Infected ants, normally black, develop a bright red abdomen, called a gaster, and tend to hold it in an elevated position, an alarm posture in ants. The ants also get sluggish, and the gaster is easily broken off, making it easy for birds to pluck. Dudley noted that birds usually don't eat ants, especially C. atratus, as the ants are heavily armored and defended by bad-tasting chemical defenses.
Yanoviak and Poinar reconstructed the life cycle of the nematode, though Yanoviak admits that they never saw a bird eat an ant's red gaster.
"Nevertheless, I definitely saw birds come in and seemingly stop and take a second look at those ants before flying off, probably because the ants were moving," Yanoviak said. "So I really suspect that these little bananaquits or tyrannids (flycatchers) are coming in and taking the ants, thinking they are fruit."
Birds apparently are merely a way to spread the parasite's eggs more broadly, since the eggs pass directly through into the feces. Ants become infected when they feed to ant larvae the bird feces containing parasite eggs. The nematodes hatch and migrate to the gaster of the ant pupae, where they mate. After the pupae become adults, the adults tend the brood while the nematode females incubate their eggs inside them, stunting the ant's growth somewhat.
Then, as the nematode eggs mature, the ants' gasters turn red and the ants start foraging outside the nest, setting the scene for fruit-eating birds to be duped into eating an ant they would normally avoid.
"This is a really great example of the kinds of complex host-parasite interactions that can co-evolve, and also of the role of serendipity in tropical biology," Dudley said.
|Contact: Robert Sanders|
University of California - Berkeley