DOI: 10.5061/dryad.dncjsxkx3
title: Temperature drives caste-specific morphological clines in ants

description: 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
description: To characterize the size and shape of ants, we used data
 previously compiled for a taxonomic revision of the Nearctic
 <i>Formica</i> species (Francoeur, 1973) within the
 <i>fusca</i> group (Creighton, 1950). It consists of
 morphological measurements taken on 3280 mounted specimens, which came
 from several museum collections (listed in Francoeur, 1973). Of these, we
 used 337 queens, 211 males and 1658 workers, for a total of 2206 specimens
 (Table S1). We had measurements on workers for all 32 species, on queens
 for 27 species, and on males for 20 species. We used the 12 measurements
 taken on the greatest number of specimens (Fig. S1), for a total of 26,472
 measurements. These traits are appropriate because they are associated
 with ecological functions (Table S2).  
  
description: missing values are reported with NA