990s, and it suggests the animal evolved at a rate similar to mammals and greater than birds and most cold-blooded vertebrates.[17] However, the full genome, published in 2014, suggests that the alligator evolved much more slowly than mammals and birds.[18]

Characteristics

Domestic American alligators range from long and slender to short and robust, possibly in response to variations in factors such as growth rate, diet, and climate.

Size

The American alligator is a relatively large species of crocodilian. On average, it is the largest species in the family Alligatoridae, with only the black caiman being possibly bigger.[19] Weight varies considerably depending on length, age, health, season, and available food sources. Similar to many other reptiles that range expansively into temperate zones, American alligators from the northern end of their range, such as southern Arkansas, Alabama, and northern North Carolina, tend to reach smaller sizes. Large adult American alligators tend to be relatively robust and bulky compared to other similar-length crocodilians; for example, captive males measuring 3 to 4 m (9 ft 10 in to 13 ft 1 in) were found to weigh 200 to 350 kg (440 to 770 lb), although captive specimens may outweigh wild specimens due to lack of hunting behavior and other stressors.[20][21]

There is a common belief stated throughout reptilian literature that crocodilians including the American alligator exhibit indeterminate growth, meaning the animal continues to grow for the duration of its life. However, these claims are largely based on assumptions and observations of juvenile and young adult crocodilians. One long-term mark-recapture study (1979-2015) done at the Tom Yawkey Wildlife Center in South Carolina found evidence to support patterns of determinate growth. The study allowed for the observation of life histories noting some American alligators can possibly live over the age of seventy. This studies prolong documentation of growth rates, along with several other growing studies point toward the more newly supported pattern of determinant growth in the American alligator.[22]

Relation to age

Large male American alligators reach an expected maximum size up to 4.6 m (15 ft 1 in) in length and weighing up to 453 kg (999 lb), while females reach a maximum of 3 m (9 ft 10 in).[5][6] On rare occasions, a large, old male may grow to an even greater length.[23][24]

Largest

During the 19th and 20th centuries, larger males reaching 5 to 6 m (16 ft 5 in to 19 ft 8 in) were reported.[25] The largest reported individual size was a male killed in 1890 on Marsh Island, Louisiana, and reportedly measured at 5.84 m (19 ft 2 in) in length, but no voucher specimen was available, since the American alligator was left on a muddy bank after having been measured due to having been too massive to relocate.[24] If the size of this animal was correct, it would have weighed about 1,000 kg (2,200 lb).[26] In Arkansas, a man killed an American alligator that was 4.04 m (13 ft 3 in) and 626 kg (1,380 lb).[27] The largest American alligator ever killed in Florida was 5.31 m (17 ft 5 in), as reported by the Everglades National Park.[28][29] The largest American alligator scientifically verified in Florida for the period from 1977 to 1993 was reportedly 4.23 m (13 ft 11 in) and weighed 473 kg (1,043 lb), although another specimen (size estimated from skull) may have measured 4.54 m (14 ft 11 in).[20] A specimen that was 4.8 m (15 ft 9 in) long and weighed 458.8 kg (1,011.5 lb) is the largest American alligator killed in Alabama and has been declared the SCI world record in 2014.[30]

Reported sizes

Average

American alligators do not normally reach such extreme sizes. In mature males, most specimens grow up to about 3.4 m (11 ft 2 in) in length, and weigh up to 360 kg (790 lb),[7] while in females, the mature size is normally around 2.6 m (8 ft 6 in), with a body weight up to 91 kg (201 lb).[8][9] In Newnans Lake, Florida, adult males averaged 73.2 kg (161 lb) in weight and 2.47 m (8 ft 1 in) in length, while adult females averaged 55.1 kg (121 lb) and measured 2.22 m (7 ft 3 in).[42] In Lake Griffin State Park, Florida, adults weighed on average 57.9 kg (128 lb).[43] Weight at sexual maturity per one study was stated as averaging 30 kg (66 lb) while adult weight was claimed as 160 kg (350 lb).[44]

