American Crocodile

Crocodylus acutus

Description

The Crocodylus acutus (American crocodile) is a large, greenish-gray crocodilian with black mottling. In Florida, adults reach lengths of about 12 ft (3.8 m) although a specimen measuring 15 ft (4.7 m) was reported in the late 1800s. In other portions of their range, individual crocodiles may reach 20 ft (6.0 m). Like all other crocodilians, males are larger than females. All adults have a hump above the eyes which may or may not be distinct, and irregular, asymmetrical dorsal armoring. Hatchlings measure approximately 10 in (27 cm) and are normally yellowish tan to gray with dark cross markings on the body and tail. These markings fade as the animal grows. A lateral indentation of the upper jaw leaves the fourth tooth of the lower jaw exposed when the mouth is closed. Compared to the alligator, the American crocodile may be distinguished by its longer, narrower, more tapered snout and the exposed fourth tooth of the lower jaw.

Behavior

Females reach sexual maturity at about 7 ft (2.25 m), a size reached at an age of about 10-13 years. It is not known at what age and size males mature. Similarly, the maximum reproductive age for either sex is not known, although it is known that captively reared crocodilians eventually fail to reproduce.

As with most crocodilians, courtship and mating are stimulated by increasing ambient water and air temperatures. Reproductive behaviors peak when body temperatures reach levels necessary to sustain hormonal activity and gametogenesis. In South Florida, temperatures sufficient to allow initiation of courtship behavior are reached by late February through March. Like all other crocodilians, the mating system of the American crocodile is polygynous; each breeding male mates with a number of females. Males typically establish and defend a breeding territory from late February through March. Vocalizations, body posturing, and outright aggression are used to maintain and defend territories and to secure mating privileges with females that roam freely between territories. Male and female American crocodiles go through a ritualistic mating sequence prior to copulation. Courtship in this species is considered to be one of the most structured of all crocodilians, with copulation predictably following precopulatory behaviors.

Following courtship and mating, females search for and eventually select a nest site in which they deposit an average of about 38 elongated oval eggs. Reported clutch size ranges from 8-56 eggs. Although American crocodile nesting is generally considered a non-social event, communal nesting is the norm in parts of the Caribbean, southeast Cuba, and Haiti. In the U. S. several incidents of two clutch nests have been reported. Nest sites are typically selected where a sandy substrate exists above the normal high water level. Nesting sites include areas of well drained sands, marl, peat, and rocky spoil and may include areas such as sand/shell beaches, stream banks, and canal spoil banks that are adjacent to relatively deep water. In some instances, where sand or river banks are not available for nesting sites, a hole will be dug in a pile of vegetation or marl the female has gathered. The use of mounds or holes for nesting is independent of the substrate type and may vary between years by the same female.

The success of American crocodile nesting in South Florida is dependent primarily on the maintenance of suitable egg cavity moisture throughout incubation, nest predation, and flooding. On Key Largo, and other island nests, failure of crocodile nests is typically attributed to dessication due to low rainfall. On Key Largo, about 52% of nests were successful in hatching at least one young. Nest failures on the mainland may be associated with flooding, desiccation, or predation. On the mainland, about 13% of nests monitored were affected by flooding or desiccation, while 13% of nests were partially or entirely depredated. In 1994, surveys found that predation rates on the mainland increased to 27%, while only nine percent of nests failed because of infertility or embryonic mortality. Incubation of the clutch takes about 86 days, during which time the female periodically visits the nest. Some females may also attend and defend their nest throughout incubation, but this behavior is highly variable among individuals and nest defense has not been observed in the U. S. or Cuba. In Florida, American crocodiles are not known to regularly defend their nest against humans. However, all females must return to the nest to excavate hatchlings since the young are unable to liberate themselves from the nest cavity. Parental care after hatching has not been reported for this species in Florida, even though this behavior has been documented in other American crocodile populations.

