"What's important here is how many times behavior changed," said Lovette. "If you find the same pattern consistently repeated, you can be confident of cause and effect. In this case, we found cooperative breeding evolved when different starling species moved from forests to savannas."
The researchers found that more than one-third of the African starlings are cooperative breeders, and all live in savanna environments - semi-arid grasslands characterized by unpredictable rainfall patterns. In contrast, they found that most of the non-cooperative species live in forests, which are much more predictable and stable environments.
"If we saw just a single evolutionary origin of the behavior and a single switch in habitat, it would be very weak evidence of a relationship between these factors. But we found that cooperative breeding and habitat changed repeatedly in the same direction at the same points in the tree, so we can make a much more powerful statistical argument that the factors are related," Rubenstein said.
Using long-term rainfall data from the National Oceanic and Atmospheric Administration collected from over 2,000 sites across Africa and going back nearly 150 years at some locations, the researchers showed that African savannas are not only highly seasonal environments, but that rainfall is unpredictable and varies greatly from year-to-year. Statistical analyses and the researchers' evolutionary tree helped them isolate the specific environmental characteristic of savannas that might be responsible for this pattern: temporal, or year-to-year, variability in rainfall.'/>"/>
|Contact: Robert Sanders|
University of California - Berkeley