CROCODILIANS: AN ORDER OF FEAR

By: Nathan Moyer

Introduction

Crocodilians are possibly the most feared order on Earth!  Crocodilians are amazingly successful animals in their natural environment.  They have an ancient complex evolutionary history, dating back to the times of the dinosaurs.  There is much to be learned from this highly evolved, immensely wonderful order of reptiles.  However, time may be running out because of the critical state of many of today’s species.

How do you conserve an animal that most people fear and have little love for?  This has been a major question and problem for the conservation of crocodilians worldwide.  This problem is especially prevalent in the conservation-orientated country of Costa Rica. 

Evolutionary History

The evolutionary history of the crocodilians and their relationship to other animals is highly unclear, although there has been extensive work on this subject.  Traditionally, the relationship between species is determined by “Linnean ranks”, which is based on shared characteristics (Britton 2002).  For instance, crocodilians and lizards have many similar characteristics and are hence closely related.  This may not be the most accurate way to examine evolutionary relationships of species; therefore a less subjective alternative rank system is increasingly used called the phylogenetic taxonomy. 

There is confusion as to where the crocodilians evolutionary fit among the reptilians.  They are in the class Reptilia, a group that excludes birds, even though it is generally understood crocodilians are more closely related to birds than snakes, lizards, or turtles (Naish 2001 and Britton 2002).  Although birds belong to the class Aves (not Reptilia), they are placed in the subgroup Archosauria with the crocodilians.  A recent molecular phylogeny study by Hedges and Poling (1999) shows that crocodilians may be more closely related to turtles than birds, although morphological and paleontological evidence is unclear.  

Figure 1. Extinct crocodilian body types (Naish 2001).

There is perhaps more confusion understanding the relationship of current crocodilians with their fossil relatives.  Crocodilians and other major groups of living reptiles formed their origins during the Triassic period (Hedges and Poling 1999).  Some fossil crocodilian’s body types varied greatly, including marine forms with paddles and fins, long-legged terrestrial forms, duck-billed aquatic giants, and burrow-dwelling herbivores (Fig. 1).  They also varied in size, with the Sarcosuchus maximum body length estimated to be at least 12m (Sereno and others 2001).  Figure 2 displays two taxonomic schemes for the crocodilians with great discrepancy among classification names and relationships.                                                                                                                                                                                                                                                                                                                                                                                                             

                                                                                                        

Figure 2. Two taxonomic schemes for crocodilians (Naish 2001, Sereno and others 2001).

Modern crocodilians are a recent evolutionary phenomenon (Naish 2001).   This opposes the popular incorrect view of crocodilian evolution as static.  Although they possess several features similar to their ancestors, such as body structure, they have constantly evolved in order to survive their continuously changing environment. 

 Order Crocodylia

 There is also confusion and debate on the classification and relation of current crocodilian species.  There are currently three surviving families of crocodilians, Alligatoridae, Crocodylidae, and Gavialidae, with between 21 and 28 species.  The reason for the variation in number is both because of confusion of names that are not worldly accepted, and classifying subspecies as individual species.  The most widely used taxonomic classification states there are 23 living Crocodilian species (Fig. 3) as well as 5 subspecies (Britton 2002).

Figure 3. Widely used taxonomic classification of crocodilians

 

Gavialidae

The family Gavialidae only has one surviving species, Gavialis gangeticus, the Indian gharial.  They inhabit only six river systems within and surrounding India.  They live in calmer deep areas of fast moving rivers and seldomly leave the water.  Gharials are identified by their long narrow snout (Fig. 4).  Members of this family historically were located in Africa and Australia with one species extinction as few as 1600 years ago (Naish 2001).  They arose in the Cretaceous period with members in South America (Grenard 1991).  