The young may remain together loosely for several days to several months following hatching, but they are rarely seen with adults. Hatchling survival appears to be low in Everglades National Park (NP) ( 5%), higher at Turkey Point (8.5%), and even higher in the more sheltered habitats of North Key Largo (20.4%). Higher survival on Key Largo has been attributed to the close proximity of nest sites to suitable nursery habitat. On the mainland, nest sites on exposed beaches are often far from nursery habitat, requiring recently hatched young to disperse long distances in unsheltered water. When available, hatchlings seek shelter during the day in beach wrack or among mangrove roots. Predation during these dispersals is probably high, although little information is available to support this conclusion.

The American crocodile is typically active from shortly before sunset to shortly after sunrise. During these times, crocodiles forage opportunistically, eating whatever animals they can catch. Juveniles typically eat fish, crabs, snakes, and other small invertebrates, while adults are known to eat fish, crabs, snakes, turtles, birds, and small mammals. American crocodiles probably feed only rarely during periods of low ambient air temperatures since metabolic and digestive systems are slowed at lower body temperatures.

American crocodiles live sympatrically with American alligators where salinities are low. Most crocodilians tolerate others of the same species and of different ages provided food and other essential habitat requirements are not limiting. Where two or more species coexist, tolerance among species is also common and is usually ensured by species specific differences in habitat utilization. In Florida, the American crocodile and alligator have probably coexisted for thousands of years and relied on changing salinity gradients of surface waters to dictate which species is dominant in certain areas. Though these species probably intermingle frequently throughout the year, there is only one known location where both species may nest side-by-side. If substantiated, the nesting sites along a canal berm in the vicinity of Marco Island, Collier County, would indicate use of a common nesting area by these species. However, the species' breeding seasons may be sufficiently asynchronous in this area to allow crocodiles to breed and nest before alligators become reproductively active.

The depredation rate of American crocodile nests by raccoons in South Florida is low compared to depredation rates other crocodilians suffer from terrestrial nest predators. Therefore, although the raccoon may locally be an important predator, their overall effect on the crocodile population is not considered limiting. Once hatched, crocodilians may be eaten by several species of wading birds and gulls, blue crabs, sharks, and other crocodiles. Though limited, survival information from Key Largo suggests that predation does not limit recruitment of juveniles in that area.

Habitat

The American crocodile is found primarily in mangrove swamps and along low energy man-grove-lined bays, creeks, and inland swamps. In Florida, patterns of crocodile habitat use shifts seasonally. During the breeding and nesting seasons, adults outside of Key Largo use the exposed shoreline of Florida Bay. Males tend to stay more inland than the females at this time, whereas during the non-nesting season, they are found primarily in the fresh and brackish-water inland swamps, creeks, and bays; retreating further into the back country in fall and winter. Along northeastern Florida Bay, crocodiles were found in inland ponds and creeks, protected coves, exposed shorelines and a small number were observed on mud flats. The high use of inland waters suggests crocodiles prefer less saline waters, using sheltered areas such as under-cut banks and mangrove snags and roots that are protected from wind and wave action. Access to deep water greater than 3 ft (1 m) is also an important component of preferred habitats.

Natural nesting habitat includes sites with sandy shorelines or raised marl creek banks adjacent to deep water. Crocodiles also nest on elevated man-made structures such as canal berms and other places where fill has been introduced. In natural nesting situations, creek bank nests are generally considered optimal since these sites provide a good incubation medium and are generally protected from wind and wave action. These nest sites also provide deep water refuge for adult females. Nests adjacent to open water provide little protection from wave action for the nest, hatchlings, or adults. Shore nests are typically not located near good nursery habitat, and mortality of hatchlings is generally higher than in inland nests. Both nesting sites are desirable as there are tradeoffs associated with each and hatching success at each type of location will vary between years depending on climatic conditions.

Distribution

The historic distribution of American crocodiles in southern Florida has been debated for many years, but the overall range of American crocodiles probably has not changed substantially over the past 200 years. Historically, American crocodiles occurred at least as far north on the Florida east coast as Lake Worth, Palm Beach County, to Tampa Bay on the west coast, and as far south as Key West. The distribution of crocodiles during the non-nesting season may vary considerably among years since adult crocodiles can disperse great distances. However, the majority of crocodiles are present in the vicinity of core nesting areas, located near Biscayne and Florida bays.

