Harsh environment created worker bees

By Emma Young 13 October 2010
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Honeybees once had equal status in society, but harsh conditions led to some societies developing worker castes.

FEMALE HONEYBEES ARE FAMOUSLY divided into breeding queens and sterile workers. Other insects, like ants and termites, also have queens and worker ‘castes’, as do the naked mole rats of East Africa.

Understanding how these worker castes evolved has been a major puzzle. But a team led by Associate Professor Michael Schwarz at Flinders University in Adelaide thinks it now has the answer – at least for a group of bees.

It turns out that in ancient bee societies, all individuals were created equal – there were no queens and worker bees. But to survive harsh conditions and food shortages, some bees evolved a caste level of their society, the researchers say.

Ancient equality

Michael and his colleagues studied allodapine bees, of which there are about 300 species living in Australia, Africa, Madagascar and southern Asia. They focused on this group because there’s a big range in how social the various species are, from ones that live almost entirely alone, to ones with complex societies and worker castes.

The team looked at how closely related 16 of the living allodapine species are to each other, and how social they are. Then they analysed the past associations, to effectively go back in time, and infer what the long-dead ancestor of this group of bees was probably like.

“We were able to identify some forms of social organisation in the ancestor to all living allodapine bees, and that ancestor lived about 47 million years ago in Africa,” Michael says.

Back then, the team thinks, there were no queens or worker castes. Instead, females shared a nest, but only one dominant female laid eggs and foraged for food. When that female died, a new female became dominant and began laying her own eggs, while also helping to rear the existing brood.

This system, which still occurs in many allodapine bee species today, ensures that a dominant female’s offspring are always cared for.

Desert evolution

So how did worker castes evolve? In the two allodapine bee species that have workers today, Exoneurella tridentata in Australia and Hasinamelissa minuta in Madgascar, subordinates don’t wait for the dominant female to die, but forage without laying eggs. And the two species have something in common – they both live in dry, desert regions.  

In these regions, rain falls for only short periods each year. This means that the flowers that the bees rely on for food live for only a short time – too short a time, in fact, for more than one female to become dominant and produce offspring each year.

So if the subordinate females want their genes to survive in any way, their only option is to help the dominant female rear her brood while the flowers last. “Although the brood the workers help rear isn’t their own, they are related to it, so workers can still help pass on copies of their own genes – albeit not as efficiently as if they could lay their own eggs,” says Michael.

The insight that harsh conditions probably led to the evolution of worker castes in these bees – may well apply to other species, like naked mole rats, where individuals have to live in groups to survive tough underground conditions, but where many ultimately miss out on becoming dominant, says Michael.

“This is a highly significant paper,” says William Wcislo of the Smithsonian Tropical Research Institute in Panama. “The evolution of queen and worker castes is one of a handful of major transitions in the evolution of how life on Earth is organised.” And, contrary to some other theories, this work puts ecology – and varying ecological challenges – at the heart of an explanation for how it happened in allodapine bees, at least, he says. “Their finding puts natural history front and centre.”

The new work is published in the journal Biology Letters.