Population structure leads to male‐biased population sex ratios under environmental sex determination

Abstract

Spatial structure has been shown to favor female‐biased sex allocation, but current theory fails to explain male biases seen in many taxa, particularly those with environmental sex determination (ESD). We present a theory and accompanying individual‐based simulation model that demonstrates how population structure leads to male‐biased population sex ratios under ESD. Our simulations agree with earlier work showing that the high productivity of female‐producing habitats creates a net influx of sex‐determining alleles into male‐producing habitats, causing larger sex ratio biases, and lower productivity in male‐producing environments (Harts et al. 2014). In contrast to previous findings, we show that male‐biasing habitats disproportionately impact the global sex ratio, resulting in stable male‐biased population sex ratios under ESD. The failure to detect a male bias in earlier work can be attributed to small subpopulation sizes leading to local mate competition, a condition unlikely to be met in most ESD systems. Simulations revealed that consistent male biases are expected over a wide range of population structures, environmental conditions, and genetic architectures of sex determination, with male excesses as large as 30 percent under some conditions. Given the ubiquity of genetic structure in natural populations, we predict that modest, enduring male biased allocation should be common in ESD species, a pattern consistent with reviews of ESD sex ratios.

Becoming creatures of habit: Among‐ and within‐individual variation in nesting behaviour shift with age

“…fish, Tyler, 1995; amphibians, Rieger, Binckley, & Resetarits, 2004; reptiles, Brown & Shine, 2004; Mitchell, Warner, & Janzen, 2013; and birds, Martin, 1998; Latif, Heath, & Rotenberry, 2012; reviewed in Mousseau & Fox, 1998) and can be both repeatable and heritable (reviewed in Janzen, Delaney, Mitchell, & Warner, 2019). The microevolutionary potential of nest‐site choice has been particularly well studied in turtles because of their unique sex‐determining mechanism and the theoretical role of maternal nesting behaviour in sex ratio evolution (Bulmer & Bull, 1982; Butka & Freedberg, 2019; Mitchell, Maciel, & Janzen, 2013; Morjan, 2003a). Specifically, thermal conditions experienced by embryos determine gonadal sex in most turtle species (i.e.…”

Evidence for a male‐biased sex ratio in the offspring of a large herbivore: The role of environmental conditions in the sex ratio variation

“…The results of this study suggest that the secondary sex ratio in a large European herbivore, roe deer, is male biased and that deviations from this bias are sensitive to variations in the weather conditions during autumn and winter. In numerous species, the secondary sex ratio (male:female) deviates from 1:1 (Butka & Freedberg, 2019; Gowaty & Lennartz, 1985; Robert & Schwanz, 2011) but there is little evidence that this deviation persists for a given species when large data sets are analyzed (Palmer, 2000) or when the variation in the secondary sex ratio of free‐ranging mammals is linked to environmental factors (but see Kruuk et al, 1999). However, our finding of a male‐biased sex ratio in roe deer was robust to the addition of data on the sex ratio of roe deer offspring (Table 2; Figure 2), which led us to investigate conditions for which the sex ratio in roe deer deviates from this expectation.…”

Evolution of Sex Determination in Amniotes: Did Stress and Sequential Hermaphroditism Produce Environmental Determination?

“…[ 49,51,58 ] On the other hand, there are ESD populations with male‐biased adult sex ratios as well. [ 4 ] Highly biased sex ratios are also found in another group of organisms with specific sex‐determining strategy—sequential hermaphrodites. [ 59 ] A recent study shows that biased sex ratios in sequential hermaphrodites have surprisingly little effect on their population effective size and thus evolvability.…”

“…The mechanism of sex determination can influence offspring sex ratio and thus possibly the dynamics including viability and evolutionary potential of a population. [ 2–5 ] It is hence striking that despite its importance and far‐reaching consequences, organisms do not share a universal optimal solution to sex determination. But why?…”