The current distribution of the American crocodile is limited to extreme South Florida, including coastal areas of Dade, Monroe, Collier, and Lee Counties. Occasional sightings are still reported farther north on the east coast, and a few isolated crocodiles still survive in remnant mangrove habitats in Broward County. Along Florida's southwest coast, several small groups and individual crocodiles have been documented from Sanibel Island, Lee County, south to Collier Seminole State Park, Collier County. Very few reliable reports are available for the Ten Thousand Islands area. Crocodiles are regularly seen in Everglades NP along the mainland shoreline of Florida Bay from the Cape Sable peninsula east to U. S. Highway 1. The species also occurs in Cuba, Hispaniola, Jamaica, Trinidad, and Margarita; and the Atlantic Coast of Mexico from the Bay of Campeche south through the offshore islands of Belize to Venezuela and Colombia. On the Pacific Coast it is found from Sinaloa, Mexico, and the Tres Marias Islands south to coastal Ecuador and the Rio Chira in Peru. Throughout their range, American crocodiles are sympatric with other crocodilians, although they tend to inhabit more saline waters than most other species. In Cuba they overlap with the Cuban crocodile (Crocodylus rhombifer ) and in Central America and southern Mexico with the common caiman (Caiman crocodylus ) and Morelet's crocodile (Crocodylus moreletti ). The American crocodile and alligator are sympatric in brackish-water portions of their range in South Florida, but, due to evolutionary divergence, no hybridization would be expected.

Crocodiles were listed as endangered throughout their range in 1975 and critical habitat was established for this species in 1979. Population declines are most likely associated with habitat alterations and direct human disturbances to crocodiles and their nests.

Historic estimates of the American crocodile population in South Florida are difficult to substantiate because many records are anecdotal and early observations may have been confused with sympatric alligators. In addition, estuarine habitats, preferred by crocodiles, were remote and inaccessible to early settlers, thereby precluding reliable and consistent observations. Between 1,000-2,000 American crocodiles were estimated to exist in South Florida in the early 20th century, but this probably underestimated the population because extensive settlement and associated hunting had already occurred by this time. During the late nineteenth century and the first half of the twentieth century, many of Florida's crocodiles were collected for museums and live exhibits. The species was also legally hunted for its hide and flesh in Florida until about 1962. By the mid 1970s, crocodile numbers had been reduced to between 100-400 non-hatchling individuals. Combined, many of the natural and anthropogenic factors have resulted in adverse effects to the American crocodile. Compared to the historical estimates of 1,000-2,000 animals, populations have declined, and shifts in the nesting distribution have likely occurred. The lowest estimated population levels apparently occurred sometime during the 1960s or 1970s, when the Florida population of the American crocodile was estimated to be between 100-400 non-hatchlings. Kushlan and Mazzotti (1989) estimated that 220-278 adult and subadult crocodiles remained in South Florida, while others believe 500-1,000 individuals persist there currently.

The American crocodile population in south Florida has increased substantially since the 1970s, while still remaining well below historic numbers. The recent increase is best represented by changes in nesting effort. Survey data gathered with consistent effort indicate that nesting has increased from about 20 nests in the late 1970's to about 48 nests in 1995. Since female crocodiles produce only one clutch per year, it follows that the population of reproductively active females has more than doubled in the last 20 years. In addition, since at least a portion of the population's sex ratio approaches 1:1, it is likely that the male portion of the population has also increased substantially.

Threats

In addition to the taking of individual crocodiles, habitat modification and destruction has been occurring since the human settlement of south Florida. Formerly occupied habitats from Lake Worth, Palm Beach County, south to central Biscayne Bay, Dade County, have been largely destroyed by urbanization, and crocodiles have been essentially extirpated from these areas. Similarly, urbanization of the middle and lower Florida Keys has led to habitat degradation and loss. Though crocodiles were never abundant in these areas, further habitat loss limits opportunities for dispersing crocodiles to persist in the middle and lower Keys. Crocodiles were also probably never common along Florida's west coast. Urbanization there has also substantially altered much of the habitat once occupied.

