Nematodes use ants for dispersal by birds

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 in The American Naturalist. This is the first example of fruit mimicry caused by a parasite, the co-authors say. 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 the University of California, Berkeley. "It's just crazy that something as dumb as a nematode can manipulate its host's exterior morphology and behavior in ways sufficient to convince a clever bird to facilitate transmission of the nematode," Dudley said.

"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," said co-author Stephen P. Yanoviak, an insect ecologist and assistant professor of biology at the University of Arkansas at Little Rock, who noted that numerous tropical plants produce small red, orange and pink berries. "When you see them in the sunlight, it's remarkable." Yanoviak collected thousands of normal and infected ants in both Panama and Peru, near the Peruvian rainforest city of Iquitos, and demonstrated that, typically, about 5 percent of worker ants in a colony 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 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. Sciencedaily News.

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