Sex Determination and Sex Ratios inCrocodylus palustris

Abstract

Incubation temperature determines sex in the mugger crocodile, Crocodylus palustris . Exclusively females are produced at constant temperatures of 28.0°C through 31°C. At 32.5°C, only males are produced. Both sexes are produced in varying proportions at 31.5, 32.0, and 33.0°C. Embryo survival is not affected within this range, but developmental rate and total incubation time are strongly temperature dependent. In natural nests laid in breeding enclosures, cool incubation temperatures produced only females whereas males were produced only in warm nests. Clutch sex ratios were female or male biased. Yearly sex ratios (=percent male) varied from 0.05 to 0.58; overall sex ratio during six nesting seasons was 0.24 (1 male: 3 females). Sex ratio and incubation time vary with nest location and temperature in a manner consistent with the constant temperature results. Incubation time decreases with increasing incubation temperature, and is an accurate predictor of sex ratio in the field and laboratory. To date, temperature-dependent sex determination (TSD) has been reported in five species of Crocodylus and in three species of Alligatorinae; but the TSD patterns in these groups differ. The TSD pattern of C. palustris is similar to that of C. porosus . Nesting in C. palustris is synchronized with the seasonal availability of thermal regimes suitable for incubation. Resultant sex ratios are a consequence of when and where eggs are laid. Early nests are located in warm, sunny sites; in contrast, late season nests are located in the shade. An egg transplant experiment demonstrated that sex ratios could be altered by simple manipulations of nest temperatures in the field. The adaptive significance of TSD in crocodilians may relate to the influence of incubation temperature on various hatchling attributes, particularly growth.