Crocodilidae  

The family Crocodilidae consists of 14 species (2 subspecies) in 2 genera.  Species from this family are considered the true crocodiles.  Crocodiles arose in the late Cretaceous era in either North America or Eurasia (Grenard 1991).  They can be recognized with their “V” shaped jaw (Fig. 4), exposed lower teeth when their mouth is closed, and the presence of sensory organs located on the body (see below).  Many species of crocodiles can grow to be very large, including the American crocodile (Crocodylus acutus).    

   Figure 4. Typical Gavialidae (top), Crocodilidae (middle) and Alligatoridae (bottom) head and  snout shape (Britton 2002). 

Alligatoridae

The Family Alligatoridae consists of 8 species (3 subspecies) in 4 genera.  This family included both alligators and caimans.  Alligators can be recognized with their wide rounded snout (Fig. 4) and lack of exposed lower teeth when their mouth is closed.  There are only 2 species of alligators, the American Alligator (Alligator mississippiensis) and the Chinese Alligator (Alligator sinensis).  Caimans may or may not have a broad snout, but never expose lower teeth when their mouth is closed.  All caimans live in South and/or Central America.  This family included the extinct species Deinosuchus who was the largest of all known crocodilians and may have reached 10 m in length (Naish 2001). 

                                                                                                                                                                                                   

Distribution                                                                                                   

Crocodilians are geographically distributed in 91 countries and islands (Fig. 5) (Britton 2002). Crocodilians are generally found in the tropical and subtropical regions, being unable to survive and reproduce successfully in cold climates (Alderton 1991).  The neotropics are home to all but one of the Alligatoridae family species (Chinese alligator) and only four species that belong to the Crocodilidae family.  The old world tropics are home to the remaining Crocodilidae species, the Chinese alligator, and the Indian Gharial. 

 

Figure 5. Geographic distribution of crocodilians (Britton 2002)

Biology

There are many biological characteristics common to all crocodilians that distinguish them from other reptiles.  The order is easily recognized by their body structure of low-walking short legs and powerful tail.  However, their most recognizable feature is the head, with huge powerful jaws filled with large pointed teeth.

Size

Crocodilian’s size is highly variable depending on species, with adults ranging from 1.4m to 7m.  Growth continues throughout the lifespan of all crocodilians (Grenard 1991).  The largest species of crocodile in the world is Crocodylus porosus, the saltwater crocodile (Britton 2002).  The largest Nile crocodile recorded in captivity (died in 1997) was Gomek who reached 5.5m in length (Fig. 6). 

Figure 6. Gomek (the largest Nile crocodile recorded in captivity) feeding (Britton 2002). 

Lifespan 

Measuring the longevity of crocodiles can be very difficult, because they live for such a long time.  Crocodilians in the wild are thought to live to about 70 years, perhaps in some cases up to 100 years.  However, many caimans are estimated to live only 30-40 years.  The oldest recorded crocodilian died at the age of 115 while in captivity (Britton 2002).                   
                                                                                                                                                                                                                                                                                                                   Sight

The eyes of crocodilians are suited for both day and night vision (Grenard 1991).  At night their eyes reflect away light because of a layer called tapetum lucidum, which causes their eyes to light up when light is shined on them.  They have a transparent eyelid, called the nictitating membrane, which allows them to have partial vision underwater. 

Hearing

Crocodilians have keen hearing.  Their ears are located just next to the eyes on each side of the head and have tissue flaps (Alderton 1991).  These tissue flaps shield the ears when the crocodilian is submerged.  They are sensitive to a wide range of frequencies, allowing them to hear their young calling from within its eggshell. 

Skin

Crocodilians skin consists of many scales, which vary in shape and strength.  They do not shed their scales all at once, as snakes do, instead they replace scutes individually (Grenard 1991).  Some scales along the back contain bony deposits called osteoderms.  The extent of osteoderms varies among species and individual populations.  The number of osteoderms reduces value in the crocodilian skin trade. 

Thermo-Regulation

All crocodilians are cold-blooded, which means they rely on their environment for warmth.  They bask in the sun in the morning to absorb heat, retreat back to the water during the extreme heat of mid-day, and bask again in the late afternoon before going back in the water overnight (Grenard 1991).  The weather influences how long they bask. 

