Pheromones as drivers of behavioural plasticity

Pheromones are key elements of animal communication: they are released to convey specific messages such as sexual attraction, aggression, recognition of conspecifics, etc., to members of the same species.

Figure: Proboscis extension: appetitive response of an immobilized bee to a sugar-solution reward that has contacted its antennae. Bees exposed to pheromones of different significance modify their proboscis extension behavior, thereby demonstrating the impact of these pheromones on the evaluation of the received food reward. © Martin Giurfa

The teams led by Martin Giurfa at the Center for Research on Animal Cognition and by Patrizia d’Ettorre at the Laboratory of Experimental and Comparative Ethology have discovered a new, previously unsuspected function of pheromones in the honeybee. Beyond the specific messages they convey, pheromones are able to change how an animal evaluates the subjective value of a desired food and, consequently, its capacity to learn about that food.

The analogy between colonies of social insects and multicellular organisms was first established a century ago, when insect colonies were described as “superorganisms.” In close analogy with multicellular organisms, which use circulating hormones to coordinate their cells, social insects use pheromones—highly volatile molecules acting as chemical messengers—to coordinate hundreds or even thousands of individuals within a colony. Pheromones are key elements of animal communication: they are released to convey specific messages such as sexual attraction, aggression, recognition of conspecifics, etc., to members of the same species. With more than 50 different pheromones described to date, bees clearly stand out from other social insects through their sophisticated communication system based on these chemical molecules.

The researchers from the teams of Martin Giurfa (CNRS, University of Toulouse) and Patrizia d’Ettorre (University Paris 13) discovered a new function of bee pheromones that changes the traditional view that these substances are merely highly specific chemical messengers. The work, led by David Baracchi, showed that pheromones can affect decision-making and foraging activity beyond the specific messages they convey: in fact, they can change how an animal evaluates the subjective value of a desired food and thus its ability to learn about that food.

The researchers exposed worker bees either to an appetitive pheromone, which in nature serves to mark profitable food sources, or to alarm pheromones, which signal aversive situations. After a few minutes, when the pheromone was no longer present in the environment, they offered the bees a sucrose reward and assessed their willingness to ingest this food by using the reflexive proboscis extension response (PER) that occurs in this insect when its antennae are stimulated by sucrose solutions.

The researchers observed that pheromones induce significant changes in sucrose responsiveness and habituation (a simple form of appetitive learning), thereby demonstrating that pheromones modulate bees’ motivation beyond the messages they provide. Interestingly, the direction of this modulation depends on the positive or negative valence of the pheromones. Alarm pheromones decrease appetitive motivation, whereas the appetitive pheromone that marks profitable nutritional resources enhances it. Thus, an alarm pheromone is not merely an alarm signal; it alters an animal’s global motivation, including its willingness to access a nutritional resource and the learning of factors associated with that resource.

These results provide a new perspective on the general assessment of pheromone effects, which are usually considered to be restricted to the triggering of stereotyped responses, since the new work by David Baracchi demonstrates their influence on simple learning. They thus highlight the important role of pheromones in the behavioral plasticity of individuals and colonies.

► Source: This study was published on August 29, 2017, in the journal Scientific Reports

► Another article by Prof. Giurfa: Mini Brain, Mega Performances