Assumptions of Mighty Males
A main assumption of Mighty Males is that temperature affects condition of adults. Empirical tests performed under the lens of Mighty Males are required to assess this assumption with certainty (Mitchell et al. 2018). However, it is well-known that incubation temperature affects a myriad of traits, including embryonic survival (Schwarzkopf & Brooks 1985; Schwanz et al. 2010), embryonic development rate (Georges et al. 2005), size at hatching (Ferguson & Joanen 1983; Janzen & Morjan 2002), juvenile growth rate (O’Steen 1998; Janzen & Morjan 2002), juvenile behaviour (Janzen 1995; Booth et al.2004), juvenile immune function (Dang et al. 2015), post-hatching survival (Dayananda et al. 2017), and gamete size, reproductive physiology and behaviour of adults (Gutzke & Crews 1988; Jonssonet al. 2014). In fact, many other adaptive explanations for TSD in reptiles (at least those based on the Charnov-Bull model) also require a persistent effect of incubation temperature on phenotype (Shine 1999). A persistent effect on condition would not be surprising, as a high-quality developmental environment would result in a high-quality phenotype that, for one, enjoys any physiological advantage provided by favourable incubation conditions, and also enjoys successive non-independent events that increase the individual’s relative health and vigor over its lifetime (Madsen & Shine 2000).
A key requirement of Mighty Males is that variation in condition exists. Any factor that increases the variance in male condition or quality will therefore favour this mechanism of TSD. For instance, rapid maturation of males and non-overlapping generations is less compatible with Mighty Males, as rapid maturation of an evenly-aged cohort results in a low variance in male quality, even assuming all males are produced in a “good” environment. Factors that increase variance in male quality in a population includes environmental stochasticity over a protracted juvenile period, late age at maturity, long lifespan and/or a protracted period of indeterminate growth, low egg-to adult survival, and overlapping generations. Interestingly, many of the aforementioned characteristics have long been known to be associated with the evolution of TSD (Bull & Bulmer 1989; Sabath et al. 2016). Classically, the association between TSD and slow life histories arises because longevity and overlapping generations reduce the influence of climate on population sex ratios (Bull & Bulmer 1989); these life-history characteristics will also favour the evolution of TSD (or at least negate selection against TSD) under Mighty Males.
Finally, Mighty Males assumes that incubation temperature, not sex, will influence fitness and fitness-related traits, as the optimum incubation temperature is the same for both sexes (Figure 3b,c). Support for this assumption can therefore be sought by examining studies that use hormonal manipulations to decouple sex and incubation temperature in TSD species. There are few such studies, but those that exist generally support the assumption. In the well-studied snapping turtleChelydra serpentina , an FMF species, it is well known that male-producing temperatures promote early juvenile growth (Brookset al. 1991; Bobyn & Brooks 1994b, a), and hormonal manipulation reveals that it is incubation temperature, not sex, that promotes early growth (Rhen & Lang 1994, 1999). In the lizard Amphibolurus muricatus , another FMF species, hormonal manipulation revealed that incubation temperature influenced phenotype independent of sex, and the treatment producing the most males never performed worse than another treatment for any phenotype measured (Warner & Shine 2005). Thus, both these studies (and a few others described in subsequent sections) suggest that temperature indeed influences phenotype independent of sex.