Palatal Valve

The palatal valve is a flap found on the back of the throat of all crocodilians (Fig. 7) (Britton 2002).  A crocodile's mouth is not watertight when closed, and water easily enters when the crocodile submerges. The palatal valve plays an essential role in preventing water in the mouth from entering the throat, esophagus and trachea when the crocodile is underwater. This is crucial during prey capture and drowning.

   

  Figure 7. Palatal valve (left) found at the back of all crocodilian throats and integumentary sense organs (right) near the jaws of all crocodilians (Britton 2002).

Sensory Organs

All crocodilians possess integumentary sense organs (ISOs) (Fig. 7). In the Alligatorinae family the only place that ISOs are found are the upper jaw, nose, around the eyes, and lower jaw.  ISOs in members of the Crocodylinae and Gavialinae family are distributed all over the body.  Their function has not been conclusively determined, but they are believed to realize pressure changes, detect prey underwater, or realize changes in the salinity of the water (Britton 2002).   

Mobility

Crocodilians are excellent swimmers.  They have webbed feet to aid in swimming, but their powerful tail provides maximum thrust.  They can either glide undetected through the water or surge forward by moving their tail side-to-side.  There are three main ways crocodiles move on land (Britton 2002).  The first is called the belly crawl, which is done by sliding on its belly while its legs and tail thrust it forward.  This often occurs on muddy riverbanks and for short distances.  Another land locomotion is referred to as the high walk.  Crocodiles will walk with their entire abdomen off the ground when traveling long distances on land.  This is not a fast means of transport.  When crocodilians need to travel a long distance on land quickly, they use a galloping stride (Fig. 8).   

   

   

Figure 8. Four stages of a crocodilian gallop (Britton 2002).

Communication

Crocodilians use a variety of calls, smells and water behavior to communicate with each other.  They are the most vocal of all reptiles.  Some species can communicate over 20 different kinds of messages through sound alone, including threat calls, distress calls, hatching calls, and a courtship bellow (Britton 2002).  Crocodilians also communicate by means of smell.  They have four scent glands, of which two are along the lower jaw and two are within the cloaca.  While in the water crocodilian can tremble their body while lying just below the surface of the water to send ripples reverberating over large distances.  It is also common to see them slap their head on the water and slash their tails, which are other communication methods (Alderton 1991).

Social Interactions

Crocodilians are generally solitary animals, though at times they do congregate (Alderton 2002).  This occurs mainly during breeding periods and dry spells when water is scarce.  Some males clearly dominate these territories, and conflicts are unlikely.  In the case of an actual fight, the weaker one usually survives.  Fights are most common during the breeding season, when individuals will guard their territory. 

Diet

Crocodilians are carnivorous animals whose diets change with age.  They kill with their powerful jaws, which can shut ay 1300 pounds per square inch (Grenard 1991).  A juveniles diet consists of insects, small amphibians and fish, while small adults eat fish, mammals and birds.  Adult crocodilians eat larger mammals and other reptiles for their main source of food.  They do not eat every day and are able to survive for months if there is no food to be found by relying on stored fat. 

Reproduction

Crocodilian reproduction is similar among all species.  The breeding periods vary among species and are dependant on both air temperatures and rainfall (Alderton 1991).  The start of the breeding period is marked with courtship rituals including bellowing and head slapping.  Mating usually occurs in the water and can last up to 3 hours.  Crocodiles take several partners each breeding season.  Larger individuals are more successful at attracting partners and will generally mate more frequently. 

Females build their nest approximately one month after mating.  Most nests are built as a mound of earth and vegetation, while others nest in flask shaped holes built into the ground.  The nests are constructed over a course of several weeks.  The nests must be above any level of likely flooding during the incubation period.  The eggs must remain between 86 and 89.5 F throughout development, as temperature during incubation determine the hatchlings sex. 

