This Spider Lassos Her Prey from the heavens above! (Monteverde Cloud Forest)
I. Crocodilians are a large family with a long history dating back to the Mesozoic. There are several subtypes of crocodilians, including crocodiles, caimans, and alligators. There are no alligators in Costa Rica, but there are several species of caiman, particularly the Spectacled Caiman (Caiman crocodylus) but also the brown or American Caiman, (Caiman crocodilus fuscus), a subspecies of C. crocodylus. This paper focuses solely on the crocodiles, of
family Crocodylidae and subfamily Crocodylinae. (Kricher) This grouping includes Old World Nile Crocodiles, (C. niliticus) and the Indo-Pacific Crocodile (C. porosus). In the Neotropics, of the American crocodile (C. acutus), the Morelet’s crocodile (C. moreletii) the Orincono crocodile (C. intermedius), and the Cuban Crocodile (C. rhombifer), only the American crocodile is found in Costa Rica. Therefore most of this paper is focused on C. actutus.
The nomenclature comes from the Greek krokodeilos which is translated as "pebble worm" (kroko = pebble; deilos = worm, also man) and refers to the elongated crocodile shape. Acutus means "sharp" or "pointed" in Latin and is probably describing the shape of the snout. (Britton, 2002) It is a large species but not the largest, is aggressive but not notably so, and has a large range. It is found in Belize, Colombia, Costa Rica, Cuba, Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Jamaica, Nicaragua, Mexico, Panama, Peru, Venezuela and the United States, (Britton, 2002) a small population in southern Florida.
Crocodiles have only recently come to prominence as an important species worthy or preservation; they have been widely hunted as a threat to humans and livestock, and killed for their skins. But today crocodiles are an identified “keystone species,” meaning one that maintains ecosystem structure and function by their activities. (Ross, J. 2000) They are known fo selective predation on fish species, recycling nutrients, and maintenance of wet areas in droughts by digging holes or burrows during dry periods. In this paper we will examine several aspects of the Costa Rican crocodile, including its genealogy, physiology, behavior, life cycle, history, and future prospects.
II. Crocodilian history is relatively well-known because their marshy habitats favor fossilization. They are the only living relatives of a large and ancient group of reptiles, the Archosauria, (“ruling reptiles,”) which also contained dinosaurs. They dominated the planet during the Mesozoic era, 245- 265 million years ago. (Ross, 1989) During this period some of their relatives reached 50 meters in length. The Archosauria was comprised of dinosaurs, pterosaurs, and the thecodontians, which may have been the precursor to modern crocodiles. This name describes the setting of teeth in the animals; thecodont teeth are set into the jaw sockets rather than fused to other sides of the jaw as is typical in other reptiles. (Ross, 1989) The earliest crocodilian and their immediate precursors existed 215 million years ago and were probably terrestrial predators, with long slender limbs and well-developed body armor. Only later did they become amphibious; in fact, they were probably fast runners. These animals, called protosuchian (suchia is a common ending of words relating to crocodiles, coming from the latin of Sobek, a crocodile god of ancient Egypt) (Ross, 1989) were hardly more than 1 meter long with a short snout.
