Morphological and behavioural differences facilitate tropical butterfly persistence in variable environments

1. The thermal biology of ectotherms largely determines their abundance
and distributions. In general, tropical species inhabiting warm and stable
thermal environments tend to have low tolerance to cold and variable
environments, which may restrict their expansion into temperate climates.
However, the distribution of some tropical species does extend into cooler
areas such as tropical borders and high elevation tropical mountains.
Behavioural and morphological differences may therefore play important
roles in facilitating tropical species to cope with cold and variable
climates at tropical edges. 2. We used field-validated biophysical models
to estimate body temperatures of butterflies across elevational gradients
at three sites in southern China and assessed the contribution of
behavioural and morphological differences in facilitating their
persistence in tropical and temperate climates. We investigated the
effects of temperature on the activity of 4,844 individuals of 144
butterfly species along thermal gradients and tested whether species of
different climatic affinities -- tropical and widespread (distributed in
both temperate and tropical regions) -- differed in their thermoregulatory
strategies (i.e. basking). In addition, we tested whether
thermally-related morphology, or the strength of solar radiation (when
butterflies were recorded) were related to such differences. 3. We found
that activities of tropical species were restricted (low abundance) at low
air temperatures compared to widespread species. Active tropical species
were also more likely to bask at cooler body temperatures than widespread
species. Heat gain from behavioural thermoregulation was higher for
tropical species (when accounting for species abundance), and heat gain
correlated with larger thorax widths but not with measured solar
radiation. 4. Our results indicate that physiological intolerance to cold
temperatures in tropical species may be compensated through behavioural
and morphological responses in thermoregulation in variable sub-tropical
environments. Increasing climatic variability with climate change may
render tropical species more vulnerable to cold weather extremes compared
to widespread species that are more physiologically suited to variable
environments.