Predation rates for hatchlings are very high.  They are preyed upon by a wide range of predators such as birds, otters, cats and older crocodilians.  In one year of growth hatchlings can triple in size.  They remain in their juvenile phase until the age of four. 

Crocodilian Ecology

Crocodilians are important species for their ecosystems (Britton 2002).  As adults they are the top predators and keep prey species populations in check.  Some are also considered “keystone” species, which are those that exert a disproportionate influence on community structure relative to its abundance or biomass and would subsequently change the composition of a community or habitat upon its removal (Zacharias and Roff 2001).  Crocodilians will burrow into the ground to create pools to raise their young.  During dry periods these pools remain as a sanctuary for many aquatic species and watering hole for others.  Removal of crocodilian species could result in detrimental effects to the entire ecosystem. 

Conservation 

Of all reptiles, human exploitation tends to be the greatest on crocodilian and turtles species.  The last few decades have shown the public turn against turtle exploitation.  Unfortunately, crocodilians are not as easy to love as turtles and their future is much less secure (Janzen 1995).

The two decades following World War II witnessed a worldwide demand for crocodilian leather, which led to a major decline in most crocodilian populations.  Hatchlings have very high mortality rates and it takes between 5-10 years to reach sexual maturity.  Therefore, killing adults, who have zero to a few predators, would cause a major decrease in reproduction (Alder and Haliday 1986). 

Many crocodilian species have faced population crashes due to human exploitation.  The Indian gharial was pushed near extinction in 1974 with as few as 50-60 adults remaining (Alder and Haliday 1986).  The American alligator (Alligator mississippientsis) and saltwater crocodile (Crocodylus porosus) both reached very low populations until their hunting was banned. 

The status of crocodilians is based on Convention of International Trade in Endangered Species of Wild Fauna and Flora (CITES) as either threatened with extinction (I) or may become threatened with extinction if trade regulations are in place (Grenard 1991).  Because of the status of crocodilian species, many countries provide legal protection by having the international trading or selling of skin banned.  Many crocodilian species populations have rebounded since their protection. 

Legal protection may not be adequate for saving some crocodilian species.  The American crocodile (below), along with other species, is threatened by opportunistic killing, accidental drowning in fishnets, and habitat destruction (Platt and Thorbjarnarson 2000).  Today, there are probably less than 150 Chinese alligators (Alligator sinensis) left in the wild and without changes towards conservation, they could disappear in as few as 15 years (Britton 2002).  Even in areas where populations are high and it is legal to commercially harvest crocodilians [such as the spectacled caiman (Caiman crocodiles) in Venezuela], an economic crisis can increase harvests, reducing populations to critical levels (Rodriguez 2000).  

Crocodilians of Costa Rica   


According to Janzen (1995) there are two species of crocodilians present in Costa Rica, the American crocodile and spectacled caiman.  The spectacled caiman are one of the smallest neotropical crocodilian species, reaching only about 2 m as adults.  They have the widest distribution of any species in the Alligatoridae family and are located from southern Mexico to the Panatal floodplain in Brazil (Fig. 9) (Britton 2002).  In Costa Rica they are found in both Atlantic and Pacific lowlands, occupying small creeks, ponds, playas, and occasionally brackish mangrove swamps and stormtide inner beach lowlands.  Their populations are higher than American crocodiles, but have recently decreased in areas near human habitation (Janzen 1995). 

Figure 9. Spectacled caiman distribution (Britton 2002).

American crocodiles occur on both coasts of Costa Rica and several miles upstream many larger river systems (Platt and Thorbjarnarson 2000).  Their population ranges from southern United States, Central America and northern South America (Fig. 10) (Britton 2002).  Crocodiles are completely protected in Costa Rica but do to some recent human fatalities, people are against their conservation and authorities are finding it difficult to enforce the laws (King 2002).  

Figure 10. American crocodile distribution (Britton 2002).