The earliest known crocodilian–like reptiles among the archosaurs are the sphenosuchians, found in rocks dating to the late Triassic (230 mya) (Ross, 1989) and the oldest known true crocodiles are 200 million years old. These sphenosuchians were found in Europe, South America and South Africa (Ross, 1989); in fact, the genus Crocodylus was found all over the world in the tertiary period. The mesosuchians, intermediate crocodilian ancestors, were much more diverse than modern eusuchians. They seem to have retreated to the equator just before the Cretacious (65 mya) which may explain how they escaped the famous K-T mass extinction of that period. The crocodiles that we know today appeared right around this period some 80 million years ago, (Ross, 1989) and have since been restricted to mainly warm or temperate regions. However, crocodilians survived in Europe until less than 5 mya. They seem to have limited success and diversity there due to the modern cooling climate. (Ross, 1989)
Because it is known that they existed in the Cretacious, their design plan has lasted about 200 million years; few changes have affected the modern crocodile since then. (Ross, 1989) The evolutionary history of modern extant crocodiles is fairly obscure, but they probably evolved originated in Eurasia. (Alderton, 1998) Speaking of C. acutus specifically, it is likely to have evolved fairly recently, because it is mostly absent from the fossil record. Scientists believe that it probably evolved in the Caribbean and migrated to its current range. (Alderton, 1998)
III. Today crocodilians are the most advanced of all reptiles. They have a unique combination of reptilian and mammalian/avian characteristics – in fact, they are more closely related to birds than lizards. (Ross, 1989) They have a bird-like brain but no bladder and a reptilian digestive system. (Ross, 1989) Like the birds, crocodiles have an elongate outer-ear canal, a muscular gizzard, and complete separation of the ventricles of the heart. They are called advanced because of their four-chambered heart, reptile diaphragm, and cerebral cortex. (Ross, 1989)
The American crocodile is a relatively large species, with males having maximum lengths of 5–6 meter range, although some 7 meter individuals have been reported. (Britton, 2002) It is two-thirds as heavy as and American Alligator of the same length, and therefore moves more quickly. (Ross, 1989) Dorsal armor is irregular and much reduced in comparison with other species, and in front of each eye is a distinctive swelling. Skin is formed from a thick dermal layer covered with non-overlapping epidermal scales. The surface scales of each scute sloughs off individually rather than shedding in large patches. Adults are olive brown all over, while the lighter-colored juveniles are tan with banding on the body and tail. It also possesses a distinctive silvery iris. Like all crocodiles, the snout is pointed rather than rounded as it in Alligators, and the fourth tooth is visible when the jaws are closed - the same tooth of an alligator (both have this extra-long tooth) fits into the upper jaw.
Crocodilians have strong muscles that allow the jaws to snap shut with incredible force – they are capable of crushing bones, skulls, and cast iron. However, the muscles for opening the jaw are relatively weak, which allows even a rubber band to keep the jaws of smaller crocodiles closed. This explains the trick of `gator wrestling. The teeth are conical in shape and anchored to the sockets by connective tissue, and in C. acutus there are between 66 and 68 individual teeth. (Britton, 2002)They fall out throughout the lifespan and are replaced in alternate rows along the jaw. (Alderton, 1998) Crocodiles have no lips, meaning that they cannot close their mouths underwater. They are prevented from ingesting water, however, by a secondary palate which blocks the throat – the region inside the mouth is actually still external to the body. Both salt and freshwater – crocodiles, unlike alligators, have maintained salt glands on the tongue, even those that do not have contact with salt water, probably a holdover from their marine ancestry. (Alderton, 1998)
The brain is small but complex, and the senses are well adapted. They have an excellent sense of smell and a fine ability to hear, as evidenced by the complex vocalizations of crocodilian species. External ear openings are covered with a flap to protect the inner ear during diving. (Ross, 1989) Both the eyes and nostrils are high on the head, so that they are above the water when the crocodile swims. The eyes are typically vertebrate, with a vertical pupil and a reflective layer behind the retina. They have moveable eyelids and a third transparent eyelid called a nicitating membrane. They apparently posses color vision but they cannot focus under water, which indicates that they use other senses when submerged. (Ross, 1989)
They are highly evolved predators, highly motile and metabolically efficient. They have fast reflexes and effective locomotor ability on land, where they walk on erect legs, and in the water, where they use the long, powerful tail to swim. They move by lateral S-shaped undulations of the tail with limbs held close to the body. (Ross, J. 2000) On land they posses two distinct types of walk: the splayed low-belly sprawl of lizards, and the more mammalian “high walk” employed for in fast travel on land. The reason for this is the unusual ankle structure that they share with the early thecodontians, unique from other reptiles. (Ross, 1989) Their feet have unusual flexibility so the can support the crocs in various positions. They have five toes on the front feet and four on the back, all of which are partially webbed to aid in swimming. Their body systems are well adapted to the marine environment, where they attempt to maintain their body temperature within narrow limits by basking in the sun when cool and seeking shade or water when hot. (Ross, J. 2000) Like all reptiles they are ectotherms, and do not regulate their body temperature internally. However, the circulatory system can divert blood away from the peripheral systems during dives or to reduce heat loss. They have well-evolved metabolisms that use and store nearly the entirety of a meal. Because of this they ingest no more than 50 full meals a year (Ross, 1989) and a large crocodile can last a year without eating. Newly hatched crocodiles do not need to eat for four months, surviving on the stored fat from the yolk – in fact, 60 percent of the fat in crocodile diets is stored. The tradeoff for this efficient system is a low oxygen count in the blood (it can also impair growth.) Crocodiles recover slowly from exercise and are easily exhausted, and have high levels of lactic acid in the blood – however, they can also survive a blood acidity that would kill most other animals. (Ross, 1989) They are, however, excellent and adaptive hunters, with an ability to consume and digest almost everything they encounter. The stomach is the most acidic of any vertebrate. (Ross, 1989) Hatchlings eat mostly insects while juveniles eat fish, frogs, tutles, small mammals, and invertebrates. Larger crocodiles also eat larger mammals and birds, as well as up to 70% fish. Crocodilian snouts are related to their diets – narrow-snouted animals tend to favor fish, and a crocodile’s narrow snout reflects this tendency towards almost any fish they encounter.
