Think you have a good bead on what reptiles are all about? Meet the leatherback.
David S. Lee
 |
The immediate striking feature is size, not only are they larger than other sea turtles, they are bigger than any living reptile. A mere seven foot leatherback has flippers which spread nine feet from tip to tip. Anacondas, several pythons, and various crocodilians can surpass the turtle's record length of 10 feet 3 inches, but in total mass leatherbacks win. Even extremely large crocodiles, ones which reach lengths of 20 feet, seldom exceed 1,500 pounds, and only an occasional giant would top the record weights of leatherbacks. The largest leatherback weighed hit the scales at 1,902 1/2 pounds. Yet, the biggest individuals have never been weighed. The problems with lifting and transporting of large turtles to scales which would register in 1000s of pounds have prevented researchers from obtaining this type of information. Louis Agassiz, a 19th century biologists and founder of the famed Museum of Comparative Zoology at Harvard, claimed he saw leatherbacks which weighed over a ton. While modern specimens of this size have not been confirmed, Agassiz was not one to misrepresent biological facts, and it is likely that individuals will someday be weighed that exceed a ton.
Leatherbacks are wide-ranging, occurring throughout the waters of the Atlantic, Pacific and Indian Oceans. In the Atlantic they have been found north to Labrador, Iceland, the British Isles, and Norway and to Japan and Alaska in the Pacific. In the southern hemisphere they are found off Chile, Australia, and the Cape of Good Hope. This composite distribution makes them the most widely-distributed reptile, and it could be argued that they are the world's most widely-distributed animal. Not only is the species cosmopolitan in distribution, but as adults individuals regularly migrate across oceans from tropical nesting beaches to sub-polar seas. Sites where adult females are tagged on nesting beaches show little relationship to where individuals are later recovered at sea. One of the most extensive tagging programs has been run continuously in French Guiana since the late 1970s. Female leatherbacks from there have been recovered off Florida, Georgia and South Carolina. The furthest distance covered in the Western North Atlantic was of a turtle caught off Newfoundland in September 1988, four months after being tagged in French Guiana. Other tagged females from this same nesting beach were reported from Spain, Morocco and France. Some of these localities represent multiple captures of different females, suggesting these movements are not exceptional, but the norm.
Leatherbacks are said to be the most aquatic of turtles and the most pelagic. Unlike other marine turtles they are not obligate bottom-feeders, while they can feed on the sea's floor, they also feed from the surface, and within the water column, and their ties to shallow shelf-waters are all but broken. They are deep divers, and with recorded dives to depths of over 3,000 feet, they are among the deepest diving of air-breathing vertebrates. Because of this they are prime subjects for physiologists interested in studies on respiration. The problem, of course, is how to conduct the studies. By placing respiratory masks over the heads of females captured on nesting beaches, and after calculating in factors for differences in size, scientists learned that leatherbacks had a respiratory rate which was 22% lower than that of active green sea turtles. They also learned that their tidal air volume was twice that of greens. Their lung capacity, however, is only 25% greater than would be predicted if they were of comparable size to other reptiles. Monitoring lactic acid levels in blood of resting vs . active leatherbacks showed minimal increase, and compared to active alligators, lizards, and other reptiles which go anaerobic during even brief periods of intense activity, leatherbacks appeared to be able to increase activity and still supply oxygen to muscles through normal breathing. These calculations alone do not tell the whole story and their sustained deep dives and respiration biology cannot be explained through what is known about reptilian physiology. The researchers concluded that leatherbacks had independently developed respiration adaptations for diving that exists in deep diving air-breathing marine mammals. They do this through storage of oxygen in active muscle tissues, enhanced circulation, large tidal air volume, and various important adaptions in blood chemistry such as high hemoglobin concentrations.
All this is necessary for the turtle's life style. These turtles are constantly diving, and sometimes stay submerged for nearly half an hour at a time. Active around the clock, they are not you typical torpid, back-yard, stereotype reptile. They spend as much or more time diving as they do at the surface, and conduct as many as 50 deep dives in a 24-hour period. If the turtles were forced to run on an anaerobic metabolism, like other reptiles do when they are highly active, they would need to spend the majority of each day just resting on the surface to allow time for their tissues to be cleansed form of lactic acid.
