WASHINGTON, Jan 16 (Reuters) - A parasitic worm can make its ant victims swell into what looks like a delicious, juicy berry to birds, which apparently eat the ants and help the worm spread and reproduce, U.S. researchers reported on Wednesday.

The nematode, a type of roundworm, changes not only the appearance of the ant but also its behavior, with the ants holding out their bloated, glowing abdomens to entice the birds, the researchers report in The American Naturalist.

Robert Dudley of the University of California Berkeley and Steve Yanoviak of the University of Arkansas said the parasite and the way it works are new to science.

The black ants, found in the forests of Panama, are foul-tasting and not usually eaten by birds, they said.

Yanoviak acknowledged the team never saw birds eating one of the swollen ants but strongly suspected that they did.

"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," he said in a statement.

"So I really suspect that these little bananaquits or tyrannids (flycatchers) are coming in and taking the ants, thinking they are fruit."

The researchers said that if the birds ate the ants, they could spread the worm's eggs in their droppings. These eggs would then be gathered by other ants who then feed and unwittingly infect their young.

"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 in the statement.

"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," added Yanoviak.

Yanoviak and George Poinar, now at Oregon State University in Corvallis, have written another study describing the nematode in the February 2008 issue of the journal Systematic Parasitology. They named it Myrmeconema neotropicum. (Reporting by Maggie Fox; Editing by John O'Callaghan)

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