IV. Crocodilians have complex behaviors including social interactions, communication, dominance hierarchies, and coordinated feeding. Socially, crocodiles have uniquely complicated lives, related more closely to birds and mammals than to other reptiles. They have a complex system of communication. It is apparent that crocodiles recognize each other and respond accordingly. Physical, vocal, and chemical factors probably all play a part, not all of which is completely understood. Chemosensory signals, for example, are widely suspected but little studied. It is likely that crocodiles, with a keen sense of smell, rely on musk for recognition and mating cues. There is much more that needs to be known in this area, but physical signals are widely reported and well understood, especially in connection with vocal and auditory clues. For example, many crocodilians are known to head-slap the water to assert dominance, snap their jaws in the water to produce a popping sound, and blow bubbles. (Ross, 1989) Tail thrashing should be interpreted as a threat and often precedes an offensive charge. Many of these cues indicate the arrival or presence of the speaker, but they are also used to attract attention or to indicate aggression. Another well-known communication, especially in American crocodiles, is the raising of the head, called snout lifting. This is a submissive signal used during courtship, mating, or between a dominant and subordinate male. In C. acutus it is used almost exclusively by females during daily social interactions. It probably conveys information about the individual’s sex. The exposure of the vulnerable underside of the throat is interpretated by researchers as a symbol of “good intentions.”
Vocally, American crocodiles do not lead the crocodilian pact. Alligators are especially vocal and are known to bellow for extended periods, and among crocodilians there are distinct and well-documented calls for a variety of purposes, including aggression, seduction, dominance, and distress. The American crocodile, however, is a particularly silent species. Some scientists theorize that the reason for this is the difference in habitats; alligators live in more densely vegetative environments that do not transmit sounds as far, resulting in more private conversation. It is also probably that much crocodilian communication is subauditory or “infrasound,” or otherwise thus far undetectable to researchers, for example auditory or aqueous vibrations. This may be particularly true of our American crocodile friends.
However, the calls of juveniles signaling distress will illicit help from even unrelated adults.
In fact, researchers playing these calls from recordings have been attacked by adult crocodiles coming to the rescue.
However they do it, it is apparent that crocodiles have used communication to set up complex dominance hierarchies. They are a regular feature of crocodilian daily life, with dominant individuals usually possessing large body size and aggressive temperaments. They are recognized by their fellow crocodiles as dangerous, and have access to the best mates, nest spaces, food, and living space. Subadult males and small females occupy low positions while large males rate the highest, as do the largest females in the female heirarchies. Dominant crocodiles are recognized by their approaching other individuals, while submissive individuals move away. It is also probable that body posturing tells individuals a lot about the status of the crocodiles they encounter. A dominant crocodile swims lazily at the surface, while lesser ones will submerge readily. Actual combat is comparatively rare, but dominant individuals grab lower-ranking ones at the base of the tail, where scarring or injury can occasionally occur. (Ross, 1989) Crocodiles are also territorial, defending large areas from males that pose a threat to their dominance or defending nest sites with ferocity. This behavior is however density-dependent and worst during the breeding season, a period of intense communication among crocodiles. The males first assert their position with increased aggression, and in American crocodiles large males fight with open jaws directed at their rivals. Actual injury is rare but the victor will win dominance, giving him access to the choicest mates. He will express his power by patrolling his territory conspicuously on the surface and self-assertion with headslaps or vocalizations. A female, once attracted, will initiate courtship by indicating submission – she must divert the male’s aggression away from herself and into the courtship display, or risk serious injury by the male. In American crocodiles the snout-lifting behavior is used by the female to signify her receptivity to the male. He will copulate for several minutes and then, depending on the density, both may proceed to a number of other mates, or may copulate together repeatedly over the next several days.