Snow leopards, giant pandas, Komodo dragons, various whales, well in fact, most of the great animals of the earth can typically be seen in some zoo or public aquaria. Not so the leatherback. They are seldom kept in captivity at all, and when attempts have been made to do so the turtles do not fare well. The record longevity for a captive is just over 3 years, and few survive more than a few months. Leatherbacks are constantly in motion, and in their world there are few barriers. In captivity leatherbacks spend every moment of the day and night swimming full speed into the sides of their containers. Resulting contusions and abrasions never heal and eventually infections take hold. In the late 1960s I once saw a small number of hatchling leatherbacks on exhibit at the Miami Seaquarium. The curators had lined all but the front of the aquarium with a air-filled plastic liner (it looked like a modified air mattress) and the little turtles were all swimming at high speed, bouncing about like half a dozen ping-pong balls. Because they present unusual problems in preservation techniques they are also seldom exhibited in museums. The flesh and skin, even the skeleton, are all saturated with oil. Mounted specimens drip oil for decades, and eventually what remains looks like misshapen, black, turtle-jerky. Today a number of museums have produced high quality fiberglass models that look for all purposes like the real turtle. The North Carolina State Museum of Natural Sciences is the only place I know where you can view an actual leatherback. It still drips grease despite a collection date of 1897. I can find no record as to how the animal was prepared but, despite showing its age, she has held up rather well, and remains one of our most interesting specimens. And if you look closely you can still see a few grease spots.
The oil has uses beyond destruction of museum specimens. In Indonesia ritual and sustenance hunting of leatherbacks still occurs, and as is typical with ancestral rights practiced in a modern world, the hunts are a blend of ancient ritual and twenty first-century equipment which eliminate any competitive edge the turtles may have had in the past. In addition to the meat, the oil is an important by- product for the villagers participating in the hunts. In fact, the use of oil from these turtles was first described in 1554 by Guillaume Rondelet in a volume which contained the original published description and illustration of a leatherback. "I saw a turtle of this sort that had hung in the sun for several months, and a pound of oil dripped out of it every day. The fellow who had caught it used this oil for his lamps" This report probably came from the French Mediterranean coast where Rondelet lived.
One of the prime characteristics of reptiles--scales -- are completely lacking in leatherbacks. Actually almost invisible, small scales cover the shell and extremities of hatchlings, but these disappear almost immediately. To say that leatherbacks grow rapidly is an understatement. Reptiles in general, and turtles in particular, are renowned for their slow growth, while leatherbacks exhibit incredibly rapid skeletal and body growth. Their growth rates are more like mammals than reptiles, and some researchers suspect that they may reach sexual maturity in 3 to 6 years, while others believe it takes 5 to 15 years. In either case this is very rapid growth; a hatchling weighs only 1.06 ounces but by maturity it achieves an 8,000-fold increase in body weight. The leatherback's pattern of bone and cartilage growth is not only unlike that of any reptile, it is totally different from any vertebrate. These turtles have vascularized cartilage superficially similar to birds and mammals. The supply of blood into cartilage and bone allow for very rapid growth, and detailed study of the cellular structure show the growing tips of bones are unique to leatherbacks. Cell biologists are preoccupied with the origins and formation of the different cell/tissue types and careful study of leatherback tissues has lead cellular biologists in many directions which each in turn spawned many unanswered questions.
They are the most streamlined of turtles. Peculiarities in shell structure make comparisons with other types of living turtles nearly impossible and the relationship of leatherbacks to other turtle families is unclear. The external and internal anatomy of leatherbacks, combined with their bizarre physiological adaptions led some herpetologists to suspect that leatherbacks were an independent evolutionary line of some freshwater ancestor to life in the sea. The species is so odd that in the past herpetologists placed it in a sister group to all other turtles (Suborder Athecoidea). The superficial resemblance to other marine turtles, both in shape and nesting behavior, may simply have been a result of the molding influences from making a living in the open sea. The 'shell' consists of thin, irregular plates of bone in the leathery skin. Unlike other sea turtles leatherbacks lack shells and don't have claws on their flippers. Seven ridges that run the length of their leathery bodies provide structural support.
