Once the team dated the evolution of the social behaviors to just 20 million to 22 million years ago, they researched what might have been going on in this time frame that would trigger the development of social behavior in so many species at the same time.
"We discovered that the Earth underwent a warming trend from 15 to 26 million years ago," Danforth said. "In modern halictid bees, social behavior varies among species and even within species as a function of latitude and altitude such that species and populations at low latitudes and in warmer regions are often fully social, whereas they are solitary at higher latitudes and altitudes, which are colder."
Warmer regions have longer growing seasons, he explained, which allows two broods to emerge instead of one. The first brood (workers) helps raise the second brood (reproductives).
Danforth, who teaches Alien Empire: Bizarre Biology of Bugs (Entomology 201), a two-credit course in insect biology, and maintains a Web site on bee phylogeny, http://www.entomology.cornell.edu/BeePhylogeny/, is also struck by the social flexibility of sweat bees.
"Other social insects (such as ants, termites, paper wasps and honey bees) have reached 'a point of no return' in social evolution in which members of the lineage are now unable to revert back to a solitary condition. These insects, however, seem to be able to revert fairly easily," he concluded.
Halictid bees, which are important native pollinators in the Northern Hemisphere, where there are about 1,000 species, are nicknamed sweat bees because they are attracted to the salts in human perspiration.