seaturtle.org : MTN : ARCHIVES : Sign In

Marine Turtle Newsletter 32:4-5, © 1985

Sex Ratio of Kemp's Ridley: Need for Evaluation

N. Mrosovsky
Department of Zoology, University of Toronto, Toronto, Canada

Thirty-nine 5-year old Kemp's ridleys at the Cayman Turtle Farm had a sex ratio of 1.OO:1.05 (M:F) on 1 April 1984 (Wood & Wood 1984, MTN 29:12). Pritchard and Frazer (1984, MTN 31:8) take this as an "important and pleasing piece of information," suggesting that the artificial hatching at Rancho Nuevo and at Padre Island is not biasing the sex ratio through abnormal thermal conditions. In fact the data from the Cayman Farm are neutral on this point because they do not include information about seasonal trends. A 1:1 ratio at a time of year when mostly one sex is being produced on the nesting beach would represent a bias. Marked seasonal trends in sex ratio are not documented for 3 species of sea turtle (Mrosovsky et al., 1984a, b) and presumably also occur in Kemp's ridleys. Two further and minor points may be noted for the sake of completeness. First, some other Kemp's ridleys were originally sent to the farm but died; necropsy showed these had a 1.4:1 (M:F) ratio (n = 22, Wood 1982, MTN 20:7-9). Second, despite statements in Márquez (1982, MTN 26:3-4) it is not clear whether the Cayman stock of Kemp's ridleys came from eggs incubated at Padre Island or at Rancho Nuevo.

To check if artificial incubation procedures are biasing sex ratio, one needs comparisons of the sex ratio with those in natural conditions at the same time of year. There are three ways of addressing this issue without sacrificing hatchlings for histology. First, one can study specimens that have died. My own efforts to help the Kemp's ridley programme with this failed because the specimens were not preserved rapidly enough. Others are trying this approach again. Second, one can keep turtles till they are mature enough to be sexed laparoscopically or by hormone assays. It would be essential to keep a comparison group of turtles hatched naturally at the same time of year. Third, much could be learned from data on incubation duration. It should be feasible from existing data to produce a seasonal chart of incubation durations for Rancho Nuevo and then add durations for artificially incubated eggs. Because procedures have varied over the years at Padre Island, years should be separately designated, as should data for styrofoam boxes kept at Rancho Nuevo. Some allowance for the time between hatching and emergence would have to be made. None of these methods would yield as quick and as reliable data as systematic sampling of hatchlings. More work on other species might provide guidance if the pivotal temperature for Kemp's ridley were known, but there is an understandable reclutance to sacrifice even enough hatchlings for that to be determined. Nevertheless, with considerable intervention at Rancho Nuevo, involving both relocation of eggs within the sqne and the use of styrofoam boxes, ongoing attempt to evaluate the possibility of sex ratio distortion need to be continued or speeded up. The production of fertile eggs at the Cayman Farm is indeed exciting and may become very important should the population of Kemp's ridleys at Rancho Nuevo decline much further. But these data provide no assurance that the Kemp's ridley programme is not adversely affecting sex ratios.

Mrosovsky, N., S. R. Hopkins-Murphy, and J. I. Richardson. 1984. Sex ratio of sea turtles: seasonal changes. Science 225:739-741.

Mrosovsky, N., P. H. Dutton and C. P. Whitmore. 1984. Sex ratio of two species of sea turtle nesting in Suriname, Can. J. Zool., 62(11):2227-2239.