Human encroachment into estuarine habitats can disturb crocodiles to such an extent that normal behavior patterns are altered. As recreational demands increase on public lands, indirect disturbance by apparently innocuous human activities such as camping, fishing, and boating are expected to increasingly affect crocodiles. Observations suggest that repeated close human presence may cause female crocodiles to abandon nests or relocate nest sites. Recreational boating including use of jet skis, has been limited in portions of the American crocodile's habitat within Everglades NP, but public demands for additional recreational opportunities will likely threaten these sanctuaries in the future.

Crocodiles are frequently killed on U. S. Highway 1, and Card Sound Road. On average, three to four crocodiles are killed annually while crossing these roads. Unfortunately, subadults and adults make up the majority of road mortalities. Efforts to preclude crocodile movement across portions of Card Sound Road by fencing sections of the road have been largely unsuccessful, due primarily to improper installation of the fence.

Natural, catastrophic, stochastic events such as hurricanes also are known to adversely affect American crocodiles and may be one of the most important factors limiting the number and distribution of this species in South Florida. Crocodiles are long lived and suffer high juvenile mortality and must, therefore, produce many young over their lifetime to ensure sufficient recruitment and population persistence. Natural events that add substantial adult mortality can result in long periods of little or no recruitment. Failure to successfully recruit age classes in consecutive years can, if repeated periodically, depress small populations.

Crocodiles undoubtedly perish during tropical storms and hurricanes that make landfall in extreme south Florida. The tidal surges, rough seas, and high winds probably result in direct mortality, but may also erode important nesting beaches, destroy nests, and alter other important habitat features. The adverse effects of tropical weather have not been quantified or reported extensively in the literature. Some suggest that the occurrence of major hurricanes at regular intervals may be a factor that serves to hold the Florida crocodile population at some depressed level. Even though extreme south Florida is considered sub-tropical, it is occasionally exposed to sub-freezing temperatures. The effect of freezing temperatures on American crocodile populations is not well known, principally because crocodiles which may be killed during freezes are rarely found. Critical minimum water temperatures are not known, but water temperatures of 55-57°F (13-14°C) in sheltered canals did not result in crocodile mortality during an extremely hard freeze in southern Florida during 1989. Unconfirmed reports identified four dead crocodiles in exposed areas after this freeze but noted that mortality was likely much higher since dead crocodiles were difficult to find. A substantial decline in nesting effort was documented during the following spring, and suggests that adult mortality during the freeze may have been responsible for the observed decline in nesting.

Water salinity affects habitat use and may be locally important especially during periods of low rainfall. Although American crocodiles have salt glands that excrete excess salt and physiological mechanisms to reduce water loss, maintenance of an osmotic balance requires access to low salinity water for juveniles. Hatchling crocodiles are particularly susceptible to osmoregulatory stress and may need to have brackish to fresh water available every four to five days. Crocodiles more than 7 oz (200 g) have sufficient mass to withstand osmoregulatory stress and are not typically believed to be affected by drought. Freshwater needs of the crocodile are usually met with frequent rainfall which results in a layer of fresh water on the surface that may persist for several days after rainfall. Hatchling crocodiles are probably stressed and occasionally die during periods of low rainfall. Anthropogenic changes in the amount and timing of freshwater flow to south Florida may have resulted in shifts in the distribution of American crocodiles. Unfortunately, detailed data on crocodile distribution is only available since the early 1970s and any changes that may have occurred due to hydrological perturbations over the past century cannot be identified with available information.

Conservation and Recovery

Protection of the American crocodile outside of the United States was enhanced when most countries throughout the range of the species became signatories to the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). CITES signatories agreed that, as an Appendix I species, the American crocodile would be afforded protection from international commerce. This protective measure has greatly reduced, and in some cases eliminated illegal harvests of the crocodile for its hide. Other protective measures include prohibitions against hunting all crocodilians in Mexico, and establishment of no-hunting areas in certain portions of Cuba. In 1984 the U.S. Fish and Wildlife Service prepared a recovery plan for the American crocodile. Numerous conservation measures were identified in the recovery plan that were needed to ensure persistence and recovery of the crocodile in south Florida, including securing habitat for all life stages and establishment of self-sustaining populations at natural carrying capacity in appropriate habitats. In addition, the recovery plan for the American crocodile called for research to determine habitat needs, habitat distribution, ownership, and habitat availability to crocodiles. Management options include control of human-related mortality, educating the public, reducing natural mortality, and protecting nest sites.

