Alligator mississippiensis

COMMON NAMES:

American alligator, Mississippi alligator, Pike-headed alligator, "gator"

DISTRIBUTION: Southeastern United States: Alabama, Arkansas, North & South Carolina, Florida, Georgia, Louisiana, Mississippi, Oklahoma, Texas

HABITAT:

Primarily freshwater swamps and marshes, but also in rivers, lakes and smaller bodies of water. They can tolerate a reasonable degree of salinity, being occasionally found in brackish water around mangrove swamps, although they lack the buccal salt-secreting glands present in crocodiles. Construction of burrows is well documented in this species. The burrows are used when the seasonal temperatures fall. Even outside their burrows, they can tolerate limited periods of freezing conditions. They modify their habitat through the creation of 'alligator holes', which provide a refuge for other animals during dry periods. These are excavated using both snout and tail. Once these dry out, however, the alligator crosses land in order to find another body of water.

APPEARANCE:

Adult males larger than 4.5 m are rare today, but were much more common earlier in the century when 5 to 6 m long males were found. Females reach lengths of just under 3 m. Snout is characteristically broad (although this varies slightly between populations), with the edge of the upper jaw overlapping the teeth of the lower jaw, which fit into depressions in the upper jaw (unlike the crocodilians and gharial). A bony nasal bridge is present. Juveniles are miniature versions of their parents, although they possess bright yellow cross-bands on a black background - disruptive camouflage. More western populations (which may have been historically isolated from eastern populations) tend to have white speckling around the jaws, although the coloured cross-bands on their bodies and tails are paler. Older animals lose the yellow banding, and turn dark green and brown. The ventral surface is a much paler yellow colour. The presence and extent of ventral osteoderms varies with different populations. Eyes are silvery. Adults generally take one of two forms: long and thin, or short and stocky. Captive individuals appear to have broader heads - a possible indication of their more sedentary lifestyle.

DIET:

Juveniles eat small invertebrates (particularly insects) and small fish. Dietary range increases substantially as they grow larger, with large adults able to tackle any aquatic or terrestrial prey which comes within range. They will also take carrion if the opportunity presents itself. Feeding activity is governed by water temperature, with foraging activity ceasing if the temperature drops below 20 to 23ºC (68 to 73ºF).

BREEDING:

Females reach sexual maturity at an average of 1.8 m. The courtship rituals, which occur when the temperatures rise in spring (this varies geographically - it is earlier further south, but the most northerly populations may not breed), have been well-studied. Both sexes communicate using aural, visual, tactile and olfactory cues. The vibrations from low-frequency bellowing travel considerable distances in water, advertising an individual's presence. <click here for male's bellow in mp3 format>. The act of rapidly swinging the head down to make contact with the water surface (head-slapping) transmits both aural and visual messages. Complex body postures communicate additional information, which is reinforced with odour from paired glands everted from under the chin and from the cloaca. Near the end of courtship, both animals will engage in a bout of snout and back rubbing. Overall, this courtship can last for several hours, and is thought to help synchronise both spermatogenesis and ovulation.

A mound nest of rotting vegetation lined with mud is constructed at the start of the summer, when it is both damp and warm. It is located on higher ground to reduce the risk of flooding. Such an event can kill all the eggs within 12 hours. The exact location of the nest depends upon a number of factors, as chosen by the female. These are often the same sites each year, and may be close to an 'alligator hole' which was constructed by the female. Between 20 and 50 eggs (mean of 40 to 45) are laid in a conical depression in the top of the nest (the egg chamber), which is subsequently covered over with more vegetation. Before completion of the nest and oviposition, the site may be abandoned for reasons unknown, or other females may lay another clutch in the same nest. The finished nest may rise 3.5 feet and be twice as wide. The females will remain near the nest throughout the incubation period (around 65 days, depending on temperature). If danger threatens, she will rapidly return to the nest to deal with the threat. Once the eggs are ready to hatch, the mother often opens the nest (which the small juveniles may have difficulty breaking out of otherwise, due to the hardened mud and dried vegetation) and carries the 8 to 10 inch babies to the water. The juveniles form a pod (which may include individuals from other nests) and remain close to the mother for a variable period of time of up to three years. This affords protection in numbers, and a swift response from the guardian female if they begin calling as a result of impending danger. Despite this protection, individuals are still vulnerable to predation by a number of other animals including raccoons, large fish and birds. Occasional cannibalism may also occur, particularly by larger, dominant males. Members of a pod may be found over-wintering in the same den as the adult female.

