A captive group of Madagascar ploughshare tortoises or angonoka (Geochelone yniphora) has been maintained at the Honolulu Zoo since April 1971 (Juvik & Blan, 1974). There was little reproductive activity in this group until the late 1970's when an additional adult female G. yniphora was secured on breeding loan from the San Antonio Zoo in Texas. Between September 1879 and September 1982 this tortoise laid a total of 25 eggs in 6 clutches. However, none of these eggs proved to be fertile. Also, over this period, the zoo herpetology staff developed techniques to encourage successful natural mating by periodically isolating the males and females and then reintroducing them to one another for short periods. More natural mating attempts followed the adoption of this management approach. McKeown (1982) and McKeown et al. (1982) have described general captive husbandry, courtship behavior and egg laying for this species.
Figure 1. Hatchling Geochelone yniphora; age one month (October 1983); carapace length50 mm; weight 32 g.
During the summer of 1982, several apparently successful courtship and mating sequences were observed involving the male and egg laying female G. yniphora. Between September 1982 and February 1983, the female laid a total of fiveegg clutches at approximately one month intervals. The egg laying dates and clutch sizes were as follows: 22 September 1982, four eggs; 25 October 1982, seven eggs; 1 December 1982, five eggs; 6 January 1983, six eggs; and 19 February 1983, three eggs. All eggs were incubated in covered plastic containers with moist vermiculite at 28.9° C (+/- 1.0° C). On 12 March 1983, rotting eggs of the 25 October clutch were opened and one revealed a dead embryo (approximately 19 mm in length). This was the first evidence of egg fertility among the many egg clutches laid by the female since 1979.
Other eggs of the 1982-83 clutches were examined and candled periodically during the summer of 1983 (two additional dead embryos were discovered). One egg of the 2 February 1983 clutch, that appeared to be developing during periodic candling, hatched successfully on 16 September 1983 after an incubation period of 226 days. At birth, the baby G. yniphora was 42 mm long and weighed 24.6 g.
|Figure 2. Electroejaculation of G. yniphora at the Honolulu Zoo (13 October 1982). Left: tortoise secured upright in working position. Right: penis relaxed and protruded, 2.5 cm diameter rectal probe (with electrodes) inserted dorsal to exposed penis into the cloaca a depth of 10-12 cm.|
Because of egg infertility in G. yniphora clutches laid in Honolulu between 1979 and 1982, Zoo staff became interested in the possible application of semen collection/evaluation and artificial insemination (AI) procedures then being developed for turtles and tortoises (Platz et al., 1980). With technical expertise and equipment provided by Dr. C. Platz, Jr. (Texas A&M University) and Dr. Greg Mengden (Australian Museum-Sydney), an AI program was initiated at the Honolulu Zoo. In October 1982, Mengden visited Honolulu, and began initial experimental work with the Zoo's G. radiata (radiated tortoise) group. The electroejaculator consisted of a variable voltage power source and a plastic rectal probe 2.6 cm in diameter with three 3.0 mm electrodes arrayed longitudinally. After first fully testing semen collection procedures on a male G. radiata, a small quantity of motile sperm was successfully collected from the male G. yniphora on 13 October, 1982 and immediately injected into the egg-laying female. This was the only AI procedure with the female prior to September 1983. The female G. yniphora had laid a clutch of 4 eggs on 23 September 1982 (all proved infertile). Twelve days after the first AI procedure, on 25 October 1982, the female laid another clutch of 7 eggs. It was from this clutch that the first evidence of egg fertility was recorded when a single dead embryo was discovered in March 1983. We conclude that the interval between AI and egg laying (12 days) for this 25 October 1982 clutch provided insufficient time to credit the AI procedure for the recorded egg fertility; fertilization was probably produced by natural inseminations from active mating episodes that occurred during the summer and fall of 1982.
Evident fertility associated with additional dead embryos recorded in egg clutches laid on 1 December 1982 and 19 February 1983, and the live hatchling (also from the 19 February 1983 clutch), is thus equivocal with respect to the possible role of the October 1982 AI procedure or concurrent natural fertility. In July 1983, one of us (McKeown) left the Honolulu Zoo, and the zoo's herpetologist position remained unfilled for almost a year. During this period AI work with G. yniphora was under the direction of the zoo veterinarian. After a further series of electroejaculation procedures were undertaken in September 1983, the male G. yniphora stopped eating and died 30 days later (necropsy was inconclusive but suggested kidney failure).
Growth data for the 1983 G. yniphora hatchling are presented in table 1.
