When it comes to greeting their fellow insects, honeybees preferentially use their right antennae. In fact, without their right antennae, the insects may not even be able to tell friend from foe, according to a new study.
Neuroscientist Giorgio Vallortigara and his colleagues suggest there may be a link between social life and this kind of directional bias in behavior and the brain, even for humans.
We've long known that humans exhibit brain lateralization. That is, the two halves of our brains are not exactly alike, with each hemisphere specializing in certain cognitive functions. For a long time people thought lateralization was reserved for humans, but we now know that the trait is actually widespread among vertebrates, existing in everything from other mammals, to frogs, to birds.
Usually, the right hemisphere is responsible for handling quick responses to novel events, such as escaping from an attacking predator, while the left hemisphere deals with routine behaviors and responses that require the consideration of alternatives, such as searching for food.
The fact that so many vertebrate species have lateralized brains suggests there is some kind of evolutionary advantage to the trait. In 2004, a research team including Giorgio Vallortigara, an expert in brain lateralization, experimentally showed that lateralization does impart a cognitive advantage. The researchers manipulated the brain development of chicks by exposing eggs to directed light within the last three days before hatching — the light exposure caused the lateralization of some visual functions to develop.
Compared with their test-mates that were incubated in the dark, the lateralized chicks were able to pick out grain from a bed of pebbles, while keeping track of an overhead predator (a flying model raptor). "This finding suggests that cerebral lateralization enhances brain efficiency in cognitive tasks that demand the simultaneous but different use of both hemispheres," Vallortigara and his colleagues write in their study.
Over the years scientists have found that numerous invertebrates, such as flies, bees and crabs, also demonstrate lateralization. Though invertebrates have much smaller brains than their backboned competition, scientists think they may still rely on brain lateralization to complete complex tasks.
"One example concerns the presence of an asymmetric structure in the fruit flies' (Drosophila) brain," Vallortigara, a neuroscientist at the University of Trento in Italy, told io9 via email. Studies show the vast majority of wild-type flies have this structure in the right side of the brain, and those without it are unable to form long-term memories. Other research in fruit flies has shown that the insects have a kind of directional bias with their antennas, which they use to smell. "Sensory signals from the left antenna contribute disproportionately more to odour tracking than do signals from the right antenna," he says.
In 2008, Vallortigara and his colleague Lesley Rogers found that honeybees — which show a range of higher cognitive functions, including recognizing human faces and solving complex maze-type problems — have antennal asymmetry that's linked to memory formation. The scientists conditioned honeybees to extend their proboscis (tongue) when they received a particular odor, and then tested how well the bees recalled what to do at various times later.
At one to two hours after conditioning, the bees showed excellent recall, but only when their right antennae were coated with the odor. They also showed good recall a day after training, but only when their left antennae were coated, suggesting that long-term olfactory memory formation is accessed mainly with the left antenna.
Vallortigara and Rogers previously theorized that directional biases in brain and behavior could develop under social pressures, to help animals cooperate with each other and other species. So the pair of scientists, along with researchers Elisa Rigosi and Elisa Frasnelli, decided to test this idea using honeybees, which have a rich social life and already exhibit lateralization. "We tried to investigate whether the left and the right antennae play different roles in social interactions in bees," Vallortigara says.
The team first collected honeybees from two different colonies. Back in the lab, they snipped off the left antennae of some of the bees and the right antennae of some of the others. They then tested the interactions between different dyads: pairs with only the left antenna, pairs with only the right antenna and pairs with both antennae. For each condition, they also tested dyads of bees from the same colony and dyads of bees from different colonies.
In their experiments, the researchers monitored three important measures of honeybee social interactions: how long the bees took to come in contact with one another, how often they interacted positively by extending their proboscis and how often they interacted negatively by making an aggressive "C-response," where they point their stinger and jaws towards another bee.
The researchers found that same-colony pairs with only the right antenna were much quicker to make contact and more likely to react positively than left-antenna dyads. Left-antenna dyads, on the other hand, were more likely react negatively than positively, even though the bees were from the same colony. Unexpectedly, right-antenna dyads from different colonies produced more C-responses than left-antenna dyads from different colonies.
The results suggest that the honeybee's right antenna controls context-appropriate social behavior, including the exchange of odoriferous information between colony mates and the control of aggressive C-responses towards outsider bees. In contrast, social behaviors mediated by the left-antenna are not context-appropriate, possibly because the bees are unable to distinguish who is and isn't a hive mate using the left antenna alone.
"Asymmetry of the brain (lateralization) seems to be a general characteristic of the animal nervous systems, even of those with less than a million neurons, like honeybees," Vallortigara says. For social animals and social interactions, lateralization appears to be particularly important.
In fact, Vallortigara adds, social pressures and the "interactions between asymmetrical individuals" likely promoted the evolution of the specific form of lateralization we see today in a number of social species — that is, where most individuals in a population show a particular direction of bias, such as right-handedness in humans and right antenna use in bees.
The team is now looking to explore the role that antennal lateralization plays in other social behaviors — the honeybee waggle dance, in particular — and how brain asymmetry is generated.
You can check out the new study in full in the journal Scientific Reports.
Top image via Allie Kade/Flickr. Inset image via Elisa Rigosi.