Sexual dimorphism

While noticeable in very mature specimens, the sexual dimorphism in size of the American alligator is relatively modest among crocodilians.[45] In the saltwater crocodile, for example, the females are only slightly larger at average (2.4 m (7 ft 10 in) in the American alligator, 2.6 m (8 ft 6 in) in the saltwater crocodile) than female American alligators, but the mature males, at 4.3 to 5.2 m (14 ft 1 in to 17 ft 1 in) typically, as opposed to 2.4 to 4 m (7 ft 10 in to 13 ft 1 in) expected in mature male American alligators, are considerably bigger than male American alligators and at median are nearly twice as long as and at least four times as heavy as the female saltwater crocodiles of the same species.[46] Given that female American alligators have relatively higher survival rates at an early age and a large percentage of given populations consists of immature or young breeding American alligators, relatively few large mature males of the expected mature length of 3.4 m (11 ft 2 in) or more are typically seen.[47]

Color

Dorsally, adult American alligators may be olive, brown, gray, or black. However, they are on average one of the most darkly colored modern crocodilians (although other alligatorid family members are also fairly dark), and can be reliably be distinguished by color via their more blackish dorsal scales against crocodiles.[23] Meanwhile, their undersides are cream-colored.[48] Some American alligators are missing or have an inhibited gene for melanin, which makes them albino. These American alligators are extremely rare and almost impossible to find in the wild. They could only survive in captivity, as they are very vulnerable to the sun and predators.[49]

Jaws, teeth, and snout

American alligators have 74-80 teeth.[25] As they grow and develop, the morphology of their teeth and jaws change significantly.[50] Juveniles have small, needle-like teeth that become much more robust and narrow snouts that become broader as the individuals develop.[50] These morphological changes correspond to shifts in the American alligators' diets, from smaller prey items such as fish and insects to larger prey items such as turtles, birds, and other large vertebrates.[50] American alligators have broad snouts, especially in captive individuals. When the jaws are closed, the edges of the upper jaws cover the lower teeth, which fit into the jaws' hollows. Like the spectacled caiman, this species has a bony nasal ridge, though it is less prominent.[25] American alligators are often mistaken for a similar animal: the American crocodile. An easy characteristic to distinguish the two is the fourth tooth. Whenever an American alligator's mouth is closed, the fourth tooth is no longer visible. It is enclosed in a pocket in the upper jaw.

Bite

Adult American alligators held the record as having the strongest laboratory-measured bite of any living animal, measured at up to 13,172 N (1,343.2 kgf; 2,961 lbf). This experiment had not been, at the time of the paper published, replicated in any other crocodilians, and the same laboratory was able to measure a greater bite force of 16,414 N (1,673.8 kgf; 3,690 lbf) in saltwater crocodiles;[51][52] notwithstanding this very high biting force, the muscles opening the American alligator's jaw are quite weak, and the jaws can be held closed by hand or tape when an American alligator is captured. No significant difference is noted between the bite forces of male and female American alligators of equal size.[50] Another study noted that as the American alligator increases in size, the force of its bite also increases.[53]

Movement

X-ray video of a female American alligator showing contraction of the lungs while breathing

When on land, an American alligator moves either by sprawling or walking, the latter involving the reptile lifting its belly off the ground. The sprawling of American alligators and other crocodylians is not similar to that of salamanders and lizards, being similar to walking. Therefore, the two forms of land locomotion can be termed the "low walk" and the "high walk". Unlike most other land vertebrates, American alligators increase their speed through the distal rather than proximal ends of their limbs.[54] In the water, American alligators swim like fish, moving their pelvic regions and tails from side to side.[55] During respiration, air flow is unidirectional, looping through the lungs during inhalation and exhalation;[56] the American alligator's abdominal muscles can alter the position of the lungs within the torso, thus shifting the center of buoyancy, which allows the American alligator to dive, rise, and roll within the water.[57]

Distribution

American alligators, being native both to the

Lowcountry in South Carolina, south to Everglades National Park in Florida, and west to the southeastern region of Texas.[58] They are found in parts of North Carolina, South Carolina, Georgia, Florida, Louisiana, Alabama, Mississippi, Arkansas, Oklahoma and Texas. Some of these locations appear to be relatively recent introductions, with often small but reproductive populations.[59] Louisiana has the largest American alligator population of any U.S. state. In the future, possible American alligator populations may be found in areas of Mexico adjacent to the Texas border.[60] Alligators have been naturally expanding their range into Tennessee,[61] and have established a small population in the southwestern part of that state via inland waterways, according to the state's wildlife agency.[62]

