On the campus of the University of Sao Paulo, Brazil, some trees buzz more than others. Like the one that behavioural ecologist Francisca Segers was walking to with a small box in her hands.
Closer to the tree, Segers caught a faint buzzing in the air. A short wax tube extended from a hole in the tree. A few small yellow-and-brown bees, not longer than 5mm, hovered above the narrow tube opening while more stood on its edge.
A colony of Tetragonisca angustula, a stingless bee species which the locals call Jatai, had built their nest in the tree hole.
Segers reached into her box and picked a tiny black lump. It was a crushed head of a robber bee, Lestrimellita limao, freshly killed that morning. She gingerly placed the robber bee head on the tube opening, and watched it rolled into the Jatai bee nest.
A cloud of bees burst from the tube. “It was just six Jatai bees flying at the entrance,” recalls Segers “then suddenly there were two hundred.”
Pheromones from the robber bee head had kicked the Jatai nest into high alert. Agitated Jatai bees flew about their tube, hunting for the intruders. Robber bees are pirates on wings—they plunder other colonies and raid their stores—and must be dealt with immediately.
Luckily for this colony, the threat was just a mock attack by Segers to test their response to robber bees . However, if the Jatai bees had found a robber bee scout, death comes quick.
The hovering Jatai guards would rush into the robber bee and bite it. Both bees hit the ground grappling as the robber bee tries to tear away the tenacious guard.
“Ideally, the Jatai bee grabs the wings of the robber bee and doesn’t let go,” says Christoph Gruter who studies Jatai bees and has witnessed many such fights.
The Jatai bee’s iron grip grounds the robber bee. Eventually, the much larger robber bee kills the Jatai bee, often decapitating it, but the dead bee’s relentless jaws clamp on to its killer.
“The Jatai bees commit suicidal defense, lying on the ground with the robber bee until maybe an ant comes along and pick them away.”
Pacifying one robber bee could save the entire Jatai bee colony. If the scout returns to its hive and recruits more robber bees to raid the Jatai bee nest, “you would get massive fights with hundreds of casualties,” says Segers. “We have lost colonies to robber bees.”
Those Jatai bees guarding the tube entrance are physically adapted for rough scuffles. They are larger and heavier than the other Jatai workers. That size difference marks Jatai bee guards as a unique physical soldier caste—the only such case discovered in bees.
When Gruter and his colleagues announced in 2012 that Jatai bees have soldiers  , they stirred up quite a buzz. Although physically distinct soldier castes are not the norm among social animals, a number of animals have evolved soldiers: Ants, termites, and even those with less complex sociality, like aphids, snapping shrimps and flatworms . Yet nobody had found bee soldiers, feeding a general assumption that bees cannot evolve soldiers.
“It is very difficult to spot the size differences in Jatai bees because they are so small,” says Martin Kaercher who had examined the defensive behaviours of Jatai guards for his Ph.D. at University of Sussex . “I had also not expected size differences in bees.”
Kaercher was not alone. Many researchers had examined the 5mm-long Jatai bees up close. Minute size difference at such scale would be almost impossible to spot if one was not looking for it. It also helps to have the sharp eyes of Cristiano Menezes, Gruter’s colleague at the University of Sao Paulo.
“We were running a study where we introduced workers from one Jatai bee colony to the nest entrance of another,” recalls Gruter. “Eventually, Cristiano noticed that the guards seemed larger than the workers we introduced.”
Meticulous measurement of more than 300 bees confirmed Menezes’ observation: Jatai bees guarding the entrance are 30% heavier than the rest and have larger hind legs.
“I was amazed and delighted [to know of Jatai bee soldiers], since we only knew about caste differences in ants and termites,” says Kaercher of Gruter’s 2012 discovery.
Although late to fly into the limelight, Jatai bee soldiers are gripping our attention as tight as they would a robber bee.
In a recent American Naturalist paper , Gruter reports that Jatai bee soldiers are quite unlike any other animal soldiers we know. While soldiers of ants and termites do little else but fight enemies, Jatai bee soldiers also partake in civilian duties: They build and clean the nest, nurse the larvae and dry the wax.
Even more surprising is that in the first half of their month-long lives, Jatai bee soldiers outwork the workers. On average, each Jatai bee soldier performs ten different behaviours to the worker’s eight, and performs those behaviours more than the workers (46 times versus 32). Most of these behaviours involve nest maintenance.
“We do not know of any other case where soldiers show more diverse behaviours than workers,” says Gruter. “In ants, soldiers are specialists that work very little and have a relatively narrow set of behaviours.”
“What we see in Jatai bee soldiers is counterintuitive. It goes against the theory of caste evolution that increasing specialization leads to narrowing behaviour sets.”
For now, Gruter and his colleagues can only speculate why Jatai bee soldiers do so many tasks. Perhaps Jatai bees have just begun to evolve more specialized soldier caste.
