Scientists have discovered a parasite that causes ants to resemble fruits, which turns them into choice morsels for birds.
Working on tree-dwelling ants in the Panama and Peruvian Amazon, University of Arkansas researcher Steve Yanoviak and his colleagues initially thought that the ants scuttling past with large red shiny abdomens were part of a new species. But careful examination of the red-reared insects, which are actually a well-known species called Cephalotes atratus, revealed that the ants were crammed with hundreds of parasite eggs, each of which contained a tiny nematode worm.
The yellow colour of the parasites, together with the natural dark colour of the ant's body, is what makes the insects turn red.
The team also found that infected individuals tend to carry their infected abdomens, known as gasters, pointing upwards, which makes them much more prominent, and that the abdomen is much easier to detach from the body, possibly because the infection also thins the tissue making up the ant's outer skeleton. The ants also seem to become less aggressive under the influence of the parasite, and also cease to produce pheromonal warnings whenever predators approach. Together with their colourful complexion, these changes make them much more conspicuous to birds, which mistake them for the small fruits prevalent in the tree canopy.
The parasite eggs can pass unharmed through the birds' digestive tracts and the infectious cycle is completed when uninfected ants subsequently scavenge the birds' faeces to feed to their young, which then become infected.
"It's phenomenal that these nematodes actually turn the ants bright red, and that they look so much like the fruits in the forest canopy," says Yanoviak, "When you see them in the sunlight, it's remarkable." The findings have been accepted for publication by the journal American Naturalist.