Conservation status

leucistic American alligator at Gatorland

American alligators are currently listed as least concern by the IUCN Red List, even though from the 1800s to the mid-1900s, they were being hunted and poached by humans unsustainably. In 1987, the U.S. Fish and Wildlife Service pronounced the American alligator fully recovered.[63]

Historically, hunting and habitat loss have severely affected American alligator populations throughout their range, and whether the species would survive was in doubt. In 1967, the American alligator was listed as an endangered species (under a law that was the precursor

Endangered Species Act of 1973), since it was believed to be in danger of extinction throughout all or a significant portion of its range.[64]

Both the United States Fish and Wildlife Service (USFWS) and state wildlife agencies in the South contributed to the American alligator's recovery. Protection under the Endangered Species Act allowed the species to recuperate in many areas where it had been depleted. States began monitoring their American alligator populations to ensure that they would continue to grow. In 1987, the USFWS removed the animal from the endangered species list, as it was considered to be fully recovered. The USFWS still regulates the legal trade in American alligators and their products to protect still endangered crocodilians that may be passed off as American alligators during trafficking.[64]

American alligators are listed under Appendix II of the

Convention on International Trade in Endangered Species (CITES) meaning that international trade in the species (including parts and derivatives) is regulated.[3]

Habitat

They inhabit swamps, streams, rivers, ponds, and lakes. A lone American alligator was spotted for over 10 years living in a river north of Atlanta, Georgia.

salt glands on their tongues do not function.[67] One study of American alligators in north-central Florida found the males preferred open lake water during the spring, while females used both swampy and open-water areas. During summer, males still preferred open water, while females remained in the swamps to construct their nests and lay their eggs. Both sexes may den underneath banks or clumps of trees during the winter.[42]

In some areas of their range, American alligators are an unusual example of urban wildlife; golf courses are often favoured by the species due to an abundance of water and a frequent supply of prey animals such as fish and birds.[68][69]

Cold tolerance

American alligators are less vulnerable to cold than American crocodiles. Unlike an American crocodile, which would immediately succumb to the cold and drown in water at 45 °F (7 °C) or less, an American alligator can survive in such temperatures for some time without displaying any signs of discomfort.[70] This adaptiveness is thought to be why American alligators are widespread further north than the American crocodile.[70] In fact, the American alligator is found farther from the equator and is more equipped to handle cooler conditions than any other crocodilian.[71] When the water begins to freeze, American alligators go into a period of brumation; they stick their snouts through the surface, which allows them to breathe above the ice,[66] and they can remain in this state for several days.[72]

Ecology and behavior

Basking

An American alligator basks on the bank of a pond in Big Cypress National Preserve
.

American alligators primarily bask on shore, but also climb into and perch on tree limbs to bask if no shoreline is available. This is not often seen, since if disturbed, they quickly retreat back into the water by jumping from their perch.[73]

Holes

American alligators modify wetland habitats, most dramatically in flat areas such as the Everglades, by constructing small ponds known as alligator holes. This behavior has qualified the American alligator to be considered a

American alligator in the Everglades

Prey

Bite and mastication

The teeth of the American alligator are designed to grip prey, but cannot rip or chew flesh like teeth of some other predators (such as

felids), and depend on their gizzard, instead, to masticate their food. The American alligator is capable of biting through a turtle's shell or a moderately sized mammal bone.[76]

Tool use

American alligators have been documented using lures to hunt prey such as birds.[77] This means they are among the first reptiles recorded to use tools. By balancing sticks and branches on their heads, American alligators are able to lure birds looking for suitable nesting material to kill and consume. This strategy, which is shared by the mugger crocodile, is particularly effective during the nesting season, in which birds are more likely to gather appropriate nesting materials.[78]

