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Ant parasite turns its host into something resembling a ripe, red berry
The better to facilitate parasite population growth through consumption of avian droppings, researchers conclude

| 16 January 2008

A newly discovered parasite so dramatically transforms its host, an ant, that the ant comes to resemble a juicy red berry ripe for picking, according to a report accepted for publication this spring in The American Naturalist. This is the first example of fruit mimicry caused by a parasite, the co-authors say.


An infected Cephalotes ant holds its nematode-ridden abdomen in an elevated "alarm posture." (Photo courtesy Stephen Yanoviak)
 

Presumably, the dramatic change in appearance and behavior tricks birds into eating infected ants - parasites and all - so that the bird can spread the parasite in its feces. The fruit-eating birds' droppings, which are mostly seeds and insect parts, are gathered by other ants who then feed and unwittingly infect their young.

This bizarre lifecycle of a parasitic nematode, or roundworm, plays out in the high canopy of tropical forests ranging from Central America to the lowland Amazon, according to Robert Dudley, a professor of integrative biology at Berkeley.

"It's just crazy that something as dumb as a nematode can manipulate its host's exterior morphology and behavior . . . to convince a clever bird to facilitate transmission of the nematode," says Dudley.

Dudley chanced upon the infected ants while he and a pair of colleagues from other universities were studying the gliding ability of a species of ant, Cephalotes atratus, common in the tropical-forest canopy. Three years ago their team described the ant's ability to make mid-air maneuvers so that, if knocked off a branch, it can glide toward the tree trunk, grab hold, and climb back up, avoiding the treacherous forest floor.

In May 2005, when searching for a colony of the ants in a downed tree on Panama's Barro Colorado Island, Dudley was puzzled to see some members of the colony with bright-red abdomens - something neither he nor his colleagues had ever seen. Taking several of the ants back to the lab and opening them up, Stephen Yanoviak, an insect ecologist and assistant professor of biology at the University of Arkansas at Little Rock, discovered that each red abdomen was full of hundreds of nematode eggs.

Because the red abdomen clearly mimicked in both size and color the many red berries that attract birds, the biologists suspected that the nematode had found a unique way to guarantee its transmission from ant host to bird host. The researchers spent the next couple of years trying to prove their hypothesis.

Yanoviak, who collected thousands of normal and infected ants in Panama and Peru, demonstrated that about 5 percent of worker ants in a colony typically are infected. Cephalotes colonies contain between a few hundred and several thousand ants.

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 employ 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 [seem to] stop and take a second look at those ants before flying off, probably because the ants were moving," Yanoviak says. "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 ants' 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," says Dudley.