Three human fatalities by American crocodile attacks occurred between September 1995 and May 1998 in Costa Rica (King 2002).  The first occurred on the Tempisque River where a fisherman’s boat overturned and he was attacked as he tried to swim across the river to get help.  Another victim was a man who was taking a swim in a lagoon when he was attacked by a gigantic crocodile nearly 6m in length.  The most recent occurred on the Jesus Maria River when a resident was attacked as he swam to untangle a fishing line.  These attacks generated a climate of insecurity, fear and hatred to all crocodiles by many Costa Ricans. 

Following these attacks, there have been many random and direct attacks on large crocodiles.  Killings were especially bad on the Tarcoles River, where tourism is big.  In 1998 there were twelve dead crocodiles found in the river with only about ten individual adults remaining at the end of the year (King 2002).  The Crocodile Specialist Group (CSG) worked with the Vice Minister of       

Wildlife, Environment, and Energy Department to propose steps to address the situation in 1999.  Some steps involved notifying judges about the laws, notify the national fishing agency asking them not to fish within two miles from the mouth of the river, environmental education, and notifying tour guides not to feed crocodiles.  Despite these actions, problems continued to escalate because of lack of funds and even the murdering of a CSG member at Tarcoles.  As of 2000, the situation was reported to be in chaos and intervention of Costa Rica’s government was anticipated.                            

Despite being urged not to, some tour groups have continued to feed large crocodiles for extra money from tourists.  The website Crocodile Safari (2002) describes an American tourists witness of such a feeding (Fig. 11).  This practice poses immediate danger to the tour guide and any human near the river, as the crocodiles associate people with food.  It is important to note that not all tour guide teams of the Tarcoles River allow such illegal practices. 

  

Figure 11. Safari guide feeding American crocodile in the Tarcoles River in Costa Rica (Crocodile Safari 2002).

It is suggested that small-scale ecotourism be used as a crocodile conservation mechanism in Belize, which could be applied in Costa Rica (Platt and Thorbjarnarson 2000).  This could create a value for living crocodiles in their natural habitat.  The Crocodilian Specialist Group promotes better understanding of crocodiles in Costa Rica and hopes that their efforts will enable crocodiles and people to continue to co-exist (King 2002). 

   

           

References:

Alder, K. and T.R. Haliday. 1986. Reptiles and Amphibians. Torstar, New York. 160 p.

Alderton, D. 1991. Crocodiles & alligators of the world. New York. 190 p.

Britton, Adam. 2002. Crocodilian Biology Database. <http://www.flmnh.ufl.edu/natsci/herpetology/brittoncrocs/cnhc.html>. 3/27/02

Crocodile Safari. 2002. Tarcoles, Costa Rica. <http://www.angelfire.com/va2/vern/#DOWN BY THE RIVER>. 5/7/02

Grenard, S. 1991. Handbook of alligators and crocodiles. Malabar, Fla. 210 p.

Hedges, S.B. and L.L. Poling. 1999. A Molecular Phylogeny of Reptiles. Science. 283:118-1001.

Janzen, D.H. 1995. Costa Rican Natural History. Chicago Press, Chicago. 816 p. 

King, F.W. 2002. Crocodilian Specialist Group Newsletter <http://www.flmnh.ufl.edu/natsci/herpetology/crocs/CSGnewsletter.htm>. 3/27/02

Naish, D. 2001. Crocodilians. Geology Today. 17:71-77.

Platt, S.G. and J.B. Thorbjarnarson. 2000. Status and conservation of the American crocodile, Crocodylus acutus, in Belize. Biological Conservation.  96:13-20.

Rodriguez, J.P. 2000. Impact of the Venezuelan economic crisis on wild populations of animals and plants. Biological Conservation 96:151-159.

Sereno, P.C., H.C. Larsson, C.A. Sidor, and B. Gado. 2001. The Giant Crocodyliform Sarcossuchus from the Cretaceous of Africa. Science. 294:1516-1519.

Zacharias, M.A. and J.C. Roff. 2001. Use of focal species in marine conservation and management: a review and critique. Aquatic Conservation: Marine and Freshwater Ecosystems. 11:59-76.