Explaining sex beyond TSD
If Mighty Males is correct, then sex determination occurs because of the
effect of the environment on phenotypic quality. Temperature just
happens to be a pervasive agent in the environment that has a profound
influence on phenotype. But if sex determination occurs because of
environmental effects on phenotypic quality, then it seems possible that
environmental factors other than temperature should affect sex in TSD
species. Or, other environmental factors might at least modify how
temperature affects sex, ultimately ensuring that high-quality
phenotypes are linked to male production. In fact, there is good
evidence that factors other than temperature affect sex ratios in
“TSD” species. A strength of Mighty Males is that it can explain
multiple forms of ESD operating within a “TSD” group or population.
In two TSD species of turtle (Trachemys and Graptemys ),
incubation of turtle eggs under low O2 and/or high
CO2 concentrations simultaneously increases mortality
(suggesting negative phenotypic effects) and results in a dramatic
overproduction of females (Etchberger et al. 2002). This suggests
multiple ESD mechanisms in the same species, and the finding is
ecologically relevant as elevated CO2 and depressed
O2 concentrations can occur naturally in turtle nests
through embryonic and microbial respiration, coupled with low gas
exchange with the surface e.g., under wet conditions (Ackerman 1980;
Ackerman & Lott 2004). If we accept that Mighty Males acts through
phenotypic quality and not necessarily temperature alone, then the
overproduction of females in low O2 environments is not
surprising, given the low phenotypic quality of individuals that likely
arise from stressful and oxygen deprived environments. As another
example, moisture in wild reptile nests affects fitness-related traits,
with moist nests tending to produce larger and more robust offspring
(reviewed by Packard 1999). Experimental addition of water to turtle
embryos during the thermosensitive period results in male-biased sex
ratios (LeBlanc & Wibbels 2009; Lolavar & Wyneken 2017), suggesting
that moisture affects both phenotype and sex in some turtle species, but
not all turtle species (Paukstis et al. 1984; Packard et
al. 1989), in the direction predicted by Mighty Males. More generally,
a multiplicity of ESD mechanisms is not unexpected under Mighty Males,
and the possibility of multiple ESD mechanisms under TSD should be
investigated more broadly (Warner et al. 2017).
Finally, in addition to accommodating multiple forms of ESD operating
within ‘TSD” species, Mighty Males can also explain forms of TSD that
are sensitive to the maternal environment. For example, Maternal effects
on TPiv occur in at least two MF species of freshwater turtle, where
embryos incubated at the population TPiv are more likely to develop into
females when eggs are laid late in the season, compared to eggs laid
early in the season. Thus a decline in TPiv across the season is
inferred, which is observed under constant conditions in the lab, and
this decline affects the sex ratio under fluctuating temperatures in the
field (Bowden et al. 2000; Carter et al. 2017). Hormonal
differences provide a proximate reason for the greater propensity of
embryos to become female as the season progresses, but no ultimate
reason has been proposed (Bowden et al. 2000). Life-history
theory and a variety of data suggest that individuals in low condition
nest later in reproductive season (Rowe et al. 1994), laying
fewer eggs that are relatively poorly provisioned (Harms et al.2005). In turtles, for instance, relatively young females nest late in
the season, and their offspring have lower survival than offspring of
the same size that are produced by older females (Paitz et al.2007; Rollinson & Brooks 2008). Female quality may be the ultimate
driver of changes TPiv across the reproductive season, with an
overproduction of females reflecting the low-quality offspring produced
by mothers in relatively poor condition (Trivers & Willard 1973).
Notably, this would not be the first example of maternal effects
modifying the extent of TSD in a reptile (Radder et al. 2009),
but it is one that involves modification of intraspecific TSD patterns
by maternal effects in a manner potentially consistent with Mighty
Males.