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GALAPAGOS TORTOISES

Galapagos Tortoises 2

The Galapagos Islands are a tropical archipelago of 13 major islands made up of craters and black lava flows, located on the equator some 600 miles off the coast of Ecuador. Since their discovery in 1535, the Galapagos Islands have been famous for their namesake - the giant tortoise (Geochelone elephantopus). The Galapagos tortoise is of great historical scientific interest since the species illustrates a correlation between geographic isolation and morphological divergence (Honegger 1997). Carapace length of members of this population may reach 4 feet and their weight may reach 500 pounds on the larger islands. Males are much larger than females. No one knows just how long these giant tortoises can live in the wild, but estimates set their life span at over 150 years, and two hundred years may not be uncommon (Roy 1997). If this is true, then some of the tortoises now living may well have been youngsters when Charles Darwin visited the islands in 1835.

The taxonomy of genus Geochelone has been much debated. Van Denbergh, in 1914, recognized 15 species; two of these are certainly extinct, one is probably extinct, and one is believed to be a human introduction, bringing the present number to 11. Hendrickson (1966) regarded all forms as belonging to the single species, Geochelone elephantopus, for which he recognized 15 subspecies or races. MacFarland et al (1974) supported the recognition of each population as a distinct subspecies (Fritts 1983). A contrasting opinion is presented by Fritts, Ernst and Barbour (quoted in Siaca-Colon 1994), who consider each geographically isolated population a distinct species. In keeping with the traditional literature, this paper will treat the populations as subspecies of the species, Geochelone elephantopus.

The subspecies of the giant tortoises vary in size and in the shape of their shells. Some populations are composed of tortoises having the dome-shaped carapace common among tortoises in many parts of the world, with an anterior opening that can be protected by retracted forelimbs. Galapagos tortoises in other populations have diverged from this common carapace shape, having a large anterior opening of the carapace and a carapace resembling a Spanish saddle. Tortoises with this carapace shape are frequently termed saddles and are known only from Galapagos and a single island in the Indian Ocean. The Indian Ocean species is now extinct, leaving the Galapagos saddle backs as the only living example of this carapace shape (Fritts 1984). Of the two Galapagos morphological types, dome tortoises attain the largest sizes. Saddle tortoises are markedly smaller in size and have proportionately longer necks and forelimbs (Van Denburgh quoted in Fritts 1983).

During the historic visit of Charles Darwin to the Galapagos Islands in 1834-35, he was informed by the Vice Governor of the Archipelago that it was possible to tell from which island a tortoise came on the basis of its appearance (quoted in Fritts 1983). With limited exceptions this is a valid observation. Body size and carapace type are correlated with elevation of the island or volcano inhabited (Fritts 1983). Large tortoises tend to occur on islands with maximal elevations and mesic habitats related to cloud capture at high elevations. Small tortoises tend to occur on islands with low elevations and markedly drier and warmer habitats (Fritts 1984). It is important to note that, on the drier islands having tortoise populations, the Opuntia cactus, a major source of water, has evolved an arborescent form (Honegger 1997). It is believed that this arborescent Opuntia form and the saddle morphology are correlated. Dawson (1966), Pritchard (1979) and Thornton (1971) suggested that the shell shape and long neck of saddles were adaptations that allowed them to feed upon the local tree-like Opuntia and, conversely, that tallness in the cacti was an adaptation to avoid the reach of the tortoises (quoted in Schafer and O'Neil 1983).

Dome populations are found on the islands of San Salvador and Santa Cruz, the southern portion of Isla Isabella, and Volcans Alcedo and Darwin on Isla Isabella. Saddle populations are found on the islands of Espanola, Pinta, Pinzon, San Cristobal, Fernandina, and Volcan Wolf on Isla Isabella (Siaca-Colon 1994). The height of the anterior carapace in relation to overall size also varies among the populations by island. Specifically, populations for Pinzon, Espanola and San Cristobal (in descending order) have the highest anterior carapaces relative to size, while tortoises from Santa Cruz tend to have the lowest anterior carapace (Fritts 1983).

