A team of biologists working in Brazil may have found the clues to resolving the longstanding mystery of why some species of bees, such as honey bees, communicate the location of food with dances in their hives and why other bees simply leave scent trails from the food source to the nest.
In the paper to appear in the October 22nd issue of the Proceedings of the Royal Society, biologists at the University of California, San Diego and the University of Sao Paulo report that one species of Brazilian stingless bee uses a slightly different form of communication, presumably in an effort to confuse its foraging competitors.
"Previously, biologists thought different species of bees either marked the food source or left an odor trail from the food all the way to the nest," says James Nieh, an assistant professor of biology at UCSD who headed the study. "We have discovered an intermediate strategy, in which bees leave an odor trail extending a short distance from the food source. This abbreviated trail may be less conspicuous to foraging competitors."
The discovery is significant because it may help scientists understand how "functionally referential communication," or the use of abstract representations to convey information about the physical world, could have evolved.
Bees and humans are among a small number of species that can use abstract representations to communicate. For example, honeybees use a "waggle dance" in which a returning forager bee runs up and down the honeycombs and shakes her abdomen to communicate distance and direction to a food source. Honeybees communicate within the nest how to get to the food source and do not leave a scent trail between the nest and the food source. However, they can mark flowers with special odors to help guide nestmates to the correct spot. Species of bees that do not have such sophisticated referential communication, leave a scent trail from the hive to the food source.
Nieh suggests that communicating the location of the food source to nestmates within the confines of the nest may have evolved as a strategy to avoid broadcasting information about the food source to competitors, which may also be able to follow the scent trail. The abbreviated scent trail strategy was observed in T. hyalinata, one species of stingless bee, a diverse group prevalent in South America. This discovery by Nieh and his colleagues Felipe Contrera and Paulo Nogueira-Neto from the University of Sao Paulo lends support to the notion of chemical eavesdropping, "espionage," driving the evolution of communication.
Besides the length of the stingless bees' scent trail the short trail has a second unique feature. The scent drops are more concentrated at one end of the trail than the other.
"All previously discovered bee scent trails had a roughly equal number of drops per unit distance," says Nieh. "Here the scent markings are highly concentrated around the feeder and they taper off at increasing distance from it, in a trail that is roughly teardrop-shaped."
Remarkably, the bees fly to the point with the highest scent concentration, and will ignore food placed at other locations along this scent trail, even if it is closer to the hive. Nieh hypothesizes that this scent gradient permits the bees to pinpoint a specific food source, enabling them to arrive en masse.
T. hyalinata is very aggressive. These bees will readily attack other bees of different species, or bees of the same species from different colonies. Because they can better compete for food, Nieh says it is advantageous for aggressive bees to arrive at a food source in large groups. Similar behavior occurs in other types of aggressive insects; for example, army ants arrive in "enormous hordes," according to Nieh. A scent trail with concentration clues, like that of T. hyalinata, he concludes, is able to provide the guidance needed for the bees to arrive as a swarm.
To test their hypotheses about how bees follow the scent trail, Nieh and his colleagues needed to be able to change the location of the food source and the trail itself. This would be extremely difficult with real plants and flowers; thus they used bowls of scented sugar water as the food source and used ropes decorated with leaves, as moveable foliage.
To analyze marking behavior in detail, the researchers also videotaped the bees as they were depositing odor marks. They found that the bees landed briefly and then rubbed their mandibles, or mouth parts, against the leaves or ropes to leave the scent. When the researchers made an extract from ground-up mandibular glands (analogous to salivary glands), they found that the extract had the same effects on bee behavior as the odor marks deposited by other bees. This confirmed that the mandibular glands are the source of the odor cues that constitute the scent trail.
These mandibular gland odors actually have a complex effect on the behavior of the bees. "The odors initially elicit an attack response, but then after 10 to 15 minutes they elicit feeding," explains Nieh. "The mandibular gland odors are actually a cocktail of chemicals, which evaporate at different rates."
This explains the dual effect on behavior because a chemical cocktail that initially stimulated the bees to attack could later change into one that stimulates bees to eat. "Thus mandibular gland odors could enable foragers to draw in reinforcements precisely where they are most needed to take over a rich food source," Nieh surmises.