A bee species with specialized soldiers

Scientists originally believed that only ants and termites had individuals adapted exclusively for defense. A study published in PNAS showed that the sole purpose of guard jataí bees is to defend the hive.

By Karina Toledo

Agência FAPESP – A new study has revealed that among the jataí bees (Tetragonisca angustula), there are individuals physically adapted to perform a single function: defending the hive.

Previously, scientists thought that the division of tasks in colonies was based solely on the age of the bees and that all bees but queen bees performed a variety of tasks. The study, conducted by scientists in Brazil and the United Kingdom, was published in the Proceedings of the National Academy of Sciences (PNAS).

“Among ants and termites, the existence of specialized and physically adapted castes is a well-known function and is described in the literature. Among bees, this is a new fact,” says Cristiano Menezes, researcher at Embrapa Amazônia Oriental and one of the authors of the article.

The study, conducted in partnership with researchers at the University of Sussex, is one of the results of the Thematic Project “Biodiversity and sustainable uses of pollinators, with emphasis on Meliponini bees,” conducted under the auspices of the BIOTA-FAPESP project and coordinated by Professor Vera Lúcia Imperatriz Fonseca, of USP’s Biosciences Institute.

“We conducted another study with jataí bees to try to understand how they identify whether or not an individual belongs to the colony. That’s when we noticed that the guards were much larger than the others. It was shocking because the difference was noticeable to the naked eye,” states Menezes.

Laboratory tests not only confirm that soldier bees are 30% heavier than foragers, which are tasked with finding food, but also revealed the morphological differences between the two castes. “We discovered that foragers have a larger head, while the guards have more developed legs. We also saw differences in the size of the thorax and the wings, albeit less significant,” he said.

It is possible that these physical variations are connected to the activities that each bee executes. Foragers leave the colony to search for food and must memorize the route home, so they need a more developed brain, but soldiers benefit from larger legs to attack the enemy and immobilize them with propolis. “New studies are needed to confirm this hypothesis,” affirms Menezes.

The researchers also verified a subdivision within the soldier bees. Some soldiers guard the entrance to the colony, whereas others fly over the site and monitor the arrival of enemies. In total, this caste represents just 1% of the colony – between 30 to 50 individuals – enough to meet the demands for defense.

Rivals

So-called robber bees, such as the iratim species (Lestrimelitta limao), are the main threats to jataís. These insects normally invade the hives of other species to steal honey, pollen, food for larvae, and even wax. But, as the researchers stressed in the article, although jataís are bees without stingers, they are not helpless.

In the second stage of the research, scientists analyzed how these soldiers behave when attacked by a robber bee. “With special tweezers, we caught an iratim and placed it in front of a colony of jataís. In an instant, the guards that flew over the hive bit the wings of the invader, temporarily impeding it from flying,” tells Menezes.

As the invading bees are larger and stronger than the jataí, they generally managed to fare better. But, the larger the guard, the longer the fight lasted and the more time the colony had to prepare for the invasion.

When the arrival of enemies was noted with substantial advance warning, explains the researcher, the soldier bees managed to prevent the pillaging. To this end, they blocked the entrance to the hive with resins, leaving all the bees confined for two days.

“Our hypothesis is that the successive attacks of robber bees were the major evolutionary force for the jataí’s development of a caste specialized in defense,” says Menezes.

But this physical differentiation, stresses the researcher, also has a cost. “Very specialized individuals cannot perform other tasks if needed. They cannot act as foragers, for example, to meet the momentary needs of the colony,” he explains.

For now, the case of jataís is unique among bees. In other species, the division of labor is based on the age of bees. Scientists call this division “age polyethism.”

The younger operators perform internal functions, such as producing and manipulating wax, cleaning combs, producing cells where they will house larvae, and manipulating trash internally. After a certain age, they assume external functions. They first take trash outside the hive, and they finally become guards and foragers.

“The riskier functions are the last, because if the bees die, they have already performed all the other functions. The loss to the colony is reduced,” explained Menezes.

Very common in São Paulo, jataí bees belong to the Meliponini tribe, which are also known as bees without stingers. This tribe was the focus of the Thematic Project coordinated by Imperatriz Fonseca and concluded in 2010.