Sex ratios of hatchlings of the green turtle, Chelonia mydas, in natural nesting grounds, in open beach hatchery, and in sheltered beach hatchery in Gulisaan Island, Sabah
The sex of the green turtle, Chelonia mydas, is determined by temperature during egg incubation. Temperatures higher than 30.0 °c generally produce female while below 30.0 °c produce male hatchlings, but the actual temperatures were dependent on where the investigations had been carried out. Guli...
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| Format: | Thesis |
| Language: | English |
| Published: |
2001
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| Online Access: | https://eprints.ums.edu.my/id/eprint/19291/1/Sex%20ratios%20of%20hatchlings%20of%20the%20green%20turtle.pdf |
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| Summary: | The sex of the green turtle, Chelonia mydas, is determined by temperature during egg
incubation. Temperatures higher than 30.0 °c generally produce female while below
30.0 °c produce male hatchlings, but the actual temperatures were dependent on
where the investigations had been carried out. Gulisaan Island, one of three islands in
the Turtle Islands Par~ is one of the important turtle hatchery sites for the green and
hawksbill turtles, Eretmochelys imbricata, in Sabah, Malaysia. A preliminary study
by the present investigator in 1996 in Gulisaan Island concluded that feminisation of
these turtles was temperature dependent during the dry period of April to July. The
present investigation was planned as an extension to the previous study to coincide
with the wet season from November 1997 to January 1998, but coincided with the El
Nino dry period. In addition, an experiment was carried out to investigate the
feasibility of lowering the incubation temperature of eggs by using different roofing
materials in the open-beach hatchery. The materials used for shading were nonporous
plastic canvas, black netting, green netting, and coconut fronds. Fifteen
hatchlings were haphazardly selected from each clutch and sacrificed for histological
examination of their gonads.
Results showed that the natural nesting grounds have slight variations ranging
30.1 ± 0.1 °c (± S.E.) for vegetated locations and 31.3 ± 0.3 °c for sandy locations.
The sex ratio of hatchlings from natural nesting grounds was 7 females to 3 males.
However, it was clear that female hatchlings were only produced from the exposed
sandy locations, while the shady vegetated locations produced balance female to male
sex ratio. The open-beach hatchery however, had consistently high incubation
temperatures (31.3 ± 0.1 °C), producing 100 % female hatchlings. The shading of the
open-beach hatchery resulted in lower incubating temperatures and significantly
different sex ratios. The plastic canvas treatment recorded lower temperature (30.7 ±
0.1 °C), but was not significant in producing males, as 100 % females were produced.
The green netting treatment lowered the temperature to 30.2 ± 0.1 °C, producing
female to male sex ratio of 7:3. The black netting treatment lowered the temperature
to 29.6 ± 0.1 DC, producing female to male sex ratio of 2:8. However, at 29.6 ± 0.2
DC, the coconut frond treatment produced female to male sex ratio of 7:3, contrary to
the generalisation that at this temperature more males will be hatched. This seen an
anomalous result since the incubation temperature, on the average, did not surpass the
estimated pivotal temperature range of 30.16 °c to 30.19 °c, but may just be within
natural variation.
These findings are significant to enable hatchery manipulation to produce the
desired sex ratios in green turtle hatcheries. Providing shade to the open-beach
hatcheries is recommended during periods of high temperature and dry weather to
keep natural ratios of female to male hatchling. In addition, the observed correlation
of sex ratios with incubation temperature and incubation duration from the three
incubation conditions can be used as a simplified method in estimating the sex ratios
produced in the open-beach hatchery. This study has provide a rare empirical estimate of the sex ratios in the wild under the impact of elevated global temperature
such as the El Nino phenomenon. These findings and its applications are relevant to
the conservation of green turtles in the Gulisaan Island and most probably throughout
the Indo-Pacific region. |
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