Horikoshi, Kazuo 



Eqcf Survivorship and Primary Sex Ratio of 

 Green Turtles (Chelonia itiydas) at Tortuquero. Costa Rica 



Center for Sea Turtle Research 



Department of Zoology 



University of Florida 

 Gainesville, Florida 32611 USA 



Number and position (either vegetation/border zone or open 

 sand zone) of green turtle nests in the central part of 

 Tortuguero Beach, Costa Rica, were recorded from 15 July - 9 

 November 1986. The number of nests was the highest recorded for 

 16 years at Tortuguero. Over 3,700 clutches per kilometer were 

 deposited in the high density areas from July to October. The 

 proportion of the nests in the vegetation/border zone and in the 

 open sand zone were 51.4% and 48.6%, respectively. These values 

 are not significantly different (chi-square test, alpha = 0.05). 



Egg survivorship was measured for 74 marked nests. In 1986, 

 hatching success rate was 46.3% +39.0 (n=32) in the 

 vegetation/border zone and 57.3% + 37.5 (n = 42) in the open sand 

 zone. These values are not significantly different (Mann-Whitney 

 test, alpha = 0.05). Flooding and high ground water caused by 

 sporadic heavy rains (>100mm per day) and erosion by waves were 

 major causes of mortality for clutches. Animal predation, mainly 

 by coatis ( Nasua narica) , and digging up of nests by adult female 

 turtles also reduced the survivorship of eggs. 



Sand temperature at the depth of the nest (60 cm deep) in 

 the different zones was monitored during the incubation period 

 from 1 July - 10 December 1986. At Tortuguero, Morreale (1983) 

 found that nests with temperatures below 28.5°C during the middle 

 third portion of development would produce nearly 100% males, 

 while nests with temperatures exceeding 30.3° C would produce 100% 

 females. For intermediate temperatures, a mixed sex ratio would 

 result. Because the sand temperature, regardless of the position 

 on the beach, remained below 28.5° C for most of the season, the 

 sex ratio of hatchlings that emerged in 1986 was probably 

 strongly male-biased. In the open sand zone, although the sand 

 temperature from September to December intermittently rose to the 

 level of producing both sexes, these periods were too short to 

 produce many females. 



Numerous rainy days and especially, sporadic heavy rain, 

 produced the low sand temperatures for the entire season. 

 Rainfall records revealed that rainfall in August (84 6 mm) and in 

 September 1986 (514 mm) were the highest recorded for the last 

 nine years. Rainfall for 1980, when Morreale (1983) found 

 differences in sex ratios depending on nest position on the 

 beach, was much lower than that in 1986. Since rainfall at 



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