Fans of Aesop know that hard-working ants are fabled targets for freeloading parasites.
But a new study of the ant species Myrmica rubra shows that sometimes the tiny laborers fight back, triggering a co-evolutionary arms race that can catch related species in the crossfire.
In parts of Denmark, Alcon blue butterflies lay their eggs on marsh plants (left) near M. rubra colonies. When the larvae hatch, their outer layers mimic the hydrocarbon chemistry of the ants' cuticles.
The trick fools foraging ants into thinking the caterpillars are their own young, so they carry the parasites into their nests (right). The butterfly larvae are then cared for at the expense of actual ant babies.
For their study, David R. Nash of the University of Copenhagen and colleagues found that the ants' genes—and thus their surface chemistries—vary by location, with infected colonies showing the greatest differences between populations.
The team also noted that Alcon blue butterflies that best matched a colony's unique signature were the most successful at infiltrating the nests.
This suggests that as the ants evolve their cuticles so that they can better recognize larval parasites, the butterflies counterattack with a more refined chemical cocktail.
At the same time, Myrmica ruginodis ants—which can also be fooled into "adopting" butterfly larvae—don't show as many genetic differences between populations.
"Alternative hosts may therefore provide an evolutionary refuge for a parasite during periods of counteradaptation by their preferred hosts," the team writes in their paper, which appears in this week's issue of the journal Science.