Fritts (1983) proposed two hypotheses to explain size variations between populations:

(1) Small tortoises have lower energy and water budgets and can use the limited shade of the sparse vegetation better than can larger tortoises; thus small tortoises occupy xeric habitats.

(2) Large tortoises can exploit the abundant food, water and shade of moist, high habitats and can use their large size to resist cooling during overcast or foggy periods; thus, large tortoises occupy the mesic habitats at high elevations.

In addition to differences in size, marked differences in behavior can be seen between saddle and dome tortoises. Saddles are less tolerant of other tortoises in close proximity, are more attentive of movements by other animals (including tortoises and man) and are more frequently involved in agonistic behavior (Fritts 1984). Agonistic behavior in Geochelone in the Galapagos appears to be a ritualistic competition for height. The competing tortoises attempt to achieve dominance by raising the head as high as possible and, if necessary, by gaping their mouths and biting each other on the head or upper jaw. The forelimbs are extended to increase the height and even more height may be gained by raising one forefoot off the ground and stretching. The height reached in the vertical neck extension ultimately determines the winner of an encounter. Winners extend their heads higher than losers. The greater vertical reach results from a variety of factors: a longer neck, an elevated anterior opening of the carapace and longer forelimbs. Schafer and O'Neil (1983) propose that the higher level of aggressiveness observed in saddles may have occurred from the increased competition for resources in the xeric environment, the expression of dominance through agonistic contacts resulting in the displacement of the submissive animal and the increased fitness of the dominant animal.

Fritts (1983) proposes that the smaller, saddle back tortoises with longer necks and limbs developed because selection for increased vertical reach, together with decreased body size, could increase fitness in xeric conditions. He further proposes that xeric habitats with limited food, water and shade and subject to climatic variation would produce intense intra-specific competition for these resources and this environmental variation could result in selective pressures that:

(1) favored increase in vertical feeding range [to reach arborescent Opuntia] without increased body size in extremely dry situations where food and water are limited, and

(2) favored increase in vertical reach to facilitate agonistic behavior important in the defense of food, water and shade in dry conditions.

The divergence in carapace shape of some populations of tortoises in Galapagos involved opening the anterior carapace, exposing the head and base of the neck and abandoning, in part,an anti-predator defense employed by terrestrial turtles in many parts of the world. No native mammalian predators occur in the Galapagos Archipelago, and Fritts (1983) proposes that the saddle shape may have been allowed to develop there due to the lack of natural predators. He points out that domed tortoises such as those from the islands of Santa Cruz, southern Isabela, and other populations, are relatively similar in shape to other tortoises of the genus Geochelone and related genera in South America, Africa, Asia and North America. Nearly all are capable of retracting the head and neck within the carapace and protecting the anterior opening of the carapace by pulling up the forelimbs, which are covered with thick scales. Nearly all tortoises passively protect themselves from vertebrate predators in this manner. He points at that the elevation of the anterior carapace of saddles is found nowhere outside of the Galapagos and is unknown in the fossil record. He suggests that, since this deviation from a "typical" tortoise shape has evolved only in the Galapagos where there are no large native terrestrial predators, that predation pressure may have prevented development of the saddle morphology in other ecosystems. Once free from predator pressure, some Galapagos tortoise populations apparently responded to selection for increased height in relation to size. By modifying previous developmental patterns, a diversity of tortoise morphotypes developed which allowed occupation of a wide array of environmental conditions in the Galapagos Islands (Fritts 1984).

In genetic sampling, Marlow and Patton (1981) determined the overall genetic similarity among the sampled races to be quite high and the very close genic similarity suggests that all 14 subspecies of Geochelone elephantopus evolved from a common ancestor, each population evolving separately in response to biological and environmental factors on individual islands or on geographically divided volcanic craters. However, their genetic sampling also suggest the possibility of two introductions, each from a different original source population. Marlow and Patton (1981) state that a choice between the one-ancestor versus two-ancestor possibilities cannot be made because comparisons to extant mainland species reveal such distant relationships, that the immediate common ancestor of the Galapagos forms must undoubtedly be extinct. The geographic pattern of heterozygosity variation across populations suggests a sequence of colonization events in which started at San Crisotbal, spread to Esapnola and Santa Cruz and from Santa Cruz to all other islands supporting tortoises. The saddle backed carapace condition is believed to have been independently derived several times in this colonization process.

