Pheidole ants, of which there are more than 1,100 known species, making the genus one of the largest in the taxonomic system, are known for their extraordinary diversity. And among their sundry forms is a “supersoldier” subcaste, a rare group of ant sumo wrestlers. But as a team of scientists from Europe, Canada, and the United States has discovered, although supersoldiers are produced by just a few Pheidole species and thus are infrequent in nature, all Pheidole ants have the potential to produce them, and they have possessed this ability since the genus evolved some 35 to 60 million years ago.
The study, published in Science, reveals that the production of Pheidole supersoldiers is the result of a developmental program that likely was present in the common ancestor of all Pheidole ants. It also suggests that the production of supersoldiers may be triggered by environmental factors and that repeated stimulation of the developmental program through time allowed the evolution of supersoldier subcastes to occur in parallel in different Pheidole species.
Supersoldiers are produced with some frequency in at least eight Pheidole species found in the southwestern United States, generally in areas also inhabited by predatory army ants. Pheidole species apparently have evolved different strategies to deal with army ant raids; for example, while some species evacuate their nests and flee, others stay put and rely on supersoldiers for defense. When army ants attack, supersoldiers block nest entrances with their large heads, preventing invaders from penetrating the colony. The giants also use their extra large size to intimidate and fight off the enemy.
The researchers began investigating the development of supersoldiers after having observed a supersoldier-like subcaste in a wild colony of Pheidole morrisi ants. The supersoldier-like individuals, the team believed, arose from abnormalities in the growth and the development of soldier larvae. To test their hypothesis, they reconstructed the evolutionary history of 11 Pheidole species, two of which, P. obtusospinosa and P. rhea, produce supersoldier subcastes. The analyses revealed that the subcastes evolved independently in these two species, meaning that the subcastes evolved in parallel. This in turn presumably enabled adaptive variation and the emergence of new phenotypes (observable traits) in each species. New phenotypes are vital in helping species' thrive in their environments.
The ability of some Pheidole species to repeatedly produce supersoldiers appears to be mediated by a substance known as juvenile hormone, the production of which is thought to be dictated by nutrition, with increased availability of nutrients facilitating the development of supersoldiers. Because the entire Pheidole genus was suspected of retaining an ancestral potential for supersoldiers, the researchers exposed P. hyatti and P. spadonia (two species that do not normally produce supersoldiers) to the chemical methoprene, which mimics juvenile hormone. Following exposure to methoprene, both species produced supersoldier-like ants, indicating that the developmental potential had in fact been retained.
The researchers suspect that the ancestral developmental program of supersoldiers in Pheidole is the result of genetic accommodation, a process characterized by the emergence and incorporation of a new phenotype into a population. Genetic accommodation occurs through natural selection, in which environmental factors control the frequency and expression of genes. Possible selection pressures for supersoldier production in Pheidole may include nutrient availability and army ant raids.
The limited number of Pheidole species that produce supersoldier subcastes suggests that selection pressures favoring the giants have lessened over time. For example, selection for supersoldiers may have been reduced in P. hyatti when the species found greater success in nest evacuation compared with supersoldier defense during army ant attacks. Still, all Pheidole retain the developmental program for supersoldiers, possibly because its loss would compromise the development of the soldier caste itself.
Kara Rogers is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is a member of the National Association of Science Writers and author of Science Up Front on the Britannica Blog. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.