Table 1. Growth of captive born Geochelone yniphora in the Honolulu Zoo, 1983-1990.
|16 SEPT 1983||1 day||24.5||42||—||36|
|27 SEPT 1983||10 day||25.6||46||25||39|
|21 MAR 1984||0.5 yr||76.6||65||41||56|
|22 SEPT 1984||1.0 yr||113||74||51||64|
|30 MAR 1984||1.5 yr||174||80||59||71|
|16 SEPT 1985||2.0 yr||280||95||71||83|
|12 MAR 1986||2.5 yr||368||103||78||91|
|10 AUG 1986||2.9 yr||512||—||—||—|
|5 NOV 1987||4.2 yr||927||140||105||118|
|19 SEPT 1988||5.0 yr||1490||—||—||—|
|15 SEPT 1989||6.0 yr||2340||—||—||—|
|23 JULY 1990||6.8 yr||3100||225||165||170|
Figure 3 illustrates a normal growth curve (made linear by log transformation of the weight values), with weight and age highly correlated (R=0.997; p<0.001) at age 2 years and above.
Figure 3. Relationship between age and weight (log scale) of the captive born G. yniphora in the Honolulu Zoo. Regression line fitted to data points beginning at two years of age.
Figure 4. Growth rates for the Honolulu captive bred G. yniphora in comparison with wild tortoises (age estimated from annual growth rings). Wild tortoise data from Juvik et al., 1981 and Curl et al., 1984.
If this growth curve was maintained over the next few years, the regression equation (log Y = 2.045 + 0.213X) predicts that typical adult weight (10-15 kg) would be achieved at between 9 and 10 years of age. Figure 4 illustrates the relationship between age and carapace length for the Honolulu captive-raised G. yniphora in comparison with growth of wild tortoises in the Baly Bay region (age estimated from annual growth rings). The linear regression equations yield annual growth increments of 25.77 mm/yr for the captive tortoise compared with 14.96 mm/yr in the wild. At the captive tortoise's current age (6 years, 10 months) its shell length (225 mm) is 57% larger than that predicted (143 mm, from regression) for a wild tortoise of comparable age.
Figure 5. Honolulu born G. yniphora at age 2.5 years (carapace length 106 mm). Abnormal "doming" of shell
Gaymer (1968) in an analysis of comparative growth in Geochelone gigantea (Aldabra tortoise) found an even larger difference in growth rates between captive-raised and wild populations. At age 7 years, the average carapace length of captive-raised G. gigantea was 94% larger than in wild populations. This extreme difference may be explained by generally suppressed growth in high density wild populations experiencing intense intraspecific competition for food and shade (see Merton et al., 1976). Such competition-related growth suppression should not be a significant factor in wild G. yniphora which now exist at very low densities.
Zovickian (1973) reported that sexual maturity in the closely related G. radiata is achieved at carapace straight length of 330 mm (male) to 340 mm (female). If these values are roughly transferable to G. yniphora (with a slight upward adjustment to compensate for the overall larger size of this species, i.e., 350 mm), then, based on the above regression equations, the Honolulu captive should reach maturity at year 12, compared to year 20 for wild tortoises.
In retrospect, the growth rate achieved in the Honolulu Zoo, captive-raised G. yniphora was too fast, and led to distortions in allometric growth. At one year of age the captive G. yniphora exhibited abnormal shell "doming". This species naturally possesses a high domed shell, particularly in adult females, where shell height averages 62% of carapace straight length (Juvik et al., 1981). By age 3 years, the captive G. yniphora shell height reached 76% of carapace length, although by age 6 this value had retreated to 73%. This shell deformity is not only of aesthetic concern, particularly if carried into adulthood. Excessively domed shell geometry in G. yniphora could be expected to lead to reduced reproductive success.
Our captive growth results in G. yniphora indicate that allometric relationships must be closely monitored during the first few years. Diet and environmental factors should be manipulated to ensure that captive growth rates do not get too far ahead of "natural" (wild) rates so as to produce shell distortions.
Figure 6. Ratio of shell height to carapace length in the Honolulu captive born G. yniphora and wild tortoises (data from Juvik et al., 18981 and Curl et al., 1984)
Shortly after the departure of the supervising herpetologist (McKeown) to the Fresno Zoo in 1983, the diet for juvenile tortoises which had consisted of chopped butter lettuce, romaine, kale, grated carrots, tomato and papaya was modified by the Honolulu Zoo veterinarian. Additional fruit in the form of banana and apple were added along with moistened (dry) dog food and later, ZU/PREEM Marmoset diet. We believe that the abnormal growth of the captive raised G. yniphora may be related to both the additional fruit, as well as the dog chow and marmoset diet provided during the first two years of its life.