Observation of Galapagos tortoises raised in captivity reveal that hatchling tortoises from all populations are similar in external appearance, but four- to five-year-old young from Espanola show an elevation of the anterior carapace and light coloration on the head and neck not present in young of the same age from Santiago or southern areas of Isabela. This observation led Fritts (1983) to suggest that there is an underlying genetic control of the phenotypic differences. His studies indicate that populational differences are (1) stable; (2) probably due to differential growth patterns; and (3) not environmentally determined as they do not vary with short-term climatic or ecological fluctuations.

In addition to the vertical neck extension and biting mentioned above, Geochelone elephantopus displays a variety of agonistic behaviors in the establishment of dominance. According to Auffenberg (1977), dominance in tortoises is determined primarily by superiority in aggressive encounters, and the order of access to food, mates and resting sites is strictly regulated by the dominance hierarchy. Neck extension is the most common interaction, followed in frequency by gaping the mouth open wide, exposing the tongue; this behavior is followed in frequency by approaching the opponent. The vertical movement of the head during neck extension, enhanced by the colorful gaping mouth, produces a conspicuous and effective threat display (Schafer and Krekorkian 1983). Hissing has also been reported during neck extension (Hendrickson, Rodhouse et al. quoted in Schafer and O'Neil 1983). Other agonistic interactions reported include leaning the extended neck and head from a vertical position toward an opponent; bumping the opponent's nose (disrupting the agonistic sequence and causing the immediate expression of submission in both animals); and pushing the front of the carapace against an opponent, forcefully moving him away. Submissive actions include retracting the neck and head into the carapace (accompanied by an audible hiss of exhaled air) and turning or moving the body away from an opponent. In studies conducted by Schafer and Krekorkian (1983), both males and females, saddles and domes, were observed to display agonistic behavior, with male to male contacts occurring significantly more often than male to female contacts and contact for individual saddles occurring significantly more often than for individual domes. The higher level of aggressiveness observed in saddles may have occurred from the increased competition for resources in the environment and perhaps the development of ritualized fighting played an important role in the exaggeration of the saddle-backed shell shape (Schafer and O'Neil 1983).

Fritts (1983) summed up the significance and relationships of all these factors by offering two hypothesis to explain the adaptive significance of increased reach of the head in saddled populations in the Galapagos: (1) Increased head reach in relation to size could function to increase the vertical feeding strata available to a tortoise. Since plant density and palatability are reduced in xeric environments where saddled tortoises occur, such an increase could enable animals to reach the upper stems of Opuntia and other plants in areas where food and water resources are limited. Tortoises are known to depend on Opuntia for sources of water during prolonged dry periods.

(2) Increasing head reach may also confer an advantage in aggressive encounters related to intraspecific, and potentially interspecific, competition. Although aggressive interactions are infrequently observed in the reduced population densities in the Galapagos today, they are frequently observed under captive or crowded conditions. During these encounters, the tortoises extend their heads and necks as high as possible, and with limbs fully extended, they ultimately resort to lifting one forelimb, thereby gaining added height. Extended to their maximum height, the tortoises may threaten each other with a gaping display of the mouth. Occasionally the highest tortoise will strike or bite the head of the other tortoise. The victor, or dominant individual, is nearly always the one that extends its head the highest. The degree of intraspecific competition in undisturbed populations of tortoises in the Galapagos is unknown, but it is expected to be greater in xeric areas where food, water and shade may be limited.

