The researchers suspect the nematodes grow up inside the ants, then cause the hard outer layer of the gasters to thin. When combined with the yellowish coloring of worm eggs, this makes the body segments appear amber.
"When you combine those two effects in the sunlight, you get a nice, bright red ant rear end," Yanoviak said.
Infected ants tend to hold their infested gasters in an elevated position over the rest of their bodies, making them "really conspicuous," he added.
The ants also become sluggish, and the gaster itself weakens, making it easily plucked off by birds, the researchers noted.
"[The ants] often feed out on leaf surfaces collecting pollen, collecting bird droppings, so they are pretty highly exposed," Yanoviak said.
Yanoviak and his colleagues hypothesize that birds are tricked into thinking infected ants are ripe berries and eating them. Birds don't normally feast on Cephalotes atratus ants, which are thickly armored and full of bad-tasting chemicals.
The eggs pass unharmed through the birds' digestive systems. Ants then eat the bird feces, giving rise to a new generation of parasites in a new population of ants.
However, the researchers have not yet observed a bird actually eat one of the ants.
To test the hypothesis in the lab, Yanoviak fed an infected ant to a chicken. The parasite eggs came out in the chicken feces unscathed, he noted.
He also examined pieces of bird droppings collected from ants marching back to their colonies and found a parasite similar to the one that turns the ant gasters red.
"No matter how we look at it, somehow that parasite has to infect new colonies, or else it would die with its host colony. So there has to be a mechanism for transport to a new colony," Yanoviak said.
"And the association of this ant species in particular with bird droppings makes the message even stronger for implicating the bird as the vector."
Janice Moore is a biologist at Colorado State University in Fort Collins and an expert on the ecology and evolution of parasitic worms.
She said the researchers have done just about everything they could to show the infected ants might be fruit mimics and how the parasite could spread among ant colonies.
"That's pretty cool," she said.
Humans notice the fruit mimic ants readily because they are readily visible, she added, but other examples of parasite-induced change probably abound.
"As humans, we are highly visual and we notice visual modifications," she wrote in a follow-up email. "We are less conscious of, say, olfactory or auditory alterations.
"We know of a few such cases," she said, "but they are thin on the ground—probably because we don't notice them."
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