Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world
Species: Trigona hyalinata, a stingless bee
Habitat: Across the tropics of Brazil, Bolivia and Paraguay
You're a bee without a sting whose home is under attack. What can you do to drive off the enemy? Bite, and never let go.
Meet Trigona hyalinata, an aggressive, 10-toothed, highly suicidal bee. Its stinger is vestigial and has lost its defensive function, but this angry and altruistic bee doesn't let that hold it back.
Stingless bees are closely related to their better-known cousins, the honeybees, which sacrifice their lives when they sting animals that pose a threat to the hive. When a honeybee deploys its sting, it self-amputates, causing lethal injury. Although stingless bees have lost this heroic ability, they still suffer predation and attack from animals ranging from anteaters to other bees – and have taken to biting instead.
"Bees are at their most aggressive when defending their colony," says Kyle Shackleton of the University of Sussex, in Brighton, UK. The nest takes months to make, contains all the colony's food stores, the queen, and all of an individual bee's siblings. "If their colony dies, they have nothing."
Defensive behaviours are well known in social insects, which share a high degree of genetic relatedness and act altruistically for the good of the hive or colony.
Now Shackleton, working with Francis Ratnieks, also of the University of Sussex, and colleagues, have identified a new self-sacrificial behaviour in these stingless bees – biting to the point of suicide.
Monstrous mandibles
Ratnieks was inspired to study aggression in stingless bees by a casual but painful encounter in 2012. "Trigona bees have painful bites and are very persistent," says Ratnieks. "I allowed a worker to bite me for as long as it wanted to. It persisted in its biting for 30 minutes and left a large red mark on my arm."
They decided to investigate this behaviour in 12 stingless bee species in Brazil. They waved small flags close to the entrance to a colony to provoke the bees, and then measured how long each bee spent attacking the flag. To gauge the level of pain inflicted by each species, the researchers offered their own forearms, and scored each bite according to a five-point scale, ranging from "could not pinch skin", to "sharper unpleasant pain and capable of breaking skin if persistent".
They found that the more aggressive a species was, the more painful its bite.
Worst of all were the three Trigona species they studied, which included an individual that attacked a flag for over an hour. Individuals from these species all scored five on the pain scale.
A closer look revealed the reason: these species have five "teeth" on their mandibles. And with tens of thousands of bees per nest, this makes for a powerful deterrent.
"I have been stung by honeybees over 10,000 times, so am pretty hardened to the pain," says Ratnieks. But he says that even though a Trigona bite is much less painful than a honeybee sting, "when dozens of them start biting you, you have to retreat. It's not nice at all."
Suicidal tendencies
To see just how far the bees were prepared to go, the team devised a test that offered the bees a choice: stop biting and survive, or stay and suffer lethal damage.
The researchers first brushed a biting bee with a paintbrush, causing no harm. They then stepped things up by gripping its wings with forceps. Lastly, they started to tug on the forceps, attempting to pull the bee away by its wings, and putting the bee in danger of losing them if it didn't loosen its bite.
"When bees were pulled by the wings, large segments of the wing membrane would tear off or the wing would separate at the joint, such that the bee could no longer fly," says Shackleton. "In this state, the bee can no longer return to the nest or function in any of its duties, and has functionally sacrificed itself."
Many species had individuals that were willing to die, but the highest proportion was seen in the super-aggressive species Trigona hyalinata, where 83 per cent of individuals would keep biting until they suffered irreparable harm.
Biting behaviour may have evolved as an adaptation to the bees' particular enemies."Stinging causes greater pain, but venom is metabolically expensive to produce," says Shackleton. While stinging is a great way to defend against larger vertebrate predators, the main threats to stingless bees are ants and other bees.
"Biting is likely more effective against these more numerous foes where the objective is not to drive off a single enemy through pain, but fight off hundreds through killing them," says Shackleton.
He admits though that from his own personal experience, biting is still a powerful deterrent to larger intruders.
But like the bees, Shackleton and the rest of the team show a persistently high level of self-sacrifice and daring.
"Despite being bitten hundreds of times and chased away on more than one occasion, I think we all thoroughly enjoyed the work," he says, adding that they intend to return to Brazil next year to find out more.
Journal reference: Behavioural Ecology and Sociobiology, DOI: 10.1007/s00265-014-1840-6
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