 |
For an animal so large, leatherbacks are relatively weak-jawed and eat mostly soft bodied prey. Their upper digestive track is quite different from most other animals. I first learned this peering into the mouth of the dead leatherback that I had found near the Virginia line on Corolla Beach. It was in an early stage of decay and a very advanced state of rigormortis, but I somehow managed to pry open the mouth with a tire iron. I was looking to see if the cause of the turtle's death was from swallowing plastic, a common form of mortality in sea turtles. Clear plastic looks like a large jellyfish and once swallowed, the undigested plastic results in intestinal blockage, or perhaps in some cases, suffocation, when the turtles try to disgorge the aberrant food item. I was surprised to see the mouth and throat was lined with long white spines, some were up to two inches long. They were thick and pointed back toward the esophagus. I later read that these spines continue down much of the length of the turtles digestive track. A leatherback's upper digestive track is simple-- a long, spine-filled, cylinder which makes more than a full loop on the left side of the turtles body cavity. This odd digestive apparatus is six or seven feet long, and certainly longer in really big individuals. The spines are linked to its habit of feeding largely on jelly fish. The turtles suck in the jellyfish, and with them large amounts of seawater. The downward pointing spines are believed to hold the jellyfish and jellyfish scraps in place when the turtles expel the water back up their digestive system. The long esophagus may also be a food storage area allowing leatherbacks with full stomachs to continue to feed. While these turtles are also reported to eat sea urchins, crustaceans, mollusks, squid, fish, tunicates, and algae, their primary food is jellyfish, and to a lesser extent the Portuguese man-of-war. On their deep dives researchers suspect they are seeking out bioluminescent abyssal jellyfish and glowing tunicates. Of all the world's 260 or so species of turtles this is one most highly specialized feeders. How an animal this large makes a living digesting the little amount of protein that is in the watery tissue of a jellyfish is a mystery to me.
Because of their tropical nesting it was long assumed that leatherbacks like our other marine turtles were tropical, and that records from northern seas were of misplaced vagrants. It is now clear that these turtles regularly follow drifting schools of jellyfish northward to high latitude seas on a regular basis, and, in that females don't nest in consecutive years, they actually spend a significant portion of their life in cold and temperate waters stalking large jellyfish. From the point of view of economics this makes sense because cool water holds more oxygen and thereby supports more life than do tropical seas. Furthermore, the long days of summer produce vast quantities of phytoplankton that in turn generate massive blooms of jellyfish. With decreasing day length the turtles retreat to temperate and sub-tropical seas.
But how can does a cold-blooded "tropical" reptile manage to stay functional in northern seas? The answer is they can't-- leaving the little untidy problem that healthy leatherbacks are presistently found in waters off New England and the eastern Canadian provinces. In the early 1970s researchers found that in 45 F waters off Nova Scotia leatherbacks maintain body temperatures of about 80 F. This in itself is quite remarkable and the findings were promptly reported in Science, a journal reserved for prestigious research of obvious importance. How is this possible? Are these turtles actually warm-blooded? They already fail to match many of the other characteristics of cold-blooded reptiles. What is going on here?
In 1972 a number of researchers descended on a leatherback which had died at the New England Aquarium seeking the answer. While to some degree the heat is controlled metabolically as it is in birds and mammat they also contribute to temperature regulation by a totally different process. The constant swimming activity of leatherbacks, the same behavior which makes them impossible to maintain in captivity keeps them warm. Muscular activity generates the heat. We already understand this. A short jog on an cool winter day and we start overheat. Swimming, however, is another story. Even in relatively warm water we eventually will become hyperthermotic. Even water somewhat cooler than 98.6 will eventually draw the heat from our bodies faster than we can produce it. On leatherbacks their large size compared to their body surface area allows for a more efficient storage of heat. The turtles leathery shell insulates the bulk of the animal from the cool water and the oils in their body act as heat reservoirs. This leaves one small, but important, obstacle to overcome. Blood circulating out into the long flipper shaped fins is quickly cooled by sub-artic waters and the cooled blood returning directly to the body would numb and eventually kill any leatherbacks making even short visits to frigid seas. The flippers are large and have a huge surface area compared to their size so the blood cools rapidly. Muscle activity alone would not generate enough heat to keep the blood from cooling.
A careful dissection of the New England Aquarium specimen showed that leatherbacks have solved this problem by having the incoming veins (cool blood) packed adjacent to the out going arteries (hot blood). The system is simple and functional. Heat is rapidly lost by the flippers, but is not critical to have the blood in the flippers at 80 F, it is only necessary to have the blood carry oxygen. The blood cools as it reaches the capillaries, but on return the veins running along side the arteries are warmed. The closer the blood gets to the turtle's body the warmer it becomes, so by the time it reaches the heart and lungs it is back up to 80 F. In addition lipids in the flippers act as antifreeze and cool much slower than similar ones found elsewhere in the turtle. The system is beautiful. The veins and arteries are wrapped in insulating fibrous bundles, designed for all purposes like telephone cables. Instead of the outgoing blood loosing all its heat to the water it transfers most of it to the returning blood as it moves out. And its all surrounded with antifreeze. Leatherbacks are believed to have a similar system in their nares enabling them to breath cold air. Mammals and birds have also made use of this system. Blood in beaver's tails and duck's feet share the same problems that it faces in the turtles flippers-- a relatively large unprotected surface area and a small amount of tissue exposed to water and heat loss. Examination of the circulatory system of these animals shows that in the exposed parts of their bodies their veins and arteries are also arranged in parallel heat transfer systems.
