Desert and tropical ecosystems of the world harbour the most complex ant societies

Morphological diversity in the worker caste is present in ants, wasps, termites and thrips, but is particularly striking in ants, as they can produce distinct or continuous worker subcastes. This is referred to as worker caste polymorphism, which has enabled efficient division of labour and consequently, ecological dominance. Previous research shows that nutrition and colony demography during larval development are important determinants of worker polymorphism, which in turn can be mediated by the abiotic environment. However, little is known about the influence of environmental gradients on the geographic distribution of worker caste polymorphism. Here, I developed and tested three hypotheses explaining the global distribution of worker polymorphism: (1) The Tropical Polymorphism Hypothesis, (2) The Desert Polymorphism Hypothesis and (3) The Extreme Climate Hypothesis. I therefore investigated the influence of geographic variation in temperature and precipitation on the distribution of worker polymorphism worldwide using 680 000 ant occurrences and identifying each occurrence point as polymorphic or not. Previous estimates suggested that 13% of ant species are polymorphic whereas my results show this number to be as high as 29%. Moreover, I found that the occurrence of polymorphism was highest in the most arid and tropical ecosystems, suggesting that it may be an adaptation to regions with extremely scarce or diverse resources. Additionally, warm climate is a pre-requisite for the evolution of worker caste polymorphism. Taken together, my work sheds light on the factors promoting complex social lifestyle in insects, and reveals that it is more common in ants than previously believed