Mammals and many species of birds and fish are among evolution's "winners," while crocodiles, alligators and a reptile cousin of snakes known as the tuatara are among the losers, according to new research by UCLA scientists and colleagues.

"Our results indicate that mammals are special," said Michael Alfaro, a UCLA assistant professor of ecology and evolutionary biology and lead author of the research.

The study, published July 24 in the early online edition of Proceedings of the National Academy of Sciences, also shows that new species emerge nearly as often as they die off.

Alfaro and his colleagues analyzed DNA sequences and fossils from 47 major vertebrate groups and used a computational approach to calculate whether the "species richness" of each group was exceptionally high or low. The research allows scientists to calculate for the first time which animal lineages have exceptional rates of success.

Among the evolutionary winners are most modern birds, including the songbirds, parrots, doves, eagles, hummingbirds and pigeons; a group that includes most mammals; and a group of fish that includes most of the fish that live on coral reefs, said Alfaro, an evolutionary biologist.