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Keywords:

  • embryogenesis;
  • incubation;
  • life history;
  • reproduction;
  • sex reversal

Abstract  Sex disparities in mean dates of hatching are widespread, but are generated by a variety of mechanisms. For example, a correlation between offspring sex and hatching date is produced by temperature-dependent sex determination (TSD) in turtles and lizards, and by laying asynchrony and differential incubation behaviours in birds. Our laboratory experiments on montane lizards (Bassiana duperreyi, Scincidae) provide the first direct evidence of an intrinsic sex effect on the date of hatching in a reptile. All eggs within a clutch are laid on the same day but, even when incubated under identical thermal and hydric conditions, male lizards hatch about 2 days earlier than do their sisters. The effects are not due to laying asynchrony, differential incubation by parents, TSD or sex differences in mean egg size. Hormonally induced sex reversal did not eliminate the sex disparity in incubation periods, suggesting that this sex disparity is due to events occurring prior to oviposition rather than to sexual differences following gonadal differentiation. In the field, the intrinsically briefer incubation of male embryos is counterbalanced by two other trends: smaller eggs more often produce males and tend to hatch later than larger eggs within the same clutch; and cooler (and thus, later-hatching) nests overproduce male offspring via thermally induced sex reversal. Thus, somewhat ironically, the sex-specific hatching asynchrony of laboratory-incubated eggs may reflect adaptation to minimize sex differences in dates of hatching.