Evolution of animal communication: functionally referential communication in highly social bees

Insect societies have evolved communication systems of remarkable complexity.  Highly social bees (honeybees and stingless bees) use sophisticated methods to exploit resources such as pollen, nectar, water, resin and nest sites.  Social bees can recruit, increase the number of nestmates at a particular location or increase the number of nestmates searching for a particular resource at non-specific locations.

Research in the Nieh lab examines the mechanisms that allow highly social bees to communicate resource location and seeks to understand how these communication systems have evolved.

Honeybees use functionally referential communication, the transfer of environmental information into coded signals understood by a conspecific receiver.  Such a system may be a sophisticated form of animal communication because of the cognitive complexities presumably involved in transforming sensory information into coded communication signals.  However, the question of how referential communication has evolved remains relatively unexplored.

Stingless bees
Stingless bees are an excellent model for the evolution of animal language because they possess the widest diversity of species and communication strategies, including the ability to acoustically encode the distance and height of food sources.  Moreover, some species may use functionally referential communication.

For example, Melipona panamica foragers can communicate the three-dimensional location of food sources.  To achieve this, foragers use a combination of mechanics.  Results from a series of removal experiments (segregating all feeder-experienced foragers from potential recruits as they left the nest) suggest that direction is communicated outside the nest whereas height and distance are communicated inside the nest (Nieh & Roubik 1998).

A recruiting forager produces a series of pulsed sounds when she unloads her food to other bees and when she begins to make clockwise and counterclockwise dance movements (Nieh 1998B).  During the food-unloading phase, she produces longer sound pulses for a food source on the canopy floor than for one 40 m up in the canopy.  During the dance phase, sound pulse duration is positively correlated with increasing distance of the food source from the nest (Nieh & Roubik 1998).  Thus M. panamica foragers appear to use sounds to communicate food height and distance.  Several other Melipona species also produce sound pulses whose durations correlate with the distance to a food source.  Direction may be communicated outside the nest through recruits initially following the recruiter for short distances.  However, recruits can still find the food source at the correct height and distance even when they are prevented from following recruiters outside the hive

When recruits are near the food source (within 6 to 12 m), they can orient towards an odor beacon deposited by experienced foragers (Nieh 1998).  Thus M. panamica's complete recruitment system uses visual, olfactory and acoustic cues to both directly guide and to provide symbolic information.

Multi-modal communication
Recruiting foragers exploit several different information channels - sound, odor, vision, thermal sense, tactile sense.  Such multi-modal communication is robust.  It provides backup in case of information corruption or transmission failures.  The lab therefore studies several modalities, focusing on acoustic, thermal and chemical information.

Chemical espionage
We have recently begun to focus on chemical espionage in stingless bees and the role this may have played in the evolution of potential "counter-espionage" strategies such as encoded communication inside the nest.  All social bees have some form of odor marking at the food source, but only the stingless bees use odor trails.  There is intriguing variation in the length of these odor trails, with some species producing complete odor trails that extend from the nest to the food source, others producing short odor trails that extend only a few meters away from the food source in the direction of the nest, and finally some that only odor mark the food source alone.  Why does this variation exist?

Some stingless bees, such as the aggressive Trigona spinipes, can evidently eavesdrop and orient towards the odor mark deposited for good food sources by other species such as Melipona rufiventris.  In experiments, this eavesdropping occurred when T. spinipes foragers were acting as scouts.  Trigona spinipes foragers were also clearly able to distinguish between their own odor marks and those of M. rufiventris.  After finding the food source marked by the "victim" species, T. spinipes attacked, drove away, and killed the M. rufiventris foragers, taking over the food.  Limiting the conspicuousness of odor marks via strategies such as decreasing odor trail length could be an effective counter strategy to such eavesdropping.  However, this would also decrease the amount of guidance information offered to recruits.  Providing encoded location information, functionally referential communication, at the nest would replace such lost odor trail information.  Interestingly, all bees for which there is some evidence for functionally referential communication, including honeybees, use only point-source odor marking.  Whether this is due to eavesdropping and aggressive competition remains to be determined.  However, stingless bees and honeybees still aggressively compete in the environments and regions in which they evolved.  Thus eavesdropping may have contributed to the evolution of functionally referential communication at the nest.