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

To characterize the size and shape of ants, we used data previously compiled for a taxonomic revision of the Nearctic Formica species (Francoeur, 1973) within the fusca 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).

 

 

missing values are reported with NA