Aquatic vs terrestrial

Fish and other aquatic prey taken in the water or at the water's edge form the major part of American alligator's diet and may be eaten at any time of the day or night. Adult American alligators also spend considerable time hunting on land, up to 160 feet (50 m) from water, ambushing terrestrial animals on trailsides and road shoulders. Usually, terrestrial hunting occurs on nights with warm temperatures.[79] When hunting terrestrial prey, American alligators may also ambush them from the edge of the water by grabbing them and pulling the prey into the water, the preferred method of predation of larger crocodiles.[23]

Additionally, American alligators have recently been filmed and documented killing and eating sharks and rays; four incidents documented indicated that bonnetheads, lemon sharks, Atlantic stingrays, and nurse sharks are components of the animal's diet. Sharks are also known to prey on American alligators, in turn, indicating that encounters between the two predators are common.[80][81]

Common

American alligators are considered an

coypu are common, they are perhaps the most regular prey for adult American alligators, although only larger adults commonly eat this species.[86][85] It has also been reported that large American alligators prey on medium-sized American alligators, which had preyed on hatchlings and smaller juveniles.[87]

If an American alligator's primary food resource is not available, it will sometimes feed on carrion and non-prey items such as rocks and artificial objects, like bottle caps. These items help the American alligator in the process of digestion by crushing up the meat and bones of animals, especially animals with shells.[88]

Large animals

Other animals may occasionally be eaten, even large deer or feral wild boars, but these are not normally part of the diet. American alligators occasionally prey on large mammals, such as deer, but usually do so when fish and smaller prey levels go down.[89] Rarely, American alligators have been observed killing and eating bobcats, but such events are not common and have little effect on bobcat populations.[90][91] Although American alligators have been listed as predators of the Nilgai and the West Indian manatees, very little evidence exists of such predation.[92] In the 2000s, when invasive Burmese pythons first occupied the Everglades, American alligators have been recorded preying on them, possibly controlling populations, thus preventing the invasive species from spreading northwards.[93][94] American alligator predation on Florida panthers is rare, but has been documented. Such incidents usually involve a panther trying to cross a waterway or coming down to a swamp or river to get a drink.[95] American alligator predation on American black bears has also been recorded.[96][97][98]

Domestic animals

Occasionally, domestic animals, including dogs, cats, and calves, are taken as available, but are secondary to wild and feral prey.[25] Other prey, including snakes, lizards, and various invertebrates, are eaten occasionally by adults.[23]

Birds

herons and egrets, storks, waterfowl and large dabbling rails such as gallinules or coots, are taken when possible. Occasionally, unwary adult birds are grabbed and eaten by American alligators, but most predation on bird species occurs with unsteady fledgling birds in late summer, as fledgling birds attempt to make their first flights near the water's edge.[23]

Fruit

In 2013, American alligators and other crocodilians were reported to also eat fruit.[99] Such behavior has been witnessed, as well as documented from stomach contents, with the American alligators eating such fruit as wild

elderberries, and citrus fruits directly from the trees. The discovery of this unexpected part of the American alligator diet further reveals that they may be responsible for spreading seeds from the fruit they consume across their habitat.[100] Additionally, American alligators engage in what seems to be cooperative hunting.[101][102]

In Florida and East Texas

The diet of adult American alligators from central Florida lakes is dominated by fish, but the species is highly opportunistically based upon local availability. In

crawfish and crabs) were found to be present in the southeastern American alligators, but largely absent in southwestern American alligators, which consumed a relatively high proportion of reptiles, although fish were the most recorded prey for adults, and adult males consumed a large portion of mammals.[104]

In East Texas, diets were diverse and adult American alligators took mammals, reptiles, amphibians, and invertebrates (e.g. snails) in often equal measure as they did fish.[105]

Vocalizations

Mechanism

An American alligator is able to

vocal folds of its larynx, but not to elongate or shorten them; yet in spite of this, it can modulate fundamental frequency very well.[106] Their vocal folds consists of epithelium, lamina propria and muscle. Sounds ranged from 50 to 1200 Hz. In one experiment conducted on the larynx, the fundamental frequency depended on both the glottal gap and stiffness of the larynx tissues. As the frequency increases, there's high tension and large strains. The fundamental frequency has been influenced by the glottal gap size and subglottal pressure and when the phonation threshold pressure has been exceeded, there will be vocal fold vibration.[107]

