Alligator mississippiensis
American alligator

Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Deuterostomia - Chordata - Craniata - Vertebrata - Gnathostomata - Teleostomi - Euteleostomi - Sarcopterygii - Tetrapoda - Amniota - Sauropsida - Sauria - Archosauria - Crocodylidae - Alligatorinae - Alligator - Alligator mississippiensis

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Brief facts

• Taxonomic position

  Alligators belong to subfamily Alligatorinae of family Crocodylidae (crocodilians). The taxonomic group includes eight living alligators and caimans and a large diversity of extinct taxa extending back to the Late Cretaceous. Extant species of genus Alligator are the two living species: Alligator mississippiensis from the southeastern United States and A. sinensis from eastern China. It was suggested that salt intolerance forced alligatoroid dispersal along land bridges. Based on known fossil occurrences, caimans and alligators diverged from each other in North America sometime during the Late Cretaceous and spread to South America by the Paleocene, when North and South America were separated. Alligatoroids include some of the largest crocodilians known, such as the giant Mio-Pliocene Purussaurus (around 12 meters or 39 feet long) and a few that diverge radically from the basic crocodilian bauplane, such as the duck-faced Mourasuchus.

• Distribution

  American alligators are the most northerly distributed of the extant crocodilians. Reproducing populations are found as far north as 35° latitude in the fresh and brackish waters of marshes and rivers of coastal North Carolina. Alligators have been heavily exploited for leather, and by the mid 20^th century, population levels had fallen sharply. State and federal protection laws succeeded and the population was restored. The highest density of these animals currently is observed in Florida and Louisiana. It was estimated that the Florida adult wild alligator population is in the range of 1-2 million.

• Sex determination

  Heteromorphic sex chromosomes that provide genetic sex determination (GSD) in mammals and birds are absent in all crocodilian species and temperature-dependent sex determination (TSD) is prevalent. Alligator mississippiensis exhibit TSD pattern similar to that of Caiman crocodilius, Crocodylus niloticus, and Crocodylus siamensis. Artificial incubation of eggs at low temperatures (not more than 30 °C in A. mississippiensis; not more than 31 °C in C. niloticus; not more than 31.5 °C in C. Crocodylus ) induces 100% females; high temperatures (not less than 33 °C) induce 100% males. At intermediate temperatures both sexes are produced although there is a strong female bias; the incidence of males increases with increasing incubation temperature. In temperature shifts from 30 °C to 33 °C the temperature-sensitive period (TSP) occurs between days 14 and 21 of a total incubation period of 65 days, but in shifts from 33 °C to 30 °C it occurs between days 28 and 35.

• Feeding

  Crocodilians, including American Alligator, approach their prey with stealth and grab it with their conical teeth. Small prey are swallowed whole. Hard-shelled prey such as turtles are tongued and tossed to the side of the mouth to crush. Large prey are bitten and shaken repeatedly to separate manageable chunks. When occasionally large and strong animal is caught, alligator performs so-called death roll, which is an example of a behavioral strategy referred as rotational feeding. Death roll is a spinning maneuver that is initiated after fore- and hind legs are pressed against the body and head and tail are thrown sideways at ~49° and ~103° to the body axis, respectively. During the death roll, head, body and tail are rotated smoothly and freely around their individual axes generating a powerful shear forces that result in dismemberment and tearing of the prey's body.

• Cannibalism

  Cannibalism in American alligator was reported by many authors. In one quantitative study it was found that once alligators grew >1.35 m in length, they start to prey on hatchlings accounting for ~50% of total hatchling mortality. Large alligators (>2.73 m) are especially cannibalistic preying exclusively on large juveniles and small adults. Overall, cannibalism accounts for up to 64% of total mortality in alligators of age 11 months and older.

• Diet

  Insects, most commonly water bugs (Belastoma spp.), constitute the greatest (>70%) food source for juvenile alligators of up to 60 cm in length. As animals grow bigger, vertebrates, especially fish, small amphibians and reptiles become prevalent. At least two studies found that diet of adult alligators consists mostly of fish (~55%), such as shad (Dorosoma spp.), bowfin (Amia calva), gar (Lepisosteus spp.), etc. and reptiles (20-40%), of which red-bellied turtle is most frequent (~8% of all food). Amphibians (Siren lacertina), birds and mammals (round-tailed muskrat) provide the rest of nutrition.
  
  American alligator cruises Blue Hole (Pine Keys, Florida). It is easy to see why fish constitutes most of its diet.
  
   
  
  During herons' and other water birds' nesting season, alligators are swimming beneath their nests built on mangrove branches in hopes of catching fallen nestlings.

