Big-Headed Ants and Their Big Heads

Pheidole is one of the most diverse ant genera in the world, with 1,004 currently described species. This genus is known for having two worker castes – a “minor” and a “major”. The major caste typically sports a head that is comically larger than minor heads. You can see why Pheidole species are called the “big-headed ants”:

Two majors and a minor of Pheidole xerophila. Photo: Alex Wild

This ant group provides an excellent study system for investigating a fundamental question in ant research: How, and why, do major and minor worker castes evolve? Dr. Jo-anne Holley and colleagues addressed just this question in a study published earlier this year.

One potential ecological function of the big major heads is to allow a colony to utilize seeds. Through a process called seed-milling, majors utilize the expanded muscles in their heads to crush seeds, a good source of food particularly in resource-poor environments. However, only some Pheidole species mill seeds. So, Dr. Holley and colleagues hypothesized that species that mill seeds, compared to those that don’t, would have (1) greater differences between major and minor head size, (2) greater differences between major and minor head shape, (3) larger major heads, and (4) faster rates of evolutionary change resulting from the utilization of a new resource.

Pheidole minor carrying a seed back to her nest. Photo: Alex Wild

Quite surprisingly, the researchers only found support for their first hypothesis. That is, they found a greater difference in head size between majors and minors in seed harvesting species. The unexpected lack of evidence for the remaining hypotheses suggests that relative size matters more than shape, and that the utilization of seeds did not in fact drive diversification in Pheidole. Furthermore, these results suggest that shape traits are highly evolutionarily stable in this group that is also highly diverse in terms of species. Therefore, species diversification may have been driven by non-morphological traits, such as behavioral differences – or something else entirely!

Dr. Holley and colleagues do suggest that a wider set of morphological traits might better capture the role of morphology in seed-milling and species diversification in Pheidole. The authors also note that some seed harvesting species may not actually mill seeds, but utilize yummy elaiosomes that some seeds produce in order to incentivize ant-mediated seed dispersal. This would complicate the assumption that majors in seed harvesting species must be adapted to milling seeds. Lastly, the function of major castes in Pheidole colonies is not only determined by morphological traits, but also by the fluctuations of major and minor worker proportions in the colony.

So… it’s complicated! But this work by Dr. Holley and others highlights the importance of some “negative data”. Even though the researchers did not find support for many of their hypotheses, their surprising results should promote the development of new hypotheses in order to better understand the evolution of these big-headed ants as well as the worker castes in other ant groups.


Editor’s Note: One of the authors of this study, Dr. Corrie Moreau, is the writer’s Ph.D. advisor.