Calls

Crocodilians are the most vocal of all non-avian reptiles and have a variety of different calls depending on the age, size, and sex of the animal.[108] The American alligator can perform specific vocalizations to declare territory, signal distress, threaten competitors, and locate suitable mates. Juveniles can perform a high-pitched hatchling call (a "yelping" trait common to many crocodilian species' hatchling young)[109] to alert their mothers when they are ready to emerge from the nest. Juveniles also make a distress call to alert their mothers if they are being threatened. Adult American alligators can growl, hiss, or cough to threaten others and declare territory.

Bellowing

Both males and females bellow loudly by sucking air into their lungs and blowing it out in intermittent, deep-toned roars to attract mates and declare territory.[110] Males are known to use infrasound during mating bellows. Bellowing is performed in a "head oblique, tail arched" posture. Infrasonic waves from a bellowing male can cause the surface of the water directly over and to either side of his back to literally "sprinkle",[111] in what is commonly called the "water dance".[112] Large bellowing "choruses" of American alligators during the breeding season are commonly initiated by females and perpetuated by males.[113] Observers of large bellowing choruses have noted they are often felt more than they are heard due to the intense infrasound emitted by males. American alligators bellow in B flat (specifically "B♭1", defined as an audio frequency of 58.27 Hz), and bellowing choruses can be induced by tuba players, sonic booms, and large aircraft.[114]

Reproduction

Breeding season

The breeding season begins in the spring. On spring nights, American alligators gather in large numbers for group courtship, in the aforementioned "water dances".[115] The female builds a nest of vegetation, sticks, leaves, and mud in a sheltered spot in or near the water.

Eggs

After the female lays her 20 to 50 white eggs, about the size of a goose egg, she covers them with more vegetation, which heats as it decays, helping to keep the eggs warm. This differs from Nile crocodiles, which lay their eggs in pits.[70] The temperature at which American alligator eggs develop determines their sex (see temperature-dependent sex determination). Studies have found that eggs hatched at a temperature below 88.7 °F or a temperature above 94.1 °F will produce female offspring, while those at a temperature between 90.5 °F to 92.3 °F will produce male offspring.[116] The nests built on levees are warmer, thus produce males, while the cooler nests of wet marsh produce females.[117] The female remains near the nest throughout the 65-day incubation period, protecting it from intruders. When the young begin to hatch — their "yelping" calls can sometimes even be heard just before hatching commences — the mother quickly digs them out and carries them to the water in her mouth,[25] as some other crocodilian species are known to do.

Young

The young are tiny replicas of adults, with a series of yellow bands around their bodies that serve as camouflage.[25] Hatchlings gather into pods and are guarded by their mother and keep in contact with her through their "yelping" vocalizations. Young American alligators eat small fish, frogs, crayfish, and insects.[118] They are preyed on by large fish, birds, raccoons, Florida panthers, and adult American alligators.[25] Mother American alligators eventually become more aggressive towards their young, which encourages them to disperse.[118] Young alligators grow 3–8 in (7.6–20.3 cm) a year and reach adulthood at 6 ft (1.8 m).[66]

Interactions with exotic species

Nutria were introduced into coastal marshes from South America in the mid-20th century, and their population has since exploded into the millions. They cause serious damage to coastal marshes and may dig burrows in levees. Hence, Louisiana has had a bounty to try to reduce nutria numbers. Large American alligators feed heavily on nutria, so American alligators may not only control nutria populations in Louisiana, but also prevent them spreading east into the Everglades. Since hunting and trapping preferentially take the large American alligators that are the most important in eating nutria, some changes in harvesting may be needed to capitalize on their ability to control nutria.[86]

Recently, a population of Burmese pythons became established in Everglades National Park. Substantial American alligator populations in the Everglades may be a contributing factor in keeping the python populations low, preventing the spread of the species north. While events of predation by Burmese pythons on young American alligators have been observed, no evidence of a net negative effect has been seen on overall American alligator populations.[119][120][121]

Indicators of environmental restoration

Avery Island, Louisiana