Larger soldiers....have a better sense of smell? A lesson from stingless bees

Neotropical stingless bees, Tetragonisca angustula, at the entrance to their nest. Photo by Alex Wild.

This week's paper is in Biology Letters, lead-authored by Christoph Grüter at the Universidade de São Paulo in Brazil. The research is with a stingless bee species, Tetragonisca angustula. "Stingless bee" is actually a technical term that refers to bee species in the tribe Meloponini, which is comprised of >500 species distributed throughout the tropics worldwide. Like other social bees they store floral nectar, and stingless bee honey is actually collected by some human cultures (often referred to as melipona or meliponine honey). In many Hymenoptera (the bees, wasps, ants, termites etc.) colonies need to defend themselves from raids on their food or larvae. Raiders might be individuals of the same species from a different colony, or individuals of other species. In ants and termites, colonies often have some individuals that develop as soldiers, who work in nest defense and often are large, with weapons of some kind such as biting mouthparts. In many of the bees and wasps, there is not this need for soldiers because all of the workers can sting. But in the stingless bees there are some individuals that are larger, but they cannot sting and don't have any biting mouthparts...so how do these larger workers defend the colony?

Grüter and his colleagues hypothesized that larger workers would be better at identifying intruders. In the Hymenoptera individuals recognize each other by scent, particularly by the cuticular hydrocarbon profile on their bodies, which they detect through pore plates on their antennae. Larger individuals have larger antennae, therefore more pore plates and a better sense of smell! Grüter and his colleagues hypothesized that the larger bees would be better at distinguishing between nest-mates and non-nest-mates. 

Grüter et al. had 10 colonies of T. augustula in the lab on campus at São Paolo. Into each colony they introduced bees that were either nest-mates, non-nestmates, or from a different stingless bee species, and recorded the responses of the guarding soldier bees. They also measured the size of the guarding bees. They found that larger bees were more likely to accurately identify and attack non-nestmates, and that the larger bees also had larger antennal surface areas and more pore plates. This confirms Grüter et al.'s hypothesis that larger bees have better senses of smell and are better able to identify intruders. I would think that the larger bees are also better able to cope with the intruders once they identify them, but that has yet to be tested.