Discussion
Association of key life history traits, such as body size, with
environmental factors shape the adaptation of species to local
environments (e.g., Blackburn et al., 1999; Freckleton et al., 2003;
Morrison and Hero, 2003; Pincheira-Donoso and Tregenza, 2011; Meiri et
al., 2013; Volynchik, 2014; Hille and Cooper, 2015; Laiolo and Obeso
2015; Lack et al., 2016 Meiri et al., 2020; Velasco et al., 2020; Wang
et al., 2021; Deme et al., 2022a; Giovanna et al., 2022; Szymkowiak and
Schmidt, 2022). Indeed, we found that geographical patterns of female
body size were influenced by the coupling effects of the seasonal and
annual changes in the climatic conditions along altitudinal gradients,
suggesting a possible adaptation of E. argus lizards to the
changing environmental conditions. The climate-body size relationship
across populations of E. argus lizards in our study showed a
reversal of Bergmann’s rule: female lizards occupying warmer
environments at low altitudes had bigger body sizes. Further, we found
that populations at low latitudes with an abundance of available
resources and highly seasonal environments, such as increased
precipitation, had significantly larger body sizes. Thus, our study
suggests that the intraspecific variation in the geographical patterns
of body size along altitudinal clines was primarily driven by
multifarious local environmental conditions such as climatic conditions,
highly seasonal environments and available resources.
Geographic patterns of body size are thought to be primarily influenced
by climatic gradients, as summarized by Bergmann’s rule (Bergmann, 1847;
Ashton and Feldman, 2003; Sears, 2005). Based on Bergmann’s rule, there
is a general understanding that the body sizes of endotherms increase
toward high latitudes or altitudes (see Ashton and Feldman, 2003; Meiri
and Dayan, 2003; Pincheira-Donoso et al., 2008; Pincheira-Donoso and
Meiri, 2013; Moreno Azocar et al., 2015). In a reversal to Bergmann’s
rule, we found evidence that female lizards at lower altitudes in warmer
environments had larger body sizes. Our finding concurs with other
previous studies showing ectotherms may sometimes reverse Bergmann’s
rule (Ashton and Feldman, 2003; Sears, 2005). In contrast studies have
shown some ectotherms follow Bergmann’s rule, possessing large body
sizes at high latitudes (e.g., Angilletta et al., 2004). The original
explanation for Bergman’s rule did not account for the peculiarity of
ectotherms (Watt et al., 2010) in their inability to generate
significant internal body heat, and consequently that a larger bodied
ectotherm would therefore heat up more slowly (Stevenson, 1985) and
would lack the ability to conserve heat in colder environments (Liang et
al., 2021). Further, possessing larger bodies in colder environments may
be deleterious to some ectotherm species (Jadin et al., 2019; Velasco et
al., 2020; Slavenko et al., 2021); since ectotherms with large body
sizes that slowly heat up in colder environments show constrained
thermoregulatory behavior (Pincheira-Donoso et al., 2008; Anderson et
al., 2022; Szymkowiak and Schmidt, 2022).
Expressly, our results found evidence for the influence of both resource
availability and seasonality (i.e., precipitation seasonality) on female
body size, with smaller body sizes associated with decreased seasonality
and lower primary productivity. Previous studies have suggested that
highly seasonal changes in rainfall significantly influence the
abundance of available resources for female lizards (Valenzuela-Sánchez
et al., 2015; Meiri et al., 2020; Slavenko et al., 2021), which is
positively related to large body sizes (Liang et al., 2021). Perhaps
this is not surprising since unpredictable seasonal changes at high
altitudes may suggest scarce resources for lizards (Deme et al., 2022a;
Anderson et al., 2022). Previous studies have shown that abundant
available resources for lizards to feed mostly impact the growth rate
along geographic clines, which may be a function of body size variations
in lizards (e.g., Lu et al., 2018a, 2018b), suggesting that non-climatic
factors such as available resources can also influence the variation in
the body sizes of lizards. For instance, available resources as a
function of habitat productivity in novel environments influenced body
size variations in other ectotherm species like insects, fishes and
amphibians (Morrison and Hero, 2003; Laiolo and Obeso, 2015; Riesch et
al., 2018; Giovanna et al., 2022). Perhaps, the variations in
non-climatic factors across environments also play significant roles in
determining shifts in phenotypic traits, such as variation in the body
size of species. However, our understanding of how the variations of
these climatic and non-climatic factors along geographic clines can
directly or indirectly impact ectotherms’ body sizes in the context of
rapidly changing climates may still be limited.
Thus, clinal variation in body sizes across populations within
ectothermic species may be a result of adaptive plasticity to changing
environmental conditions (Riesch
et al., 2018; Giovanna et al., 2022). Organismal body size, as a
function of growth and development rates, is influenced by the interplay
of intrinsic and extrinsic factors (Duellman and Trueb 1986). For
example, extrinsic and intrinsic factors such as temperature, habitat
type, food availability, egg size, yolk reserves, and competition have
all been shown to influence the body sizes of ectotherms across
environments (Fischer et al., 2003; Laiolo and Obeso, 2015; Riesch et
al., 2018), which ultimately can affect the reproductive ecology of
ectotherms (Fielding et al., 1999; Morrison and Hero, 2003; Deme et al.,
2022a). While the patterns of female body size across altitudes and
environments found in our study may be a results of non-adaptive
plasticity, or even fixed genetic differences between populations, we
suggest that this pattern may be a result of adaptive phenotypic
plasticity (Ghalambor et al., 2007; Szymkowiak and Schmidt, 2022).
However, further experiments, such as common garden studies, would be
needed to test this hypothesis. Understanding the underlying cause of
the altitudinal body size differences is important in order to predict
how these populations will response to future changes in climate (Merilä
and Hendry, 2014). The pace of climate change is expected to be more
rapid at high altitudes (Pepin et al. , 2015). Phenotypic
plasticity in body size may allow lizard populations to quickly respond
to changes in climatic conditions across populations, but may
consequently shield body size from selection, slowing the pace of
evolutionary change (Diamond and Martin 2021). In contrast if body size
differences between populations is largely due to genetic divergence,
these population may evolve on response to changing climates, but it is
unclear whether the rate of evolution could keep pace with the rate of
climatic change (Diamond and Martin 2020).