Much of the crocodile brain is occupied with feeding itself, and it is logical that much of their behavior is geared to this end. Crocodiles are generally idle hunters that lie in wait for their pray. They are adept at stealth and camouflage. Once they lunge, they are truly impressive predators; they can leap, run, and are incredibly fast and strong. They save energy in this manner. Cooperative hunting among crocodiles has been recorded, although not specifically among American crocodiles. In Africa they line up to block rivers so that migrating fish will be trapped against a barrier of crocodiles. They have been observed sharing foods that are too big for one animal to tear apart alone without apparent hostility, and they have been seen taking turns to dismember and eat a carcass. (Ross, 1989) These same crocodile barricades can be fatal to human beings, and human-crocodile interactions are generally unpleasant on both sides. American crocodiles are less aggressive than other crocodilian species and are not frequently implicated in man-eating attacks. American alligators, Nile crocodiles, and Indo-Pacific species are all known to attack humans more frequently than this species.
V. A crocodile’s life begins in the egg, buried with a clutch on the banks of a river or lake. Crocodiles lay eggs that are less brittle than bird eggs and bury them in mounds of rubbish and vegetation or sand. American Crocodiles are particularly known for digging hole nests, especially in Florida, but the extent of this behavior is not well known. Clutch size is typically in the 30–60 range, although in some populations mean clutch size is in the low 20s. As with most hole nesting species, C. acutus nests during the annual dry season with eggs hatching around the beginning of the annual rainy period. (Ross, J. 2000) The extent of parental care is varied – many females guard the nest assiduously, fasting until they hear the grunts of the young, and then dig them out, even cracking the eggs to release the hatchlings. In laboratory studies, the eggs of American crocodiles were seen to communicate even before hatching, where tapping sounds within an egg was replied to by neighboring eggs. It is possible that this communication facilitates the synchronized hatching of the clutch.
Between 90 and 100 days after being laid the eggs hatch, right at the beginning of the rainy season. The young crocodiles are around 25 centimeters in length. They are less vocal than other crocodilians, and within a few days they move away from the nesting area. Other crocodilians have a much higher level of parental involvement, with the mothers guarding the juveniles and keeping them together for up to several years in family groups. It is possible that this behavior in American crocodiles is an adaptation to the high degree hunting in the second quarter of the century (Alderton, 1998) and maternal care was more intensive in the past – groups are more easily hunted than individuals.
Even once the hatchlings are free of the nest, they remain highly threatened for their vulnerable period of youth. Fish, birds, small mammals (especially raccoons and small cat species) all eat the young crocodiles. (Ross, 1989) Beyond one meter in length, mortality of crocodiles decreases. They have few natural predators, especially if they can avoid being caught on land, although occasionally subadult animals can be killed by snakes or large predators such as leopards.
Females grow more slowly and reach maturity at a smaller size than males, who continue growing and usually exceed females in maximum size. Crocodilians can be long lived in the wild and there are records of particular individuals residing for decades (Ross, 1989) They will spend much of their time basking in the sun and hunting; feeding is lowest in the cool periods of winter but increases with temperature during the spring and summer months. Crocodiles do not chew their food; large teeth impale the animals and then the crocodile positions it for swallowing and depends on gravity to move food down the gullet with a toss of their head.