Or maybe Jatai soldiers can work like workers do because they are not starkly different in size and shape. In insects like the army ants or leafcutter ants, soldiers dwarf the workers like tigers to cats, while Jatai soldiers stand next to workers like horses to donkeys.
“Maybe there are developmental limits that make it impossible for Jatai bees to evolve extreme soldier types,” Gruter says.
All Jatai bees are born in rigid cells of brood combs in the hive center . Worker bees regurgitate food into each cell, the queen lays an egg in it and the worker seals the cell. The egg hatches and the larva eats the provisioned food, developing in the cell until it emerges as an adult.
In contrast, ant and termite larvae grow in open space and are fed continuously by their carers.
Segers found that Jatai soldiers are reared in specific cells right in the center of the brood. There the cells are bigger and receive more food, giving rise to larger and heavier bees. Scientists call these larger worker bees ‘majors’ and the rest of the smaller workers ‘minors’. Jatai bee majors make up less than 6% of the workers and most end up as soldiers.
“The brood cells are neatly arranged,” explains Gruter. Growing a major that is 50-times larger would need a brood cell so big it might “completely disrupt the nest structure. This could act against the evolution of a more extreme soldier type in Jatai bees.”
Newly emerged majors and minors start work in the brood, deep in the hive. As the workers age, they work outwards, tending to waste, wax and resin. At this stage, majors work a lot more and do a couple more tasks than minors.
But by the third week of their lives, when their workplace moves to the periphery of the hive, minors and majors wear completely different uniforms. Minors forage for food while majors guard the hive, becoming the soldiers that Gruter and Segers studied.
“In a way, soldiers are really just hardworking workers until they reach the entrance,” says Gruter. “It makes sense to call them majors and refer to them as soldiers only when they pick up the defense at the end of their lives.”
Despite the majors defending the nests and outworking the minors, they make up less than one-tenth of the workforce. So why doesn’t a Jatai bee colony produce more majors?
For one, majors are expensive. A growing major eats 20% more food than minors and they likely eat more as adults too. Yet majors do not collect food for the nest.
The value of majors is reflected in how Jatai bee colonies respond to limited food and threats. In an experiment, Segers moved Jatai bee colonies into areas of intense food competition . Five months later, the colonies produced smaller soldiers but minors kept their size.
In areas of low robber bee numbers, Jatai bee colonies deploy fewer soldiers. But jack up the threat for several weeks (robber bee heads rolling into the nest, anyone?) and Jatai bee colonies ramp up their soldiers.
If majors do not forage, “then they can be quite useless when there is no threat,” says Segers. It appears that Jatai bees can tune production of majors and minors according to needs.
Furthermore, Jatai bee majors work more than minors but nobody has compared their efficiency. “If majors are less efficient, they might not be worth the extra cost,” says Gruter.
Gruter also suspects that the majors’ industrious efforts might be a side-effect of their larger size instead of a direct result of natural selection.
Other scientists have shown that in honey bees, biochemicals call juvenile hormones affect behaviours and a worker’s progress from one task to another.
Gruter speculates that in Jatai bees, the majors might have larger brain structures that produce more juvenile hormones. As a side-effect of elevated juvenile hormones, “majors might mature faster and work harder.”
So far, scientists have uncovered a comprehensive nest defense system in Jatai bees—one scientist  calls it “one of the most sophisticated defense among social bees.” Like many stingless bees, Jatai bees seal their nests at night, and use wax and resin on complex nest tunnels to entangle enemies.
But Jatai bees alone recruit hovering and standing soldiers to fight enemies. Hovering guards expand the defense perimeter, intercepting robber bee scouts before they reach the nest . A long-running conflict with robber bees could be the substantial force driving such arsenal.
“I’ve read that Jatai bees and robber bees overlap greatly in their distribution,” says Gruter. “If so, this would mean a long history of co-evolution and arms race between the bees.”
Paul Cunningham, an insect behavioural ecologist who has documented a four-month long war between stingless bees in Australia, thinks that the evolution of defense depends on the main enemy.
“If your enemy are larger bees and you are more likely to overpower them with individual strength, then having strong soldiers is an advantage,” says Cunningham. “This is the case with Jatai bees against robber bees.”
On the other hand, if both sides are similarly matched in strength, then the colony with the most workers win. “Having stronger fighters isn’t always the best strategy.”
Going forward, Gruter and his team will scrutinize other bee species for soldiers. If they discover soldiers in other bees, they could better understand how soldiers evolve in bees.
The diverse behaviours of Jatai bee soldiers “draw a big question mark,” says Gruter “not just about the evolution of soldiers in this species but also in general about the evolution of physical caste in social insects.”
Perhaps other bees have also evolved hardworking soldiers juggling many tasks. But that is not going to bother the Jatai bees—they worry about robber bees and honey. As new cohorts of Jatai bee majors emerge from brood cells, they will live as their dead sisters had: Work harder than the rest and die guarding the nest.
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