In warm US regions, some urban-dwelling butterflies emerge later than their rural-born peers, according to researchers.

A later-than-usual appearance of urban butterflies was documented in seven of 20 species studied by a team from North Carolina State University, Case Western Reserve University, the Institute for Ecological Research in Brazil, and the University of Maryland.

"We know that butterflies emerge earlier in North Carolina than they do in New England, because it's warmer," said study co-author Tyson Wepprich, a Ph.D. student at NC State. "We also know that cities are heat sinks that are warmer than outlying areas. So we wanted to see whether butterflies would emerge earlier in cities than they do in more rural environments."

However, among all the species observed, the most cohesive trend Wepprich and his collaborators observed was that butterflies were emerging later than usual, instead of earlier.

In some parts of Ohio where tests were done, some urban dwelling butterflies emerged weeks after rural specimens of the same species.

"Even though butterflies often change their emergence predictably to small increases in temperature, these species responded in unexpected ways to larger increases in temperature," Wepprich said in a statement, adding that the revelation may shed light on which species are more vulnerable to climate change.

"We don't really know precisely where the tipping point is, or why only some species respond this way, but something is happening here," he said. "We're still working to better understand what's going on with these butterfly species and what consequences there may be for their populations."

Wepprich and his collaborators published their work in the journal Ecology.