The Galapagos tortoise is a herbivore and depends on grass or low shrubs for food. Almost any kind of green vegetation is take as food, including Hippomane mancinella which is highly poisonous to most animals. On the higher, lusher islands, the dome-shaped tortoises feed on grasses, low-growing plants and fallen fruit. On the lower, drier island, the saddles prefer to eat fallen cactus pads and low-growing vegetation; when these are unavailable, they reach up for the leaves of taller shrubs such as Opuntia, Scalesia, and Bursera (Hamann 1993). During the dry season they may go without food or water for months.

As with other reptiles, thermoregulation is a dynamic and integral part of tortoise survival; G. elephantopus is dependent on sun, shade and mud wallows for thermoregulation and, whenever possible, spends long periods of the hot day partially submerged in pools or protected by shade. During the dry season, as the ground dries and the shade-giving vegetation shrivels, competition increases for space in the shade and the shrinking pools. Depending on the time of day, inactive tortoises assume two distinctive postures: (1) with head and limbs extended to either bask in the morning hours or to offload heat at midday; (2) during cooler evening hours, sleeping tortoises usually withdraw the head and limbs and face into a soil bank, vegetation or another tortoise (forms) to conserve body heat (Hayes et al. 1992). Interference with the Galapagos tortoise's thermoregulatory capability, especially with its ability to offload excess heat, can prove to be fatal for the animal.

Giant tortoises reach maturity at 20 to 30 years. They mate during the rainy season, usually between January and June. Then, between June and December, the females migrate to the arid zones to nest. While digging the hole with her hind legs, a task that can take several hours, the female urinates frequently to soften and bind the soil. Between two and twenty eggs are laid, each about the size of a tennis ball. After covering the nest, the female returns to the highlands, leaving the eggs to incubate for the next four to eight months. Nest temperature determines the sex of the hatchlings, with lower temperatures producing more males. The young tortoises hatch between November and April, taking a month to dig their way out of the nest. These hatchlings weigh 1/1000th of what they will weigh as an adult (Galapagos.com). The mortality rate for the hatchlings in the wild is high under the best of conditions. Predators and food stress can place the hatchling mortality rate at 80% to 94% (Swingland and Coe quoted in De Neira and Roe 1984). Even in areas devoid of feral mammalian predators, MacFarland et al. (1974) reported a 50% hatchling mortality rate in a year of plentiful rain and a 95 to 100% morality rate within four months of emergence in a dry year.

These slow-moving giants have few natural enemies; hawks are probably the only native predator of young tortoises. Food and water scarcity are additional major obstacles during the difficult first few years of life. However, introduced species, including man, have proven to be a far more devastating threat to the tortoises. Whalers and colonists killed over 100,000 giant tortoises for meat and oil during the 19th and 20th centuries. Visiting ships prized them as a source of fresh meat on long voyages - stored in the hold, they survived up to a year without food or water. Beebe (1988, p. 210) provided the following quote of Captain David Porter, whose ship visited James Island (San Salvador) in 1813, as evidence of the exploitation of the Galapagos tortoise:

" we began to lay in our stock of tortoises, the grand object for which every vessel anchors at the Gallipagos [sic] Islands. Four boats were despatched [sic] every morning for this purpose, and returned at night, bringing with them from twenty to thirty each, averaging about sixty pounds. In four days we had as many on board as would weigh about fourteen tons, which was as much as we could conveniently stow."

In the early 20th century, the tortoises were harvested extensively for their meat and oil by natives of Ecuador. R. H. Beck wrote of seeing one-hundred and fifty tortoise skeletons at a pool, one hundred more skeletons at another pool a half mile away, and ten or fifteen skeletons frequently seen in other pools (Beebe 1988).

