A sweat bee

Warm weather makes for a sociable sweat bee - and for some surprising behavioural flexibility that may help these bees adapt to changing environmental conditions, according to new findings.

Research led by Sussex evolutionary biologist Professor Jeremy Field, and published in the journal Current Biology, now shows that a species of sweat bee, Halictus rubicundus, flexibly shifts between solitary and social life in response to changes in the environment.

Most wild bee species are solitary: each female builds her own nest and rears her own offspring. However, a minority of species, such as the familiar bumblebee and honeybee, live in societies where only a single female, the 'queen', reproduces. Other members of the society, known as 'workers', forsake their own reproduction to help rear the queen's offspring 

Sweat bees (so-called because they are attracted to the salt in human perspiration) are of particular interest to biologists because they provide insights into a long-standing question: how societies evolve from solitary living.

While all bumblebees, honeybees and ants are social, sweat bees evolved sociality more recently, and exhibit the full range of behaviour, from strictly solitary to strictly social species, with more than 20 different species in the UK alone. Easily overlooked, these small bees form tiny colonies, consisting of a queen and just a handful of workers, which live in burrows in the soil.

Of special interest are 'socially polymorphic' sweat bees, who take more than one social form within the same species. The socially polymorphic sweat bee Halictus rubicundus is the subject of Jeremy's study, in collaboration with colleagues at Queen's University Belfast.

In Britain and Ireland, northern or high-altitude populations of this bee are solitary, while southern, low-altitude populations of the same species are social.

The key experiment aimed to test whether individual bees could switch behaviour. The researchers transported bees between southern and northern locations - for example from Belfast in Northern Ireland, where bees are always solitary, to warm and sunny Brighton on the English south coast.

The result was that the previously solitary bees switched to social behaviour. And in the reverse experiment, southern, social bees became solitary when transplanted to Peebles in Scotland.

Jeremy says: "Sweat bee sociality is thought to occur only where the growing season is long enough to permit two annual broods. First, the queen must rear some workers. Then the workers must help to rear a second brood of new queens. A bee that finds itself too far North may not have a long enough summer to do this.

"Our findings suggest that individual bees adjust their strategies according to the time remaining for queen production: the earlier a nest produces its offspring, the more likely those offspring are to become workers. The queen may also have an influence, because more of her offspring become workers if she is still alive when they are born."

He adds: "Origins of sweat bee sociality are linked with past episodes of climatic warming, and sociality is today associated with warmer conditions. Rapid switching could thus help some sweat bees adapt to future warming, and has probably allowed them to invade new biogeographic regions."