Darwin's Galapagos

Indeed, the saddle-back shape of the shell in many of the tortoise species 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 vigilance 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 populations 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 Geochelonewere 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:

species	Island
C. hoodensis	Española
C. duncanensis	Pinzón
C. chathamensis	San Cristóbal
C. darwini	Santiago
C. becki	Volcán Wolf, Isabela
C. microphyes	Volcán Darwin, Isabela
C. vandenburghi	Volcán Alcedo, Isabela
C. guntheri	Sierra Negra, Isabela
C. vicina	Cerro Azul, Isabela
C. porteri	Western Santa Cruz
C. donfaustoi	Eastern Santa Cruz
C, phantasticus	Fernandina (survived by one female)
C. abingdonii	Pinta (extinct after death of Lonesome George
C. niger	Floreana (extinct)
undescribed	Santa Fé (extinct)
C. wallacei	Rabida (doubtful species)

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 Rabidabecause it resembled the one removed in 1906. Thus G. e.wallaceicould 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.

These differences are related to the environments in which the respective tortoises live, and the types of food they eat. The domed tortoises tend to live in the moist high-lands and take their food from grasses and low-lying shrubs. The saddle-backed tortoises, on the other hand, live in arid regions and feed on plants that are mostly above their head, most notably the tree-like Opuntia (prickly pear) cactuses. The arched shell permits them to stretch their heads high, giving them a longer vertical reach.

There is a very interesting relationship between tortoises and the various Opuntia species that inhabit the various islands. On those islands where there have never been any evidence of resident tortoise populations (e.g. Genovesa), the resident species O. helleri tends to spread low over the ground and has soft, flexible spines, whereas on islands that have established tortoise populations, the resident species, like O. echios on Santa Cruz tends to grow as trees. As young plants, the trunks are thickly covered with long, sharp spines which give way in older plants to a tough, tree-like bark. Thus, these Opuntias are inaccessable to the tortoise until the fruits, laden with seeds, fall to the ground, whereupon the tortoises eat them. The seeds pass unharmed through the tortoise’s gut and are dropped as the tortoise wanders from place to place.

There are several other interesting differences between the two morphotypes. Domed tortoises tend to be larger than saddle-backed tortoises. Two hypotheses have been presented to explain the size differences:

1. small tortoises have require less water and energy, and can make better use of limited shade in dry environments, and
2. large tortoises are more resistant to cooling during prolonged cloud cover typical of the highlands

A second difference is that saddle-backed tortoises tend to be more aggressive and solitary while domed tortoises tend to be more gregarious. Aggressive displays are contests to see which of two combatants can raise their head the highest. On one visit to the Galapagos, I was sitting in the tortoise compound on a cool, cloudy morning. As expected, there was little activity, but as the sun came out and everything warmed up, I noticed three tortoises, side-by-side waking up. As they became aware of each other, they begain to raise their necks with gaping mouths, each trying to out-do the other. This aggressiveness might force the tortoises to become more solitary, thereby avoiding competition for food. Thus natural selection might favor the arched shell in dryer areas because it provides for greater reach while foraging as well as greater reach during combat while protecting food.

Tortoises mate during the the rainy season, beginning in January. On the large islands, they gather in the wet highlands and begin copulating. The male mounts the female from behind (his plastron, the lower side of the shell, is concave to permit him to balance on top of his mate) and then, during copulation emits a loud groaning sound which can carry quite far. The significance of the sound is unclear. After copulation, the females make their way down to the lower, coastal regions where there are good nesting grounds. Using their bodies, they scoop out a shallow depression, 30-40 cm deep. The soil is softened with urine during digging. Finally, 2-16 round, hard-shelled eggs are laid and the nest is sealed with a cap of mud moistened with urine. A female can lay 4-5 clutches per season, usually between June and December. The eggs hatch 4-5 months later.

In the pristine state, tortoises had few natural enemies – mostly Galapagos hawks, and possibly owls, that preyed on the hatchlings. But in the age of man, tortoise populations have been depleted by human activity both directly and indirectly. During the whaling era, the tortoises were highly prized by sailors for provisions because they could live for long periods of time without food or water, with little deterioration. In the days before preserved food, this was of great value to sailors who visited the Galapagos and carried off many thousands of tortoises. Because interiors of the islands were so inaccessible, the sailors concentrated on the coasts and took away, therefore, massive numbers of breeding females. Feral animals have also wrought havoc on tortoise populations. Pigs, dogs, and rats dig up nests and eat eggs and young. Goats directly compete for food and donkeys trample nesting areas and change the topography from forested areas to grassy plains. Well into this century, people have continued to exploit the tortoises for oil. The photo below showing the remains of hundreds of tortoises slaughtered on Isabela for oil, was taken in 1906 by a member of the California Academy of Sciences expedition, and this activity has gone on up until the foundation of the National Park in 1959.

Since1959 scientists at the Charles Darwin Research Station have begun a very successful two-pronged tortoise reclamation program. Eggs from both wild tortoises and breeding colonies at the station are hatched in incubators and the hatchlings cared for until they are big enough to be released with a good chance of survival. The other part of the program is to eradicate introduced animals on the various islands. As a result, the Espanola population, for example, reduced to less than 15 individuals, is now flourishing.

But despite the advances, tortoises are still in a precarious situation. In 1995 fishermen who were unhappy with new government regulations on sea cucumber fishing killed tortoises on Isabela and forcibly occupied the Charles Darwin Research Station on Santa Cruz. If their demands were not met, they threatened, they would kill Lonesome George, noted above as the sole surviving member of the Pinta island race G. e. abindoni, and the symbol of the conservation efforts in the Galapagos. Volcan Alcedo, Isabela, long considered to have the healthiest population of tortoises, is being over-run by feral goats. Alcedo is now closed to visitors and the Darwin Station is waging the “Alcedo Campaign” to kill the goats. As we left the rim of Alcedo on my 1991 trip, I looked back and saw my last tortoise for the day, slowly being enveloped by the gathering mists of low-hanging clouds. The Galapagos tortoise is as endangered now as it has ever been.