Abstract

What regulates the distribution of plants and animals on Earth remains a long-standing question in ecology and biogeography. Since an organism’s morphology relates to virtually all aspects of its life history, understanding what drives size and shape variation is fundamental to elucidate the factors that limit species distribution. Eusocial insects, especially ants, epitomize morphological variation with their unique castes; workers are typically wingless sterile females whereas queens and males are fertile and winged. Thus, the environment may select for different morphologies in each caste. As such, ants offer a unique opportunity to assess how selection operates on different components of insect societies. Here, I use global climate data and over 40,000 morphometric measurements on multiple species of closely related Formica ants to assess how abiotic factors affect morphology at the continental scale. Specifically, I examine caste-specific variation in morphology along broad-scale climatic gradients. My results show that (1) body size is strongly and positively related to geographic variation in temperature, especially in queens and males (2) temperature drives geographic variation in shape in the worker caste, but not in queens or males. Specifically, workers had relatively thinner and smaller heads in warmer environments, whereas their legs, antennae and thorax were longer. This study highlights the importance of temperature in regulating species distributions and shows the value of simultaneously examining all castes in ant ecology.