So how does the leatherback's specialized heating system work in the tropics where the seas are warm? Its not a real problem. The turtles hunt mostly at depths where the water is less than 80 F and of course they can cut back on muscle activity. Their large size not only stores heat, but prevents a rapid build up of heat. Leatherback's visits to the warm surface waters are brief and the females crawl ashore to lay their eggs at night when air temperatures are cool.
In the mid 1970s when I was first starting sea bird surveys in the Gulf Stream off Cape Hatteras we were finding all sorts of interesting birds, a few of which were unknown from North America, let alone North Carolina. We also saw flying fish, sperm whales and other large whales on a regular basis, but what excited me the most were the first several leatherbacks we encountered. They were creatures I had read about since childhood and I felt honored to be able to see them in the open sea. These leatherbacks did not bob on the surface like typical sea turtles. On calm days it was not unusual to spot loggerheads and Kemp's turtles basking on the surface, but the leatherbacks were always several feet below it. Their dark head and shells, sprinkled with white specks were hard to see and often we would be right on top of them before we knew what they were. They had no need to surface bask since they maintained a constant body temperature with their odd arterial plumbing. I was interested in their seasonal distribution. Our museum had accumulated newspaper accounts and other reports of leatherbacks and I scoured the scientific literature and other museums and located all the reports I could find for individuals between New Jersey and South Carolina. Even a few Bermuda records were obtained. A total of sixty seven records which accumulated ranged from 1835 to 1980. The cumulative information showed that these turtles to be present throughout the year. The higher concentrations were between May and October and the most were recorded from mid-summer. I now think this showed little more than the time of year most people are boating. I was surprised to see that Bermuda had few records, and that most of my North Carolina sightings were from shallow shelf waters while the majority of my effort was along the outer continental shelf where the water was deeper. Perhaps this turtle is not as highly pelagic as existing literature indicates.
Despite all their bizarre differences, today most biologists do not think that the marine adaptions of leatherbacks are independently derived from those of other marine turtles. The leatherback just took on a far greater degree of specialization than any other of our living, more lethargic, turtles. The fossil history of leatherbacks is far from complete and the parts that are involved in the fossilization process do not tell much of animals which are mostly held together by oily leather and cartilage. However, it is apparent from the fossil record that even though only one extant species exist today, in the past several different leatherback lineages existed at one time or another, and several occurred concurrently. Over the past 50 million years these various lineages of leatherbacks have become extinct leaving only Dermochelys (our living leatherback), a genus which appears nowhere in the fossil record. Did all these various lineages share a similar anatomy and physiology? For the anatomy the answer is clearly yes, for it is the anatomy that allows paleontologists to recognize they are all leatherback. Physiology? Well, who knows, but fossil fragments from New Zealand and Antarctica suggest that the hot-blooded physiology is not something unique to Dermochelys. And if this is so where did these hot-blooded leatherbacks of the late Cretaceous ( 100,000,000 YBP) come from? There was nothing else like them then, just as there is nothing like them now. As one learns more about things patterns should emerge, and eventually an understanding of how all these sundry leatherback facts fit together should become apparent. But a lot of things about leatherbacks just don't seem to jell. The questions are not so extreme that I want to declare that I have become a Creationist, but spontaneous creation would clearly be the easiest explanation for this reptilian oddity. This turtle may be our best evidence yet for the existence of aliens. Perhaps leatherbacks are ancestors of unwanted pets dumped off on Earth eons ago by space visitors who grew tired of the turtle's constant moaning and whistling.
 |
Its exciting to think of the sea as the biologically unexplored world that it is. To some this means mysteries of Vernian depths, to others man-eating sharks, or sea monsters and other real, or mythical, beasts. For me, just knowing of the existence of jellyfish stalking, gigantic, oily turtles, with their warm blood-pumping hearts, makes the sea seem a little wider and a little wilder. A huge, warm blooded, scaleless, vocal reptile housed in the body of a fast-growing, fast-swimming, turtle is a paradox which shatters nearly all our simplistic, generalized concepts of reptiles. Leatherbacks are a living reptilian counter-culture, and give cause to pause, wonder, and smile. In breaking all the rules, these guys have by-passed reptilian stereotypes, and for eons they have been highly successful while doing so.