• Social displays

  Adult alligators perform two conspicuous social displays, (1) bellows and (2) headslaps. Both of these behaviors are performed from a head oblique tail arched (HOTA) posture. Bellow displays, by both males and females, involve the production of a loud, roaring vocalization. Male alligators also produce a infrasonic signal, termed subaudible vibrations (SAV), just prior to the audible bellow. Bellowing is most frequent during the courting season, when alligators bellow daily in choruses. Bellowing may serve to attract alligators of the opposite sex and possibly to space out animals of the same sex. The headslap display is an assertion display consisting of eight component behavioral acts: (1) the elevated posture, (2) HOTA posture, (3) SAV, (4) headslap, (5) jawclap, (6) growl, (7) inflated posture, and (8) tail wag. Each act component is variable in presence and intensity. The alligator typically remains motionless in the HOTA posture for about 16 secnds before executing the headslap/jawclap acts. The headslap display involves a rapid clapping shut of the jaws as the undersurface of the head is slapped against the water surface. Head slapping is most common in early morning and afternoon hours. Majority (>90%) of the observed displays are performed by males. Headslap displays are generally performed from sites which the alligator chooses carefully prior to the display. Responses to headslap displays include headslapping by others, lunges, approaches, and growling. The headslap display functions as a declaration of presence. A musky odor is commonly detected in association with both of these social displays, suggesting a possibly important, but little understood, pheromonal component of these behaviors.
  Most of the behaviors described are shared with many other species of crocodilians.

• Overwintering

  Alligators stop eating when ambient temperature drops below 16°C. The anorexia lasts at least 6 months at 35° latitude. In southwest Louisiana alligators stop feeding in October and do not resume feeding until late March or early April.
   
  Adult alligators usually spend coldest winter days in their dens, which they sometime share with hatchlings and yearlings. On warmer winter days adult alligators (>1.8 m) bask under the sun and in response to sudden freezing weather exhibit submerged breathing posture in which the tip of their snout is kept out of the water while the rest of the body is extended into deeper warmer water. Alligators make effort to maintain air holes in the ice of frozen ponds to avoid drowning. In contrast, hatchlings and yearlings are unable to maintain air holes and often drown. Larger juveniles (>50 cm) trapped under ice can survive for up to 12 hours. Overexposed alligators can succumb to pneumonia.

• Ecological importance

  American Alligators are ecological engineers because they alter physical and vegetative structure of the marsh landscape through creation and maintenance of small ponds called alligator holes or alligator dens. Alligator holes are circular ponds ranging from 17 to 110 cm in depth and from 2.5 to 15.5 m in diameter. The holes are filled with water even during most dry seasons. Areas of higher elevations are also built when vegetation and sediment are excavated and piled next to alligator holes. Alligators therefore play a key ecological role by providing both drier and wetter habitats that benefit many marsh-dwelling organisms, including plants, fish, invertebrates, amphibians and reptiles, wading and passerine birds, and deer and small mammals. By providing a gradient of different hydrological and, possibly, nutrient conditions, alligator holes make habitat available for plants and animals not found in the surrounding marsh, increasing habitat and species diversity.
   
  Alligator nest mounds are used extensively by oviparous reptiles such as Florida soft-shell turtles Trionyx ferox, Florida red-bellied slider Chrysemys nelsoni, the mud turtle (Kinosternon baurii), and Carolina anole (Anolis carolinensis). Use of alligator nest mounds as egg deposition sites by other reptiles may increase hatching success comparing to other sites such as tree islands or levee sites that are more accessible to predators. In addition, alligator nests maintain a constant temperature of 29-31°C and a constant high humidity - conditions are highly favorable for incubation of turtle eggs.
   
  Alligator eggs are eaten by many marsh-dwelling animals such as raccoons (Procyon lotor), otters (Lutra canadensis), bears, and birds. Hatchlings are also very vulnerable to predation.

• Alligator attacks

  Alligator attacks are relatively rare because the animals instinctively avoid humans. The Florida Fish and Wildlife Conservation Commission has documented 300+ attacks on humans for about 5 decades since 1948. Some of these attacks were fatal. Alligators that were desensitized to the presence of humans are most frequent offenders. Most often this is a result of the illegal feeding or discarding of food or animal scraps into nearby bodies of water. Attacks may also occur during the nesting season in late spring and summer, when alligators are most territorial.

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American alligator at Alligator Farm (St. Augustine, Florida)

10/11/2010

Swimming (if you don't see the video below)

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More about crocodilians at GeoChemBio.com

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