Recovery efforts for the American crocodile are underway and are likely responsible for increases in the number of crocodiles in South Florida. About 6,500 acres (2640 hectares) have been acquired for protection of crocodiles and other imperiled species at Crocodile Lake National Wildlife Refuge (NWR). Six hundred fifty acres (262 hectares) of this area are wetlands and open water habitats that directly support crocodile conservation. Crocodile habitat is also protected in Everglades NP and Key Biscayne NP, J. N. Ding Darling NWR, Collier Seminole SP, and Key Largo Hammocks State Botanical Preserve. The only habitat extensively used by crocodiles that is not under public ownership is the habitat created by construction of Florida Power and Light's Turkey Point electrical generating facility.

Crocodile nesting continues to be monitored by the Game and Fish Commission, Florida Power and Light, and National Park Service. In 1984, crocodile crossing signs were erected along U. S. Highway 1 to provide public awareness and reduce automobile/crocodile collisions. During future road widening of U. S. Highway 1, box culverts will replace existing small diameter culverts to allow crocodiles to pass under the highway. Fencing also may be erected along portions of U. S. Highway 1 to discourage crocodile movement over the road.

The timing and frequency of the freshwater hydroperiod substantially influences the health of the estuarine environment in south Florida and may be one of the most important large scale factors influencing crocodile populations on the mainland. It is well known that historic alterations to the natural flow have directly affected plant and animal communities. Although there is no direct causal relationship between freshwater flow alterations and American crocodile numbers, some of the population decline witnessed through the 1970s probably was attributable to changes in the amount and timing of surface water flow to South Florida. Future changes in hydrology that mimic natural flow conditions are likely to benefit crocodiles in the long-term, but care should be taken to ensure that changes in the delivery of water do not result in catastrophic, short-term, adverse effects. When added to all other natural and anthropogenic sources of mortality, such habitat changes could have substantial impacts on crocodile nesting and hatchling survival. As advances in water management are made in south Florida, research is expected to continue to assess the effects of changes in the amount and timing of water delivery on the American crocodile.

The availability of fresh water is essential to hatchling crocodile survival. Instream freshwater flow and rainfall provide this water to hatchlings emerging from mainland nests, but hatchlings from islands depend solely on rainfall. During periods of low rainfall, island hatchlings do not gain mass and are less likely to survive during winter months. To increase hatchling survival and recruitment, it was suggested that supplemental sources of fresh water be provided during the three to four month period following hatching. Supplemental sources of fresh water may be particularly important since recent efforts to restore functioning mangrove wetlands in Crocodile Lake NWR has increased salinities in an important crocodile nursery area. Restoration of suitable salinities in this area should be considered if future monitoring indicates low hatchling growth and survival.

Encroachment of exotic vegetation has degraded thousands of hectares of wildlife habitat in south Florida. In coastal areas, and on Key Largo, Australian pine, cajeput, and Brazilian pepper aggressively invade levees and berms. Widespread invasion of C. equisetifolia and to a lesser extent M. quinquenervia and S. terebinthifolia was found at crocodile nesting sites on Key Largo. Many of the exotics were removed during habitat restoration efforts in 1994, but vigorous regrowth and reinvasion is inevitable, and periodic efforts to control exotic vegetation will likely be required to maintain suitability of crocodile nesting sites. Invasive exotics are also encroaching on crocodile nest sites at Turkey Point. However, if measures outlined in Florida Power and Light's crocodile management plan are followed, exotic vegetation would be controlled before it threatened crocodile nesting sites. Renewed efforts may be needed to control exotic plants at Turkey Point. Exotic plant control in Everglades NP should continue. Australian pine has been found, and destroyed by Park staff, on nesting beaches and keys. Management programs or land-use restrictions are used on some public lands to protect and conserve natural resources. In Everglades NP, closure of water bodies has reduced boat traffic and minimized human-crocodile encounters. Unfortunately, restrictions on land and water use are now being challenged and increasing demands for recreational opportunities may threaten crocodiles in some areas. Although human exclusion may be the best management technique for protecting crocodiles and their habitat, it is clear that increasing numbers of the general public do not support this management alternative.

