Remarkable adventurers, ants have settled on all continents except Antarctica; few animals have adapted to such a wide range of environments. This ecological success is largely explained by their sociality. Yet this sociality can take very diverse forms: some species live in colonies of barely a dozen individuals, while others form nests with millions of members; some are monogynous, with a single queen, whereas others are polygynous, with several hundred queens; in some species, all workers look alike (monomorphism), while others have workers that differ morphologically (polymorphism), belonging to sub-castes that each perform a specific role (defending the nest, searching for or cutting up food, for example).
Each environment has its own type of society
Research conducted by the team of Cleo Bertelsmeier , associate professor at the Department of Ecology and Evolution (DEE) of the Faculty of Biology and Medicine at the University of Lausanne, shows that habitat type and environmental conditions such as temperature and seasonal variation influence this diversity of ant societies on a global scale.
The researchers used data on 3,299 species to understand how three key social traits – colony size, reproductive structure (one or several queens), and worker polymorphism – vary. By grouping ants according to these characteristics, the biologists at the University of Lausanne identified three major types of societies. They then quantified how frequently each type occurs within a given biome (a vast region with a homogeneous climate, soil, and vegetation). The result: the same types of social organization are found in the same types of environments, even across different continents.
A solo queen in the tropics
The first group consists of small colonies with a single queen and workers of different shapes and sizes. It dominates tropical regions , characterized by high temperatures and low seasonal fluctuations. “At these latitudes, the diversity and abundance of ants increase competition for space, particularly for nesting sites, favoring small-sized societies. In this competitive context, having multiple worker castes is an advantage for exploiting a wider range of resources”, explains Eddie Pérochon , first author of the article published in PNAS and a PhD student at the DEE. The relatively constant climate does not represent a particular challenge: a single queen is sufficient. The explanation is simple: ants are ectotherms, meaning their body temperature depends on that of their environment. Strong fluctuations in external conditions therefore reduce the chances of survival of a queen setting out to found a new colony. Polygyny then becomes an advantage. In the tropics, by contrast, climatic stability favors monogyny.
Large colonies in deserts
In the second group , colonies are large, and queens are multiple, as are worker morphologies. This type of social composition is found mainly in deserts . Why? In very arid environments, where food is scarce and competition therefore fierce, having different types of workers makes it possible to exploit limited resources more efficiently. Living in large collectives also reduces the risks of predation and desiccation, since each worker spends less time outside the nest. Unlike tropical regions, establishing a colony in the desert – where temperature differences between day and night can approach 50°C – is a real danger. Better to do it together: founding a new nest is then more likely to succeed.
Uniform workers in temperate regions
Finally, the third group , with societies that have multiple queens and morphologically uniform workers (monomorphism), dominates temperate regions , associated with lower temperatures and strong seasonal contrasts. It is found from Spain to Japan, as well as in certain areas of the Southern Hemisphere (notably New Zealand and southern Chile). As in deserts, exposure to strong thermal variations – here on an annual rather than daily basis – increases the risks associated with founding new nests, favoring polygynous colonies. This seasonality may also constrain the division of labor, explaining the presence of a single worker caste: “In conditions where food sources can change from one moment to the next, it is better to be a versatile ant rather than an ultra-specialized one”, summarizes Cleo Bertelsmeier, the study’s senior author, to explain monomorphism in this group.
A globe-spanning study
Obtained in collaboration with the University of Hong Kong, these results highlight the crucial role of the environment, which favors certain types of social organization over others. Over the course of evolution, similar ant societies have thus emerged independently in distant regions exposed to the same constraints. “Our study offers new insights into the evolution of complex animal societies”, says Cleo Bertelsmeier.
“We already knew that the advantages offered by different types of societies can depend on temperature, humidity, or seasonality, for example,” notes Eddie Pérochon. “However, previous work on this topic focused on a limited number of species, a single social trait, or restricted geographic scales. Our analysis is different in that it combines several social traits that have evolved together and, importantly, brings together thousands of ant species distributed across the entire planet, a first.”
Proceedings of the National Academy of Sciences
Animals
Environmental conditions shape the global distribution of ant societies