Similarities to existing theory
Two existing hypotheses, both conceived in the Charnov-Bull framework, exhibit some similarities to Mighty Males. The Sexual Dimorphism hypothesis (Head et al. 1987) proposes that incubation temperature affects post-hatching growth, and TSD evolves to maximize post-hatching growth rate of the larger sex. The evidence does not support this hypothesis broadly in reptiles (Janzen & Paukstis 1991a, b), although male-producing temperatures enhance growth rate relative to female-producing temperatures in some turtles and crocodilians with male-biased size dimorphism, potentially allowing males to grow large and guard a harem of females (Deeming et al. 1988; Deeming & Ferguson 1989; Bobyn & Brooks 1994a; Rhen & Lang 1994). The Mighty Males hypothesis is formulated in a spirit that is similar to the Sexual Dimorphism hypothesis, but the former is more general. Specifically, Mighty Males recognizes that males are generally mate-limited and experience stronger sexual selection than females regardless of size dimorphism (Singh & Punzalan 2018). Critically, Mighty Males does not predict that growth rate of males is always greater than that of females. Although the quality of the embryonic environment may affect growth rate, growth rate may also be associated with asymptotic size in a sex-specific manner (Le Galliard et al. 2005), and size dimorphism can evolve for reasons that are divorced from the focal hypothesis (De Lisle & Rowe 2013, 2015). Thus, evaluation of Mighty Males with respect to juvenile growth rate must be done by decoupling sex and environment.
The Sex-Specific Survival to Maturity hypothesis proposes that TSD will evolve when incubation temperature affects juvenile survival equivalently for males and females, and the sexes differ in age at maturity (Schwanz et al. 2016). A sex by environment interaction for fitness occurs when the favourable (high survival) incubation environment produces the sex maturing at a later age, and the less favourable environment (low survival) produces the sex maturing earlier. This idea is similar to Mighty Males in that incubation environments vary in quality and the incubation environment affects the quality of male and female phenotypes equally; however, Mighty Males emphasizes fertility at adulthood, whereas Sex-Specific Survival to Maturity emphasizes its namesake. The generality of Sex-Specific Survival to Maturity may be limited because there is no systematic pattern in sex-specific maturational ages across turtle species (Berry & Shine 1980; Bókony et al. 2019) and possibly crocodilians as well (Coxet al. 2007). For the hypothesis to be valid, female-producing temperatures would represent high-quality (high-survival) incubation environments in female-size (age) dimorphic species, and male-producing temperatures must be high-quality (high-survival) environments in male-size (age) dimorphic species. So, while the Sex-Specific Survival to Maturity hypothesis should be more broadly tested, the underlying assumptions are perhaps more complex and less supported than in Mighty Males. Indeed, Mighty Males makes no assumption regarding sexual differences in size or age, and under Mighty Males, high-quality incubation environments are always those that produce males.