Man brought domestic animals such as pigs, donkeys, goats and dogs, as well as black rats, to the islands and, over time, these introduced animals formed wild populations throughout the islands with devastating results. Feral pigs root up tortoise nests to eat the eggs. For nearly a hundred years on Pinzon Island, rats have eaten every giant tortoise hatchling, leaving only an ever-aging adult population of that subspecies. Dogs kill tortoises up to four years old, and burros trample the nests and destroy the eggs before they hatch. Beebe (1983) gave the following quote of the account of R H. Beck as published in the seventh annual report of the New York Zoological Society in 1905: "After seeing on this mountain dozens of tortoises of good size, one wonders where the small ones are; but after spending a few days a-foot and seeing the many wild dogs in that region - descendants of those left by sailing vessels - we can only wonder that so many of the large ones remain. From the time that the egg is laid until the tortoise is a foot long, the wild dogs are a constant menace, and it is doubtful if more than one out of ten thousand escaped. We certainly saw none, and the natives told us that the dogs ate them as fast as they were hatched."

Feral goats present a different type of threat to the tortoises' existence than that presented by dogs, rats and pigs, but just as devastating. While dogs and rats eat the eggs and young, the goats eat the foliage, competing with the tortoises for food and destroying their habitat. Goats wipe out huge stands of native plants, grazing the vegetation down to lifeless stubs. This robs native wildlife of food and shade, as well as causing severe soil erosion, destroying the tortoises’ wallows. The goat's rapid reproduction rate magnifies the problem. the introduction of three goats to Pinta Island in the late 1950's resulted in a goat population of over 40,000 by 1970 (Galapagos.com 1974). On the island of Isabela, alone, there are an estimated 100,000 feral goats (Butler 1996). Volcan Alcedo, on Isabela Island, harbors the largest remaining population of Galapagos tortoise, Geochelone elephantopus vandenburghi, of 3,000 to 5,000 individuals. (Fowler and Roe 1984). Feral goats, traveling across bare lava fields from the southern part of the island, have invaded and devastated the area which was once a haven for the last large population of the giant tortoises. Over the past few years, thousands of goats have destroyed the fragile vegetation, causing massive soil erosion. In the denuded landscape, the giant tortoises are now deprived of their food plants, their bathing pools, and shade from the equatorial sun. Faced with starving to death or baking to death in the equatorial sun, the numbers of tortoises in this last stronghold are decreasing at an unnaturally rapid pace (Roy 1997).

From the several hundred thousand tortoises that roamed the islands before man's arrival, fewer than 15,000 survive today. By 1906, the endemic populations on Santa Fe and Floreana were extinct, and populations native to San Cristobal, Espanola, and Fernandina were approaching extinction. Only one known survivor of the subspecies native to Isla Pinta, "Lonesome George," is known to remain in existence. The Ecuadorian government, the Galapagos National Park Service and several conservation agencies have taken measures to help protect tortoise populations. With the declaration of Galapagos as a National Park in 1959, the islands' native wildlife became legally protected. Shortly thereafter, the Charles Darwin Research Station and the Galapagos National Park Service began intensive efforts to further protect endangered tortoise populations with the establishment of a captive rearing center on Santa Cruz. Friends of Galapagos, World Wide Find for Nature, United Nations Development Program, and Frankfurt Zoological Society Help for Threatened Wildlife, are among the concerned institutions and individuals supporting the giant tortoise research and rearing program.

By 1963, the Espanola subspecies had been reduced to only two males and twelve females, and little or no natural breeding was taking place because the tortoises were too widely dispersed on the island. Between 1963 and 1974, the Espanola tortoises were brought to the Charles Darwin Research Center on Santa Cruz. There, all eggs laid were carefully placed in incubators until hatching. The young tortoises remained at the rearing center for about three years, until large enough for a safe return to the wild. By 1995, nearly 700 Espanola tortoises had been returned to their island (CDRS 1994). Similar programs protect other tortoise populations from the threat of introduced animals. Eggs are collected from natural nests and brought to the center for incubation, rearing and eventual release. Today, nearly 2,000 tortoises have returned to their island of origin as a result of the Darwin Center efforts (CDRS 1994).

