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.