Reptiles


 


Giant Tortoise

No animal is more synonymous with the Galapagos Islands than the giant tortoise. Indeed, the saddle-back shape of the shell in many of the tortoise races reminded the early Spaniards of a type of riding saddle called "galapago", and this term is also applied to the tortoises. Hence, by calling the islands the Galapagos, we are, in essence, calling them "The Islands of the Giant Tortoises"!, The giant tortoise is the symbol of both the Charles Darwin Research Station and the Galapagos National Park Service. In the form of one particular individual, Lonesome George, the sole surviving member of the Pinta Island race, the giant tortoise is the symbol of extreme fragility of the Galapagos islands, and a reminder of the need for vigilence and conservation.

It was also the giant tortoise that tipped Darwin off to the incredible diversity of the Galapagos fauna and flora. In the "Voyage of the Beagle," he noted:

I have not as yet noticed by far the most remarkable feature in the natural history of this archipelago; it is, that the different islands to a considerable extent are inhabited by a different set of beings. My attention was first called to this fact by the Vice-Governor, Mr. Lawson, declaring that the tortoises differed from the different islands, and that he could with certainty tell from which island any one was brought. I did not for some time pay sufficient attention to this statement, and I had already mingled together the collections from two of the islands. I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted; but we shall soon see that this is the case. It is the fate of most voyagers, no sooner to discover what is most interesting in any locality, than they are hurried from it; but I ought, perhaps, to be thankful that I obtained sufficient materials to establish this most remarkable fact in the distribution of organic beings.

There are 15 recognized races of tortoise, all generally considered to be members of the single species Geochelone elephantopus.. The genus Geochelone itself is represented by a cluster of species of small to medium-sized tortoises in South America, Africa, Madagascar, and Asia. In the past, giant species of Geochelone were once found on all continents except Australasia, but today the giant forms are restricted to G. elephantopus in the Galapagos and G. gigantea on the island of Aldabara. There are a number of captive G. elephantopus populations in zoos around the world and it seems that fertility is lower in those zoos that permit breeding between members of different races. The reasons for this reduced fertility are unclear, but it is not possible at this point to rule out genetic factors. The basic definition of the term "species" includes reproductive incompatibility with other species. If this reduced fertility does indeed turn out to be genetic, then the the degree of relatedness between the 15 Galapagos races and their taxonomic status become unclear.

Of the 15 races of Galapagos tortoises, four are extinct. Because of breeding and release efforts on the part of the Charles Darwin Research Station, most of the remaining races are holding their own. However, there is still on-going poaching of tortoises by local residents. One race, that from Pinta is represented by a single surviving male, aptly named Lonesome George. The 15 races are:

 Genus


species


race

Island


 Geochelone

elephantopus

elephantopus Floreana (extinct)

"

"

not described Santa Fe (extinct)

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"

phantastica Fernandina (extinct)

"

"

wallacei Rabida (extinct)

"

"

hoodensis Espanola

"

"

abingdoni Pinta

"

"

ephippium Pinzon

"

"

chatamensis San Cristobal

"

"

darwini Santiago

"

"

vicina Volcan Cerro Azul Isabela

"

"

guntheri Volcan Sierra Negra, Isabela

"

"

vandenburghi Volcan Alcedo, Isabela

"

"

microphyes Volcan Darwin, Isabela

"

"

becki Volcan Wolf, Isabela

"

"

porteri Santa Cruz

 

The circumstances surrounding three of the four extinct races are unclear:

G. e. phantastica, the Fernandina form, is known from only one specimen, a male, found by members of the 1906 San Francisco Academy of Sciences expedition. Nothing more turned up until 1964 (!) with the discovery of putative tortoise droppings. However, no other tortoise, living or dead, has been found on Fernandina and it is entirely possible that that one lone male was a stray or a release. Fernandina is the most pristine of the islands and any tortoise population would not be likely to have become extinct at the hands of introduced animals. If G. e. phantastica is, indeed, a real race, then it is the only one to become extinct by natural means.

Similarly, G. e. wallacei, from Rabida is known from only one specimen. Tracks were seen on Rabida in 1897 and a single individual was removed by the Academy of Sciences in 1906. No logs from whaling or sealing vessels which are known to have collected tortoises for food make any mention of collecting at Rabida. On the other hand, Rabida has a good anchorage and nearby is the remains of a corall in which tortoises, perhaps from other islands were temporarily held. The type specimen of G. e. wallacei, the individual from which the race was named, actually has an unknown provenance: it was assigned to Rabida because it resembled the one removed in 1906. Thus G. e.wallacei could be an escapee from another island.

The situation on Santa Fe is equally unclear. Like Rabida, Santa Fe has a good harbor and the terrain is fairly gentle. But there are only 2 records of whalers removing tortoises, and there are two eye-witness accounts of locals removing tortoises in 1876 and 1890. These accounts, however, were given 15 and 30 years after the fact. The academy expedition found old bones (but no shell fragments, the most durable part of a tortoise skeleton). Given the confusion over island names, it could well be that the reports are mistaken, and the bones the remains of butchered tortoises taken ashore by visitors. There has never been enough material to describe a race.

Thus although Fernandina, Rabida, and Santa Fe are listed as having extinct races, the races are questionable.

The extinct race on Floreana, on the other hand, is far from questionable. Darwin saw them in 1835, and noted that tortoises comprised the main food item in the Floreana colony; "two days hunting will find food for the other five in the week." Although he commented on how the numbers had been obviously reduced from those in years past ("not many years since the Ship's company of a Frigate brought down to the Beach in one day more then 200"), he did mention Vice Governor Lawson's prediction that "there is yet sufficient for 20 years." Indeed there is a well-documented record of heavy collecting in the years leading up to Darwin's visit, but then just three years later, a visiting ship could find no tortoises and in 1846, another visitor declared them extinct. Descriptions of the Floreana race, G. e. elephantopus are based on skeletal material from individuals who fell down into lava tubes and died. On my first visit to Galapagos in 1989 I saw such bones in the cave near the Post Office Barrel.

The 15 races of tortoises can be divided into two general morphotypes: domed and saddle-backed. In the domed tortoises, the front edge of the shell forms a low line over the neck while in saddle-backed tortoises, the front edge arches high over the neck.

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    for more info, contact Dr. Robert Rothman: rhrsbi@rit.edu