CONSERVATION:

Probably the best studied species of crocodilian, and a large amount of literature is available on various aspects of biology, behaviour and ecology. Population surveys have been extensive, and data are available throughout the alligators' range. While populations were severely affected in the early parts of the century (with protection occurring in the early 1960's), the recovery of this species has been highly encouraging in many areas - thanks mainly to properly controlled and monitored conservation and sustainable use programs. The belly skin of the alligator produces a high-quality leather, and this resulted in considerable hunting pressure earlier in the century, particularly in Louisiana and Florida. Even after hunting was prohibited in Florida, illegal poaching continued into the 1970s. Were it not for additional changes in the law to control the movement of hides, extinction would have been possible in this area. Since this time, populations have improved considerably, and are now only considered to be threatened in some areas (including other states) by habitat destruction (including water management programs). Alligators have more recently (1979) been downgraded from the IUCN listing, while CITES Appendix II classification remains to assist control in trade of other crocodilian species (whose skins are similar in appearance). In some areas, increasing alligator populations cause problems with human populations on the edge of alligator habitat, and 'nuisance alligator' programs are in effect, where individuals which have roamed too far are relocated. In many areas, however, relocated animals are found to return to their home range within a matter of days, and more recent programs either sell the animals to a farm, or kill them - the sale of the skin being used to fund the programme. Given the high degree of human-alligator contact, some incidences have been reported, but these are very rarely serious (there have been only a handful of alligator fatalities recorded in the USA since the 1950s). Increased education is the best long-term solution to this problem.

Large-scale sustainable yield programs (over 150 farms) are well established in several states (e.g. Florida, Louisiana, Texas), involving captive rearing, ranching and direct cropping of wild populations (eggs and adults) - all linked to proper monitoring programmes. Cropping is only allowed from certain populations, as some of the more peripheral populations are still recovering, and hence are fully protected. Ranching programs usually have to return a high percentage (17% in Louisiana) of juveniles back into wild populations, although recovery in these areas has now been documented. Alligators have been successfully reintroduced or restocked in several states (e.g. Arkansas, Mississippi). In some states, alligator hunting is allowed under strict quota or licence guidelines. In Florida, the results of harvesting have shown that up to 13% of subadult to adult animals, plus all the eggs from 50% of all located nests, can be safely removed from the alligator population annually without affecting population stability. These kinds of figures are vitally important for proper management programmes.

Despite the amount of research conducted, there are still areas which need to be more thoroughly studied. Population dynamics are relatively poorly understood, but are essential in order to improve management programs. The state of the wild alligator populations provides ample opportunity for such research to be undertaken. An examination of the effects of cropping and ranching is also possible. Other research taking place involves looking at captive husbandry techniques. These findings have implications for other crocodilian species. Although habitat modification is often to the detriment of crocodilians, proper management can benefit local populations. In Louisiana, weirs and impoundments have been constructed to allow for better water and salinity control of areas of marshland. Increased alligator populations are the result in areas where these controls have been implemented.

Alligators have been shown to be an important part of their ecosystem, and are thus regarded by many as a 'keystone' species. This encompasses many areas from control of prey species to the creation of peat through their nesting activities. Several other species benefit from the presence of alligator nests, not least the Florida Red-bellied turtle (Chrysemys nelsoni) which incubates its own eggs there (up to 200, from more than one individual). The creation of 'alligator holes' is of great value not only to the alligators, but to the other species of animals which use them. For these animals, the value of the refuge outweighs any additional risks from their creators. Alligators in some areas are also showing greatly increased levels of mercury, an indicator of the state of the ecosystem.