The greatest threat to adult crocodiles, besides human influence (the subject of another paragraph or probably another paper of its own) is other crocodiles, particularly for young males by larger males. However, cannibalism seems to be rarer than popularly believed. It is more frequently social than nutritional and often occurs in the breeding season. Females reach sexual maturity at lengths of 2.5 meters, regardless of age. When the mating season begins, an extended courtship period can last up to two months. (Britton, 2002)
When it becomes time for the female to nest, she must be extremely careful in selecting the site. Nest mortality is high; even during the dry season, flooding can kill an entire clutch, especially in hole nests which can fall below the water level during periods of heavy rains. (Zappalorti, 1976) Crocodiles cannot adjust the temperature and humidity of nests like birds can, and the nest cannot be rebuilt. There is only one large clutch a year, unlike birds which lay several small clutches and the eggs often flood or drop in temperature below the required for the embryos to survive. (Ross, 1989) Eggs require oxygen diffusion through the porous shell, so many can asphyxiate under imperfect conditions. Likewise, although many crocodiles guard the nest assiduously, predation is common among crocodile clutches. Small mammals, fungus, insects, and reptiles all dig out and consume the eggs. If the female can successfully protect her nest to the hatch, another cycle of crocodile life can begin.
VI. The characteristics of the American crocodile described so far are especially significant when considering their needs for conservation. It is only through understanding the behavior and physiology of any species that we can know how best to serve them, and this species of crocodile is no exception. Unfortunately, the problems that loom large in the global environmental battlefront are especially relevant to crocodilians, which means there are no easy answers to their problems. For example, almost any issue of water quality has a direct impact on the American crocodile. Because of their almost exclusively fish diet, (Alderton, 1998) any influence on the aquatic food chain will work its way up to crocodiles. For example, eutrophication of lakes kills of algae and fish species, so even though crocodiles can endure brackish water quite well, they may still be lost if their prey species are eliminated. Furthermore, salinization of water resources is a problem for crocodiles; even though adults are quite adept in both salt and fresh water, the juveniles have larger surface area ratio and cannot survive in salty water. Salinization occurs as water sources are depleted, a universal problem in our water-guzzling world. Even if the water can be to some degree protected, they are in an unfortunate situation: they require hundreds of kilometers of undisturbed wetlands (Ross, J. 2000) and large shallow bodies of water such as slow rivers, swamps, and marshes, because they require such a large territory for each individual. This is unfortunate because wetlands are being lost at a high rate all over the world. They are drained for farming or for pasturelands. Also, crocodiles avoid strong wind and wave action because they require calm water to swim effectively. It is theorized that the still water allows them to keep only the nostrils above water, but if it is windy the snout must be raised at a steeper angle. This makes it more difficult to swim. (Ross, 1989) The crocodiles therefore need protected waters and will avoid the open water if it is rough. This means that tree cover is important and deforestation can disturb their habitats even if water quality is preserved.
VII. Crocodiles have faced over-hunting threats since the very dawn of human civilization. In fact at least one primitive terrestrial crocodilian, Mekosuches inexpectatus, was probably due to human influence. Its extinction, in New Caledonia, occurred in less than 2000 years – it was gone before the arrival of Europeans and its rapid disappearance points to hunting by primitive peoples. (Ross, J. 2000) Our record has not improved much since then. Crocodilians of all sorts have been widely hunted for hides, made into leather shoes and handbags, and have also been used for meat and traditional remedies and folk medicine. Of the 22 recognized varieties of crocodilians, at least 15 have been commercially exploited. (Thorbjarnarson, 1999) The effect has been universal and devastating. During the 1950s nearly 60,000 Nile Crocodiles skins were exported from East Africa. In 1926 36,000 American Alligators were bought in Lousinana. In 1888 a single hunter took 500 in one season, and another took 42 in a single night. (Ross, 1989) In South America it is largely Caimans that have been hunted for their “classic” skins and that species has proportionally lost thousands of individuals. The American crocodile has not escaped this exploitation: the population in Isla Fuerta, on the Caribbean coast of Columbia, was driven to extinction when the mangrove forest was destroyed. Cattle ranching has removed nearly two-thirds of Central America’s primary forest, pushing the American crocodile there to the periphery of its range. The current low population size can be attributed to the extensive commercial harvesting that occurred from the 1930s into the 1960s.
