Figure 4: Onan
Figure 4. Onan close-up.

The future for the surviving Galápagos tortoise [Geochelone nigra (formerly G. elephantopus)] population looked bleak in 1974 when MacFarland et al. (1974a) estimated their numbers at 14,000 (Table 1). Between 100-200,000 tortoises had been slaughtered by whalers, fur sealers and colonists for their meat and/or oil during the last two centuries (from the early 1800's to mid 1900's). Three of the original 14 races are extinct (Figure 1) and one G. n. abingdonii has only a single known survivor, "Lonesome George", who is now housed at the Charles Darwin Research Station (CDRS) on the island of Santa Cruz. Of the remaining 10 races those from the three volcanoes on northern Isabela Island (Alcedo, Darwin and Wolf) and the highlands of Santa Cruz have relatively stable populations capable of natural replacement. As MacFarland and Reeder (1975) reported, the other races are still threatened by the presence of feral mammals or have a drastically reduced population (Table 1).

Map of the Galapagos Islands

Figure 1. Distribution of the originally recognized 15 races of Geochelone nigra.

Thanks to the joint efforts of the CDRS and the Galápagos National Park Service (GNPS), a tortoise conservation program was established in 1965. That year native tortoise eggs from Pinzón Island were brought to the CDRS for hatching and rearing to avoid black rat (Rattus rattus) predation. In December 1970, the first group of 20 captive-raised tortoises were repatriated and most years since then other groups have also been released on Pinzón.

The tortoise conservation program has been plagued by a lack of staff and resources. Nonetheless it continues to be a major success with all but one of the remaining tortoise populations improving (Metzger and Marlow, 1986). In 1984, de Vries listed 541 repatriates from 6 races that had been reared at the CDRS and released. Marquez, et al. (1987) reported that since 1970, 893 young tortoises, from 8 different races had been released into their native habitats, and Cayot (1991) noted that 1,216 young have been repatriated as of January, 1991 (Table 2).

The situation has improved for most of the 11 known surviving races (Table 2). Noteworthy, is the fact that as of January 1991, 328 Española Island tortoises (G. n. hoodensis) and 268 Pinzón tortoises (G. n. ephippium) have been repatriated. And some of the oldest repatriates (hatched in 1965-70) of both races were observed in normal breeding and preliminary nest building activity (MacFarland personal comm., 1989). In fact, in late 1990 the carcasses of two G. n. hoodensis hatchlings, killed by hawks, were discovered on Española (Cayot, personal comm., 1991). On one hand, it is sad that two hatchlings were lost but more importantly the thrilling news is that a population of tortoises capable of natural replacement is in place on Española. Perhaps after a few more years of head starting this aspect of the program can be halted because the population will be self sustaining.

The Pinzón Island Tortoise Situation

During August 1986 I had the pleasure of visiting Pinzón Island with Dr. Peter C. H. Pritchard as part of a crew filming a documentary on the turtles and tortoises of the world. Pinzón is off limits to all tourists and home to some of the oldest tortoises in Galápagos and therefore on earth. Their history, marred by exposure to man, took them to the brink of extinction!

Figure 2: Map of Pinzon Island

Figure 2. Map of Pinzón Island

 

Pinzón is a low (458 m), dry and relatively small island (18.05 km²) in the center of the archipelago (Figure 2). Tierney (1985) said that, "Pinzón Island is basically one large thornbush, and the bush covers a 1,300-foot-high volcano -- nine square miles of lava rocks and parched thickets." The trip from the small inlet on the northeast side of the island where boats can safely anchor, to the area the tortoises inhabit is a rigorous one. The highest concentration of repatriated tortoises is at the South Crater area, while the native tortoises and many other repatriates live on the outer western and southern slopes of Pinzón (Figure 2).

Figure 3: The author and Onan
Figure 3. Onan and the author
(Photo by K. Switak).

The Pinzón tortoises, G. n. ephippium, are saddlebacked, relatively small (adults curved carapace length (CCL) = 61 cm; Santa Cruz adults, one of the races typically found in U.S. zoos have a CCL = 75-150 cm) and light (maximum weight = 76 kg; Santa Cruz = 290 kg). In the 18th and 19th centuries this was certainly a liability as whalers preferred tortoises that could be carried by one man. The sparse vegetation offered less concealment than on other islands that have more moisture. These facts encouraged the collection of large numbers of tortoises and it was only the collapse of the whaling industry in the latter part of the 19th century that prevented the extinction of the Pinzón population (Metzger and Marlow, 1986).

