Bee Evolution Linked to Ancient Warming Period

A period of global warming may have triggered a widespread social behavior change in sweat bees 20 million years ago, a new study suggests.

Sweat bees are eusocial, meaning they have permanently sterile worker castes. Eusocial animals include honey, bumble, carpenter and sweat (halictid) bees, ants, termites, many wasps as well as certain kinds of shrimp and the naked mole rat.

But this social evolution occurred much more recently than scientists ever thought--only 20 million to 22 million years ago. Other insects socially evolved more than 65 million years ago.

"We believe that climatic change was a critical factor in the evolution of social behavior in these bees," said Bryan Danforth of Cornell University. "What's so interesting is that the social behavior that's characteristic of these insects arose 'simultaneously,' that is, within 2 million years, which is a very narrow window in evolutionary terms."

Once the researchers established when the change took place, they searched for what may have triggered the development of eusocial 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, which allows two broods to emerge instead of one. The first brood (workers) helps raise the second brood (reproducers).

Halictid bees are important native pollinators in the Northern Hemisphere, where there are about 1,000 species. They get their nickname because they are attracted to the salts in human perspiration.

This research is detailed in an upcoming issue of the Proceedings of the Royal Society of London, Series B, Biological Sciences.

--Bjorn Carey

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