Temperature drives caste-specific morphological clines in ants

The morphology of organisms relates to most aspects of their life history
and autecology. In particular, morphology can reflect adaptation to the
abiotic environment in which species occur. As such, elucidating the
drivers of morphological variation along environmental gradients might
give insight into processes limiting species distributions. In eusocial
organisms, the concept of morphology is more complex than in solitary
organisms. Eusocial insects such as ants exhibit drastic morphological
differences between reproductive and worker castes. How environmental
selection operates on the morphology of each caste, and whether
caste-specific selection has fitness consequences is largely unknown, but
potentially crucial to understand what limits ant species’ distributions.
Here, we used 26,472 georeferenced morphometric measurements from 2206
individual ants belonging to 32 closely related North American species in
the genus Formica to assess how ant morphology relates to geographic
variation in the abiotic environment. Although precipitation and
seasonality explained some of the geographic variation in morphology,
temperature was the best predictor. Specifically, geographic variation in
body size was positively related to temperature, meaning that ants are
smaller in cold than in warm environments. Moreover, the strength of the
relationship between size and temperature was stronger for the
reproductive castes (i.e. queens and males) than for the worker caste. The
shape of workers and males also varied along these large-scale abiotic
gradients. Specifically, the relative length of workers’ legs, thoraxes
and antennae positively related to temperature, meaning that they had
shorter appendages in cold environments. In contrast, males had smaller
heads, but larger thoraxes in more seasonal environments. Overall, our
results suggest that geographic variation in ambient temperature
influences the morphology of ants, but that the strength of this effect is
caste-specific. The effect of temperature on the queen caste may play an
important role in determining colony fitness, and perhaps, limiting
northern distributional limits. In conclusion, whereas ant ecology has
traditionally focused on the worker caste, our study shows that
considering the ecology of the reproductive castes is imperative to move
forward in this field. 04-Aug-2020