Limited sex differences in plastic responses suggest evolutionary conservatism of thermal reaction norms: A meta-analysis in insects

Temperature has a profound effect on the growth and development of
ectothermic animals. However, the extent to which ecologically-driven
selection pressures can adjust thermal plastic responses in growth
schedules is not well understood. Comparing temperature-induced plastic
responses between sexes provides a promising but underexploited approach
to evaluating the evolvability of thermal reaction norms: males and
females share largely the same genes and immature environments but
typically experience different ecological selection pressures. We proceed
from the idea that substantial sex differences in plastic responses could
be interpreted as resulting from sex-specific life-history optimization,
whereas similarity among the sexes should rather be seen as evidence of an
essential role of physiological constraints. In this study, we performed a
meta-analysis of sex-specific thermal responses in insect development
times, using data on 161 species with comprehensive phylogenetic and
ecological coverage. As a reference for judging the magnitude of
sex-specificity in thermal plasticity, we compared the magnitude of sex
differences in plastic responses to temperature with those in response to
diet. We show that sex-specific responses of development times to
temperature variation are broadly similar. We also found no strong
evidence for sex-specificity in thermal responses to depend on the
magnitude or direction of sex differences in development time. Sex
differences in temperature-induced plastic responses were systematically
less pronounced than sex differences in responses induced by variations in
larval diet. Our results point to the existence of substantial constraints
on the evolvability of thermal reaction norms in insects as the most
likely explanation. If confirmed, the low evolvability of thermal response
is an essential aspect to consider in predicting evolutionary responses to
climate warming.