The elimination of introduced animals is also critical to the survival of Geochelone elephantopus. Since 1961, eradication programs have concentrated on wild pigs, goats, dogs and rats. There have been notable successes. Goats have been eradicated from six of the smaller islands. Black rats are gone on a few of the islets. Wild dog populations are now controlled on Isabela. A yearly trapping and poisoning campaign for rats has reduced the numbers of these threatening animals on Floreana and Santa Cruz. Although efforts have been successful on some islands, many serious problems still exist. Hunting is extremely difficult due to the rough lava terrain and dense tangles of spiny plants. Trapping and the use of poisons are also difficult and extreme caution must be practiced to ensure that native animals are not harmed. Fencing is expensive and not effective against smaller animals. The use of biological controls involves careful research and carries the risk of creating new problems in the islands.

In 1996, authorities in the Galapagos Islands launched a new hunting and trapping campaign to eradicate non-native animal species that threaten food supplies for the native fauna. The campaign's goal is for the wild goats and burros to be eradicated, or at least to reduce their numbers considerably so that they will not represent a threat to the islands' fragile ecosystem. Park authorities have approached Ecuador’s air force and navy to see if they are interested in transporting captured animals to the Ecuadorian mainland. If not, the park authorities will proceed to hunt the feral animals (World Wide Forest/Biodiversity News 1996).

Through protective legislation, captive breeding programs, eradication of predators and exotic species, and habitat recovery, the Galapagos tortoises have, thus far, been saved from extinction. It will take a continued combination of all these efforts to save these magnificent examples of evolution and island biodiversity. With continued work, the tortoises may again be established as the giants of the Galapagos islands.

 

REFERENCES

 

Beebe, W., 1988. Galapagos: World's End. New York: Dover Publications, Inc.

 

Charles Darwin Research Station, 1997. "Giant Tortoises." Online. Internet. Available: http://www.polaris.net/~jpinson/tortoises.html

 

De Neira, L. E. F. and Roe, J. H., 1984. "Emergence success of tortoise nests and the effect of burros on nest success on Volcan Alcedo, Galapagos." Copeia, 3: 702-7.

 

Fritts, T. H., 1983. "Morphometrics of Galapagos tortoises: Evolutionary implications." In R. I. Bowman, M. Berson & A. Leviton (Eds), Patterns of Evolution in Galapagos Organisms: 107-122. San Francisco, California: Pacific Division of the American Association for the Advancement of Science.

 

Fritts, T. H., 1984. "Evolutionary divergence of giant tortoises in Galapagos." Biological Journal of the Linnean Society, 21: 165-176.

 

Hamann, O., 1993. "On vegetation recovery, goats and giant tortoises on Pinta Island, Galapagos, Ecuador." Biodiversity Conservation, 2: 138-151.

 

Hayes, F. E.; Beaman, K. R.; Hayes, W. K.; Harris, L. E. Jr., 1988. "Defensive behavior in the Galapagos tortoise (Geochelone elephantopus), with comments on the evolution of insular gigantism." Herpetologica, 44: 11-17.

 

Hayes, F. E.; Hayes, W. K.; Beaman, K. R.; Harris, L. E. Jr., 1992. "Sleep-like behaviour in the Galapagos tortoise (Geochelone elephantopus)." Herpetological Journal, 2: 51-53.

 

Honegger, R., 1997. "The endangered Galapagos giant tortoise." Online. World Conservation Monitoring Centre. Internet. http://www.dicsovergalapagos.com/tortoise.html

 

Howes, C. 1994. "Galapagos giant tortoises: Inching away from extinction." Bison Brookfield Zoo, 8: 14-17.

 

Marlow, R. W. & Patton, J. L., 1981. "Biochemical relationships of the Galapagos giant tortoises (Geochelone elephantopus)." Journal of Zoology, London, 195: 413-422.

 

Roy, T. D., 1997. "Where Giants Roam." Natural History, 106: 26-28.

 

Schafer, S. F. & O'Neil, K. C., 1983. "Agonistic behavior of the Galapagos tortoise, Geochelone elephantopus, with emphasis on its relationship to saddle-backed shell shape." Herpetologica, 39: 448-456.

 

Siaca-Colon, R. A., 1994. Patterns of growth and maturation in giant tortoises from Pinzon and Espanola Islands, Galapagos, Ecuador. Thesis. George Mason U., 1994.


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