Though management of the physical components of crocodile habitat are essential to the continued survival of this species, emphasis must be placed on minimizing the potential for human-crocodile encounters. Human tolerance for and acceptance of increasing crocodile numbers is one of the primary reasons for the increase in population numbers over the last 20 years. However, as the crocodile population continues to increase, we anticipate an increasing number of human-crocodile conflicts. Unfortunately, dredging of shallow waters and creation of exposed shorelines have resulted in artificial habitats that attract crocodiles to areas adjacent to human habitation. Although American crocodiles are generally considered to be non-aggressive, the public's perception of them is that of a large, dangerous carnivore. If crocodile numbers continue to increase, more encounters will result in an increasing intolerance of crocodiles and more demands for action to reduce human-crocodile conflicts.

Part of the reason for increasing conflicts is that humans have altered the landscape for residential, commercial, or recreational purposes without completely rendering this formerly potential crocodile habitat completely unsuitable. The expanding crocodile population has exploited, and will continue to move into, these habitats and occasionally come into conflict with humans. In order to reduce conflict, one or more of the following must occur: crocodiles must stop using artificial habitats, artificial habitats must be removed or made unavailable, disruptive recreational uses must be seasonally excluded from sensitive habitats, or public education must increase human tolerance of crocodiles.

It is unlikely that the expanding crocodile population will cease using artificial habitats. These areas provide important components of crocodile habitat including basking, nesting, nursery, and deep water refuge. We are even less likely to substantially modify human use of already altered land. For example, homeowners are not likely to abandon their houses because crocodiles bask or nest in their yards. Similarly, filling of deep water channels is improbable since these provide water craft access to waterfront home sites. Seasonal restrictions for disruptive recreational uses such as power boating, jet skis, camping, etc. may be appropriate near crocodile nesting locations. In other areas, new or increased recreational access may not be appropriate since recreational use could result in greater human-crocodile conflict. Implementing recreational restrictions will be difficult, as demands for access continue to increase. However, public education can provide the foundation for developing positive, pro-active attitudes about crocodile conservation. Aggressive public education is probably the most effective method available to ensure the continued growth and recovery of south Florida's American crocodile population.

Throughout the remainder of its range, the American crocodile has suffered from threats similar to those that have adversely affected the species in south Florida. Unfortunately, only Costa Rica and Venezuela have adequately protected the American crocodile and its habitat, although Cuba protects a number of areas with large crocodile populations. Other countries have few or no laws to protect them or are unable to enforce conservation laws that do exist. Current threats to the continued survival of the American crocodile outside of the United States include changes in agricultural, ranching, and forestry practices that affect coastal habitats; developing tourism industries that seek to benefit from tropical, beachfront properties; and legal and illegal hunting. As natural habitats are destroyed and replaced with landscapes that benefit humans, American crocodiles will become increasingly susceptible to the public's intolerance of human/crocodile conflicts.

Contacts

U. S. Fish and Wildlife
Service South Florida Ecological Services
1360 U. S. Highway 1, Suite 5
Vero Beach, FL 32960-4725
Telephone: (561) 562-3909
Fax: (561) 562-4288

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

References

Britton, A. 1999. Crocodylus acutus (Cuvier, 1807). http://www.flmnh.ufl.edu/natsci/herpetology/brittoncrocs/csp_cacu.html.

Kushlan, J. A. and F. Mazotti. 1989. "Historic and present distribution of the American crocodile in Florda." Journal of Herpetology 23: 1-7.

Kushlan, J. A. and F. Mazotti. 1989. "Population biology of the American crocodile." Journal of Herpetology 23 (1): 7-21

Thorbjarnarson, J. 1989. "Ecology of the American crocodile, Crocodylus acutus." In: Crocodiles. Their Ecology, Management and Conservation. A Special Publication of the Crocodile Specialist Group. World Conservation Union (IUCN), Gland, Switzerland.

U. S. Fish and Wildlife Service. 1997. "Crocodile, American (Crocodylus acutus )." http://endangered.fws.gov/i/c0u.html