Radiated tortoise

Adult radiated tortoise
Photograph by Fred Caporaso

The New York Zoological Society (NYZS) has maintained a herd of Madagascan radiated tortoises (Geochelone radiata) at its Wildlife Survival Center on St. Catherine's Island, Georgia, since 1979. Since the program was initiated, various management schemes have been employed and modified as dictated by observed social and reproductive behaviors (see Behler and Valenzuela, 1983; and Iaderosa, et al., 1990a). Approximately 200 hatchlings have occurred with more than 100 of these taking place during the past three seasons as a result of refinements to our management protocol. Current practices are discussed below. Consistent successful captive propagation of this species has hinged upon the development of an NYZS directed Species Survival Plan program, careful attention to the health and nutritional status of the herd, and a planned annual calendar of social interactions among members of the breeding groups.

Facilities and Environment

The 2,800 m tortoise propagation complex includes a 6.1 x 16.5 m heated wintering barn and a series of 15 x 15 m to 15 x 25 m breeding and management yards surrounded by a 2.4 m high vinyl-clad chain-link fence. St. Catherine's Island is situated at 31° 40' N latitude, and the core range of G. radiata falls between 23° 30' and 25° 30' S on Madagascar. Observed temperature extremes on St. Catherine's Island have ranged from -15° C to 38° C during the past decade. The annual average temperature for nearby Savannah, Georgia, is 18.9° C; average rainfall = 1,262 mm. By comparison, the natural environment of G. radiata is characteristically tropical despite its position below the Tropic of Capricorn. Donque (1972) notes that the low-lying southern coastal areas experience rather uniform temperatures over the entire year. The difference between monthly average temperatures for summer and winter months is less than ten degrees. Temperature and rainfall data for Tulear and Ft. Dauphin, which represent the northwest and southeast ends of the radiated tortoises' range, are 22.8° C and 342 mm, and 23.8° C and 1,530 mm, respectively. Despite differences, seasonality of both St. Catherine's and native haunts of the radiated tortoise is marked by dry and cool seasons, prolonged periods of very warm humid days, and severe summer tropical storms.

Management practices on St. Catherine's attempt to keep environmental parameters within bounds of those the tortoises would experience in nature. The references cited in the introduction describe the management facility in greater detail, and offer additional information on the character of St. Catherine's Island, and the history of the tortoise propagation program there.

Management of Female Tortoises

Female radiated tortoises are housed together from mid-November until the end of April. Then they are assigned breeding partners according to Species Survival Plan directives. On 1 May, individual females are placed in an outdoor breeding pen with a previously stimulated male. Females typically are very actively courted for 2-3 days post introduction. When courtship and mounting behaviors have stopped, females are removed from breeding pens and placed in a nearby yard with other females. One to two weeks later, they are reintroduced to their assigned breeding partner.

Management of Male Tortoises

Like females, males are housed together during the cold weather, non-breeding season. In late April assigned breeders are separated from the male herd and placed in one of the pens with one or two beta males. The alpha animal will remain in the pen until mid-November. Aggressive interaction between alpha and beta individuals typically takes place in the pens and serves to stimulate the breeder. Just before introduction of the females on 1 May, beta males are removed to the bachelor pen. When reproduction activity wanes, the females are removed and beta males are reintroduced. The addition of beta individuals again provokes combat activities. The importance of these aggressive interactions between male "rivals" to program success can't be too strongly emphasized. On the other hand, breeding groups maintained together in a single enclosure during the breeding season may not reproduce well because males are preoccupied with agonistic behaviors and miss out on breeding opportunities.

Egg Production and Incubation

Since 1987, nearly 500 eggs have been laid by five females. Iaderosa, et al, (1990b) report recent data. Egg deposition has occurred in all months of the year but more than 90% of these events took place between 1 September and 30 April. Only 8% of the eggs laid before 15 October (= 30% of total eggs deposited) are fertile, while 41% of eggs laid after that date to the end of the season are fertile. Overall fertility rate has been 32%. Clutch size has ranged from 2-9 eggs (mean = 5.1). A given female may nest as many as seven times per year but 5-6 nestings/producing female/season is considered normal at St. Catherine's. Interclutch interval has ranged from 21 days to 3 years. Occasionally a female has stopped producing eggs for two or more seasons and then commenced laying again normally. Nesting times have clustered around midday. Eggs are excavated from the nest, numbered with a soft lead pencil, weighed and measured, and artificially incubated in air-tight 4 l containers, 1/3 filled with moistened vermiculite (vermiculite/water = 1/1 by weight) in a commercial wooden chick incubator (GQF #25). Incubation length, across all temperatures employed, ranged from 79-273 days (means for 28.9° C, 30.0° C, and 31.1° C = 121, 112, and 114 days, respectively).