Worker insects often exhibit extreme self-sacrificial behaviors—such as defending their nest to the death—when both genetic relatedness and benefits to the colony are high. But what if you’re a bee without a stinger? They may lack the traditional defenses of honeybees, but stingless bees have evolved a new self-destructive behavior to protect their home: They engage in suicidal biting when a predator intrudes, according to a new study published in Behavioral Ecology and Sociobiology.
As we’ve all learned from camp, when honeybees deploy their sting, that’s it. They basically self-amputated a part of their body, causing mortal injury. Stingless bees living in eusocial colonies, on the other hand, have vestigial stingers that have lost their defensive function—but they’re every bit as threatening.
To study nest defense behaviors in stingless bees, an international team led by Kyle Shackleton from the University of Sussex placed a black flag at the entrance of nests in order to provoke defensive responses in 12 stingless bee species in São Paulo State, Brazil. Using these responses, they quantified four measures of defensivity: attack probability, attack latency, biting duration of individual bees, and number of attackers. Then they ran a “suicide bioassay” on half a dozen of the most aggressive species: All six species had at least some suicidal individuals.
“Bees are at their most aggressive when defending their colony,” Shackleton says in a news release. “If their colony dies, they have nothing.”
The most fearsome of all was Trigona hyalinata. In this species, the proportion of workers willing to suffer fatal damage, rather than disengage from an intruder, reached 83 percent. “When near a colony, they attack you quickly and in their dozens,” Shackleton says. “They aim for the head and don’t leave you alone for a long time afterwards.”
After a microscopic examination, the researchers found all three Trigona species have five sharp teeth per lower jaw—a possible defensive adaptation and cause of increased pain. To the right is a close-up of Trigona hyalinata. They can sink their little teeth in for half an hour, no problem.
In another experiment, the team offered the bees a choice: Stop biting and live, or keep biting and suffer lethal damage. While the bees were clamped down, the researchers tried to pull them away by using forceps to tug at the wings. "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," Shackleton explains to New Scientist. "In this state, the bee can no longer return to the nest or function in any of its duties, and has functionally sacrificed itself."
Even though a Trigona bite is still less painful than a honeybee sting, Francis Ratnieks also from University of Sussex adds: “When dozens of them start biting you, you have to retreat.”
Images: University of Sussex