IX. Before Spanish explorers came to Costa Rica, in the person of one Christopher Columbus in 1502, the native peoples were relatively scattered and, despite at least one well-developed city in the past, had a low degree of organization. The Spaniards killed most of the natives with their foreign diseases when they came to the “rich coast,” although in actuality there was not much easily available gold. (Rosera and Palloni, 1999) The local species have been under assault ever since. Hunting was a fairly widespread problem in the country in the past, but today around twenty-five percent is protected lands and the American crocodiles are being given a chance to recover. In Costa Rica there’s a reported population of around 300 individuals in the Rio Grande de Tarcoles and around 35 in Estero Roto. (Ross, J. 2000) While these populations are considered viable, the standard minimum variable population (MVP) for any animal species is 500 individuals. (Rosera and Palloni, 1999) Habitat degradation continues, even in the well-protected island, and poaching problems can persist. In 1993 Costa Rican officials were forced to act when crocodiles were being killed following some attacks on humans. (Moyen, 2000) Because the large animals are predators and carnivores, it is natural for them to kill and even consume humans if given the opportunity. Public sentiment was strongly anti-crocodile, but it is slowly softening. The collection of adult breeders to stock farms could become a serious problem (see next paragraph) if not carefully regulated by the appropriate authorities. (Ross, J. 2000)
X. The current estimated wild population of American crocodiles is between 10,000 to 20,000 individuals. (Britton, 2002) There are only about 500 in the state of Florida, at the edge of the species range, (Alderton, 1998) and the rest are spread mostly (but not entirely) across Central America. Although the species has the widest range of any New World crocodile, it is depleted to a 'significant' extent over most of its range and in a third of its range is quite dangerously low. The populations of a few regions, especially Belize and Cuba, are somewhat safe, and the Costa Rican range is to a lesser extent the same, but there is still much to be desired. C. acutus is completely protected in most countries where it occurs, (Britton, 2002) but the enforcement of this protection is often inadequate - although there are programs in 8 countries within its range, legislation is ineffective or poorly enforced.
Crocodilians as a whole are threatened by hunting and habitat loss, and the black caimen, broad-snouted caimen, American crocodile, the Orinoco crocodile, the Morelet’s crocodile, Nile crocodile and the Cuban crocodile are all listed on the CITES appendix of endangered animals. (Kricher, 2002) There are currently 12 species of crocodiles remaining, most of which are to some degree threatened by human activities. Since the increase in international conservation legislation from 1973 (Ross, 1989) numbers are increasing, but they are still poorly protected throughout most of their range. Today two million skins are harvested annually, mostly caiman from south America, more than half taken from wild populations in violation of national wildlife regulations. Even though poaching is widespread, the real killer for crocodilians today is habitat destruction, and that will be the real battleground to protect crocodiles of all species in the future.
XI. All crocodiles, if given the chance, would choose to avoid humans; there is no need for our two species to compete. Today, however, because of increasing populations, increasing needs, and an ever-widening ecological footprint, we do not give crocodiles that chance. We can no longer afford to; in most cases human intervention has become necessary to protect genetic diversity and healthy populations for most crocodile species. Simply preserving the animals and their habitats is an important first step, certainly; since the 1970’s, when we become aware that the devastation of the past was causing serious problems for crocodiles, many species have benefited improved protection and reduced exploitation. But it is widely recognized that “the majority of the species require a more creative approach that provides incentives to people living with crocodiles to offset their real and perceived costs. Sustainable use has become a key element in the conservation of crocodilian species.” (Ross, J. 2000) Crocodiles are not flagship species for conservation efforts. Local people tend to regard them, not unreasonably, as dangerous and unattractive to live near. When conservation programs succeed, human problems with crocodiles frequently escalate, (Ross, J. 2000) and the crocodiles often need continued protection. But today new methods of utilization and protection combined are providing a chance for sustainability. Ranching (i.e. bringing eggs or hatchlings from the wild and raising them in captivity) and captive breeding (farming) by local people can contribute to local economies and also to crocodile populations. These farms are allowed to raise crocodiles for market as long as they release sufficient numbers
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