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.