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Individual study: Incubation of green turtle Chelonia mydas eggs in styrofoam boxes results in a male sex ratio bias of hatchlings, Wia-Wia Reserve, Surinam

Published source details

Mrosovsky N. (1982) Sex ratio bias in hatchling sea turtles from artificially incubated eggs. Biological Conservation, 23, 309-314


Many populations of marine turtles are declining. Conservation measures include incubating wild-laid eggs in hatcheries where protected from predation. Placing eggs in styrofoam boxes and housing these in a simple out-building is convenient and gives high hatch rates. Incubation times using this method are usually longer than in the wild, reflecting lower temperatures; also temperatures tend to vary more. However, incubation temperature affects sexual differentiation with lower temperatures producing more males. The use of styrofoam boxes for incubation of green turtles Chelonia mydas eggs and effect on hatchling sex ratio was tested at Wia-Wia Reserve, Surinam.

In March-April 1981, at Wia-Wia, around 88 turtle eggs (usually slightly less than a full clutch) from each of 10 clutches were taken from the nesting beach; each clutch was placed in a styrofoam box and incubated (following methods described by Schulz 1975).
Upon hatching in May-June, samples of these hatchlings (10/clutch selected at random) were killed and dissected to assess sex. Samples of hatchlings were also taken from 12 clutches that had incubated naturally on the beach.
Temperatures within boxes were measured and compared with those at 80 cm sand depth (nest chamber depth) where green turtles nests were present on the beach using a tele-thermometer probe (readings taken at 3-h intervals in early June).

The styrofoam boxes were 1-1.5°C (average 1.4°C) cooler than the sand on the beach at  natural nest depth (which remained within a quarter of a degree of 28.75°C). Although this does indicate the actual difference for the two groups (as they came from clutches earlier in the season), it does provide a good indication of the approximate temperature differences that prevail between styrofoam boxes and natural nests. Average incubation time for the boxes was 63 days; for the natural nests (five monitored) it was 57 days.
Incubation in the styrofoam boxes resulted in a 23% drop in the number of hatchlings with ovaries compared with those emerging from natural nests. The resultant male-biased sex ratio, is consistent with the well-known effects of temperature on sexual differentiation and verifies previous concerns about using styrofoam boxes for incubation of sea turtle, and other reptile, eggs.
Schulz J.P. (1975). Sea turtles nesting in Surinam. Zool. Verh., 143, 1-143.
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