New research on ants has revealed that larger group sizes have favoured the evolution of more worker castes, with larger colonies evolving before multiple worker castes evolved. The results may also offer insights about the evolution of multicellular organisms such as animals and plants.
Division of labour is found in several insect societies. In leafcutter ants, there are four different worker castes which perform different jobs within the colony. The smallest is responsible for maintaining the fungus garden and rearing young, the middle two forage for and cut leaves, while the largest are soldiers who defend the nest. In contrast, in some other ant species, there is only one worker caste, which must perform all the jobs required by the colony.
Similarly, a body is made up of many cells with different functions, such as those in organs and tissues specialised for reproduction, the immune response, or movement. This likeness has meant that ant colonies are often referred to as 'superorganisms', as natural selection and evolution can operate at the level of the whole colony to influence group-level traits. Lead author Louis Bell-Roberts said:
“The size–complexity hypothesis is a leading explanation for the evolution of complex life on earth. It predicts that when groups contain larger numbers of individuals, this selects for increased division of labour. In larger groups, there are more jobs that need doing. In line with this, our research showed that larger colony sizes favoured the evolution of increased division of labour, resulting in more worker castes and greater variation in worker size.”
Understanding what has influenced division of labour in ant colonies could help us to learn more about the evolution of complexity in multicellular organisms – larger numbers of cells in multicellular organisms may have promoted the evolution of a greater number of specialised cell types.
Ant species also differ in the level of reproductive division of labour – where queens are specialised for reproduction and workers for non-reproductive tasks. In some species, queens and workers are highly similar in terms of size and physiology. In others, queens can be many times larger than the smallest workers, capable of producing millions of workers throughout their lifetime, along with thousands of new queens and drones (males) each year. The researchers are now interested in exploring which factors influence this.
To read more about this research, published in Nature Ecology & Evolution, visit: https://www.nature.com/articles/s41559-024-02512-7
