1 Department of Biology, Concordia University, 7141 Sherbrooke Street West, Montreal, Canada, H4B 1R6.
2 Department of Biology, McGill University, 1205 Docteur Penfield, Montreal, Canada, H3A 1B1.
3 School of Biological Sciences, University of Hong Kong, Pok Fu Lam Road, Hong Kong SAR, People's Republic of China.
4 Department of Applied Ecology, North Carolina State University, 3510 Thomas Hall, Raleigh, NC, 27695, USA.
5 Biodiversity and Biocomplexity Unit, Okinawa Institute of Science and Technology Graduate University, Okinawa 904-0495, Japan.
6 Department of Biological Sciences, George Washington University, Washington, DC, USA.
7 Department of Ecology and Evolutionary Biology, 1105 North University Ave Biological Sciences Building, University of Michigan Ann Arbor, MI 48109-1085, USA.
8 Geographical Ecology Group, Department of Biology, University o

Biologists have long been fascinated by the processes that give rise to phenotypic complexity of organisms, yet whether there exist geographical hotspots of phenotypic complexity remains poorly explored. Phenotypic complexity can be readily observed in ant colonies, which are superorganisms with morphologically differentiated queen and worker castes analogous to the germline and soma of multicellular organisms. Several ant species have evolved 'worker polymorphism', where workers in a single colony show quantifiable differences in size and head-to-body scaling. Here, we use 256 754 occurrence points from 8990 ant species to investigate the geography of worker polymorphism. We show that arid regions of the world are the hotspots of superorganism complexity. Tropical savannahs and deserts, which are typically species-poor relative to tropical or even temperate forests, harbour the highest densities of polymorphic ants. We discuss the possible adaptive advantages that worker polymorphism provides in arid environments. Our work may provide a window into the environmental conditions that promote the emergence of highly complex phenotypes.