Figure 5: Onan assumes the male tortoise challenge position
Figure 3. Onan assumes the male
tortoise challenge position.

In 1970 the Pinzón population was estimated to be 150-200 native adults (MacFarland and Reeder, 1975). While today the native adult population is believed to be 80-100 (Morillo and Cayot, 1990) with a few more of the old individuals dying each year. [Note: Unfortunately, Onan, one of the oldest, most "colorful," and often photographed tortoises in Galápagos (see Caporaso, 1989) died in May, 1990. He was an aggressive male Pinzón tortoise (Figures 3, 4 and 5)]. Black rats were introduced to Pinzón before 1891 (the date they were first recorded; Patton, et. al., 1975), preying heavily on hatchling tortoises to the extent that it was thought that virtually no recruitment occurred during this century (MacFarland, et al., 1974a). In fact, the only reported observation of a wild Pinzón tortoise hatchling, prior to 1988 was made by de Vries (1984). Imagine any animal population (an entire race in this case) having no surviving hatchlings for more than 70 years! This would insure extinction for most. With the sparse rugged habitat, human slaughter and rat predation, one can easily see why Pritchard (1985) referred to the Pinzón Island saddlebacks as the "toughest tortoises in the Galápagos."

The Conservation Program

The Plan of Attack

The CDRS personnel began collecting eggs from natural nests on Pinzón in 1985 and transferring them to the Darwin Station on Santa Cruz Island for hatching and head starting until the young were large enough to be safe from rat predation (4-5 years). Since 1968 the program has been a co-operative effort between the CDRS and GNPS. In December 1970 the first group or tortoises was repatriated, and as of December 1990, 268 repatriates have been returned to Pinzón (Table 3).

Reproduction of the repatriated tortoises and elimination of the black rats are the final stages to total recovery. And recovery seems quite plausible; if the rats can be eliminated.

Egg Collection and Handling

Nesting areas, which are limited in number, are visited 2-3 times/year by trained park wardens or station personnel. The nest is opened near the end of the incubation season. After marking, weighing and measuring the eggs they are transported to the CDRS by padded backpack (1-2 hours) and boat (5-6 hours) (MacFarland and Reeder, 1975).

In the first years of the program, in order to evaluate the relationship between age at transport and addling (refers to a liquefied egg, i.e., either infertile or the embryo having died before attaining sufficient size to be detectable), Pinzón tortoise eggs were brought to the CDRS for incubation at various ages (0-15 weeks). The results of this study (Table 4) determined that transport at early stages of development destroyed a large percentage of the eggs. However, when eggs were transported at 10-15 weeks of age, at which time the embryos were well developed, the percentage of fertile and hatched eggs was much closer to that of eggs left in the wild. Hatching occurred at 12-17 weeks of age, the variability being due to the time when the nests were made and the continually rising temperature of Pinzón from August to March as the garua season (June-December, frequent cloud cover and misty rain) ends and the hot season (January-May; infrequent cloud cover, occasional heavy showers [only in some years] and intense solar radiation) takes over (MacFarland and Reeder, 1975). Eggs laid later in the nesting season develop more rapidly because of the higher temperatures.

Incubation and Hatching Rates

Originally, solar heated, wooden incubators were kept in cement-lined cavities as described previously (MacFarland and Reeder, 1975; Caporaso, 1989). In the 1980's the CDRS began using electric incubators to better maintain temperature control (28-29.5° C). The incubators were built at the station and consist of a shelved, wooden cupboard inside another wooden cupboard to limit temperature fluctuations. The cupboards are heated by a hair dryer and cooled by a ventilator both thermostatically controlled.

The eggs are placed in covered plastic containers with a vermiculite substrate maintained at a water potential of -206 Kpa (obtained by mixing 60 cc of water with 1,000 g of vermiculite). During incubation the eggs are checked Monday-Friday for signs of fungus, and later on, on a daily basis to remove hatchlings. The hatchlings are kept together in a dark box on Pinzón Island soil for a few days after hatching to better simulate nest conditions at hatching.

Experimentation during the later half of the 1980's determined that the use of electric incubators set at a temperature of 28-29.5° C yielded the best hatching success with approximately 66% of the hatchlings being female.