Interactions among thermal parameters determine offspring sex under temperature-dependent sex determination

Abstract

In many animals, temperatures experienced by developing embryos determine offspring sex (e.g. temperature-dependent sex determination, TSD), but most studies focus strictly on the effects of mean temperature, with little emphasis on the importance of thermal fluctuations. In the jacky dragon (Amphibolurus muricatus), an Australian lizard with TSD, data from nests in the field demonstrate that offspring sex ratios are predictable from thermal fluctuations but not from mean nest temperatures. To clarify this paradox, we incubated eggs in a factorial experiment with two levels of mean temperature and three levels of diel fluctuation. We show that offspring sex is determined by an interaction between these critical thermal parameters. Intriguingly, because these two thermal descriptors shift in opposing directions throughout the incubation season, this interactive effect inhibits seasonal shifts in sex ratio. Hence, our results suggest that TSD can yield offspring sex ratios that resemble those produced under genotypic sex-determining systems. These findings raise important considerations for understanding the diversity of TSD reaction norms, for designing experiments that evaluate the evolutionary significance of TSD, and for predicting sex ratios under past and future climate change scenarios.

Figure 1.

Experimental design illustrating three levels of daily temperature fluctuations around two mean temperatures (25°C or 28°C). Eggs were exposed to six temperature regimes using programmable incubators. Dashed lines represent the mean incubation temperature in the fluctuating treatments. Shaded areas of the graph illustrate ranges of incubation temperature that produced female-biased sex ratios under constant temperature incubation. Unshaded areas represent constant incubation temperatures that produce about 1 : 1 sex ratios [15]. The x-axis represents only about 6 days of the total incubation period.

Figure 2.

Relationships among oviposition date and sex ratio with thermal parameters in natural nests of the jacky dragon (Amphibolurus muricatus). (a) Oviposition date versus mean nest temperature, (b) oviposition date versus mean daily thermal fluctuation, (c) oviposition date versus constant temperature equivalent (CTE) and (d) mean daily thermal fluctuation versus sex ratio. Statistics are reported in the text.

Figure 3.

Effects of thermal means and diel fluctuations during laboratory egg incubation on (a) incubation duration and (b) hatchling sex ratios. Error bars represent 1 s.e. (filled circles, 25°C mean; open circles, 28°C mean).

Figure 4.

Effects of thermal means and diel fluctuations during laboratory egg incubation on running speed (over 25 cm) for (a) male and (b) female hatchlings. Least-squares means are reported. Error bars represent 1 s.e. (filled circles, 25°C mean; open circles, 28°C mean).

Figure 5.

Three-dimensional reaction norm describing the relationship between egg incubation thermal regimes and offspring sex ratios of jacky dragons (Amphibolurus muricatus). Data points represent single nests in the field and thermal parameters were calculated only from temperature data during the first 60% of incubation (i.e. during the thermo-sensitive period for sex determination; [18]). The plane (i.e. reaction norm) represents a parabaloid equation. The fluctuating thermal treatments from the laboratory experiment are represented by the diamond symbols (with drop lines). Note that the sex ratio produced under the 25 ± 8°C fluctuating treatment was substantially less female-biased than that predicted from the field data. This inconsistency may be explained by the relatively high egg mortality in nests with relatively high levels of thermal fluctuation [16]. Because of egg mortality in natural nests, field-based data are based on secondary sex ratios and laboratory data are based on primary sex ratios.