Evolution causes bad-tasting butterflies to share appearance and habitat, UF study shows

December 4, 2008

GAINESVILLE, Fla. — Competition for resources can cause animal species in an ecological community to evolve away from each other, becoming less similar — but University of Florida research shows that sometimes mutual benefit causes just the opposite.

Case in point: South American butterflies that discourage predators by advertising the message “we all taste bad” via similar, conspicuous color patterns.

A study published this week in the journal PLoS Biology shows that groups of butterfly species in the subfamily Ithomiinae evolved to share not only color patterns, but also preferences for habitat, including the places they fly, court and rest.

Both factors probably reduce the risk of being eaten, said Keith Willmott, one of the paper’s authors and an affiliate assistant professor with UF’s entomology and nematology department, part of the Institute of Food and Agricultural Sciences.

For example, Willmott says, if four bad-tasting butterfly species live in one area but they all look different, then theoretically every local insect-eating bird will have to kill an individual from each species before realizing that all four taste bad.

“Each time, the bird decides ‘I won’t do that again’ but it kills four butterflies as it learns,” said Willmott, also the assistant curator of Lepidoptera with the Florida Museum of Natural History.

If, instead, the butterflies have evolved to share a common wing pattern, each bird may only need to eat a single individual before learning to avoid all four species, thus reducing the likelihood of attack for all individuals. By sharing the same habitat, the butterflies improve their chances of educating predators quickly, because predators tend to forage in specific habitats.

The notion that mutual benefit can be a powerful evolutionary force isn’t new, but scientists are now reassessing just how prevalent it may be, said Marianne Elias, the paper’s first author and a postdoctoral associate with the University of Edinburgh in Scotland.

“Positive interactions actually happen quite a lot in various organisms,” Elias said.

For example, mammals or birds sometimes form mixed-species groups for better protection from predators and increased foraging efficiency, she said. Some flowering plants have evolved to look similar and bloom simultaneously, to maximize the chances of attracting animals that will pollinate them.

The researchers studied a diverse community of ithomiine butterflies in lowland Ecuadorian rainforest, recording the insects’ habitat use and behavior.

The study broke new ground by using genetic analysis to show that the similarities in appearance and behavior were not just due to common ancestry. Instead, natural selection has driven unrelated species to converge in both their wing patterns and the places that they favor — shady areas or sunny, valleys or ridges, treetops or the forest floor.

Upcoming research will investigate populations on a larger geographic scale, to see if look-alike species generally occur together across regions. If so, it would strengthen the notion that mutual benefit has been an important force in the evolution of these species.

Another project will be aimed at clarifying the role played by competition among members of the subfamily Ithomiinae, which includes about 370 species with a collective range reaching from the southwestern United States to Argentina.

The UF research is impressive because it helps illustrate the role that mutual benefit plays in structuring an ecological community, said James Mallet, a professor of biological diversity with University College London and a leading expert on ithomiine butterflies.

Scientists debate whether ecological communities are made up of random groups of species, or if communities are structured by interactions between species, such as predation, competition and mutual benefit. The UF paper shows that positive interactions among species can indeed have a significant effect in structuring communities, Mallet said.

Other authors of the paper include Zachariah Gompert of the University of Wyoming’s botany department and Chris Jiggins of the University of Cambridge’s zoology department.