To prick or not to prick? How bees organize defensive behavior


Honey bee alerted (Apis mellifera). Credit: Morgane Nouvian

When do bees sting and how do they organize their collective defense behavior against predators? An interdisciplinary team of researchers from the Universities of Konstanz and Innsbruck provided new perspectives on these questions. Their study, published in BMC Biology, combined behavioral experiments with an innovative theoretical modeling approach based on projective simulation. It shows that each bee decides to sting – or not – depending on the presence and concentration of an alarm pheromone. Scientists suggest that each bee has a sting probability that is not constant, but shows at least two internal thresholds for pheromone concentration: one to start stinging and one to stop stinging. Computer modeling has also revealed how several environmental factors, such as the rate of predator attacks and the diversity of predators, likely led to the evolution of bees’ pheromone-based communication in their defensive behavior.

High concentrations of pheromone alarm as a stop signal

When a bee colony is attacked by a predator or seriously disturbed by a human who – accidentally or intentionally – has come too close to the hive, the bees in the colony launch a coordinated counterattack to defend the colony and scare off the intruder. An important stimulus for them to start hunting and stinging the intruder is the presence of an alarm pheromone, which the bees wear on their sting. In the event of an attack, the pheromone is dispersed either actively – by the guard bees – or automatically in the event of a sting – by the recruited soldiers. Thus, it contains information not only about the presence of an attacker, but also about the extent of the colony’s counterattack. “The more the bees stung the intruder, the more the alarm pheromone was released with each sting and the higher its local concentration”, specifies Dr Morgane Nouvian, biologist of Constance and co-author of the study with Andrea López- Incera from Innsbruck.

To understand how individual bees in the hive can use this information to make the final sting decision for the sake of the colony and possibly die as a result, scientists have observed individual sting reactions of western honey bees (Apis mellifera) of three colonies. Using different concentrations of natural and synthetic alarm pheromones and a dummy predator, they revealed that aggression towards the dummy – measured as the likelihood of stinging – initially increases with the concentration of the alarm pheromones until ‘she peaks. However, at high concentrations, aggression drops to a low level.

This is the first time that decreasing aggressiveness at high concentrations of pheromones has been demonstrated under controlled experimental conditions. “One possible function of this ‘shutdown’ effect of high concentrations of the alarm pheromone might be to avoid excessive stinging and unnecessary sacrifice when attacking an already defeated intruder,” suggests Nouvian.

The “superorganism” as an evolutionary unit

In social insects, whether honey bees or other social species such as military ants, individuals often coordinate their actions for the benefit and survival of the colony. For this reason, the evolutionary selection processes of these insects operate at the group level rather than the individual level. “Normally, if an organism dies, it cannot pass on its genes to the next generation. In a bee colony, however, it is the queen who is responsible for reproduction. If another bee dies defending the hive but save the queen in the process, the colony will continue to reproduce “, illustrates Nouvian. Because the bee colony functions as a single” superorganism “, the behaviors of the individuals belonging can only be understood by the collective result to which they contribute.

To further analyze their experimental results and address this particularity of the evolution of collective behaviors, the scientists used computer modeling based on the so-called projective simulation, an approach originally developed by the co-author, Professor Hans Briegel and his colleagues from Innsbruck. In their agent-based model, each agent, or bee, has a very limited set of percepts – the concentration of the alarm pheromone and a signal that the predator is leaving – and actions – to sting or not to sting. – relevant to behavioral defense. “Our idea was to build a realistic, but minimalist model,” explains Professor Thomas Müller, professor of philosophy at the University of Constance and co-author of the study. He continues: “We simulated a collective of these agents, who were called upon one after the other to perceive the current level of alarm pheromone at a given point in time. When a bee stings in response, the concentration of the pheromone increases and the response of the next bee depends on this new level of pheromones. “

A second important aspect of the model is that it includes a learning component: neither the responses of individual bees nor the rules of interaction between them are predetermined. Instead, they “evolve” over many simulation cycles or, in other words, over many generations of the collective. “If the decisions of individual agents are beneficial for the collective under certain environmental pressures, they are positively reinforced. This increases the likelihood that the next generation of agents will act the same under identical conditions, ”says Müller. Taken together, the agent-based approach with reinforcement learning at the group level allowed to model the observed defensive behavior of bees from the perspective of individual bees and the collective.

Put the model to the test

Using the model and different combinations of parameters, predictions could be made on the possible influence of environmental pressures on the defensive behavior of bees. For example, simulations suggest that colonies adapt to the most powerful predator they encounter. This means that colonies that encounter mostly weak predators, such as mice or toads, are less likely to sting at high concentrations of pheromones than colonies that more frequently encounter strong and hard-to-deter predators, such as bears. . “For the survival of the colony, it makes perfect sense to be able to deal with the world’s worst predator, even if that means over-stinging some of the weakest predators,” Nouvian says.

Scientists also applied their model to the notoriously aggressive “African bee”, a subspecies of the western bee. It has previously been suggested that the very aggressive behavior of this subspecies evolved in response to higher predation rates in the tropics and to highly specialized and difficult to deter predators, such as honey badgers. This is because the simulation predicted that bee populations suffering from a high predation rate and predators that take a high number of stings before stopping their attack develop stronger defense responses than those that do. not.

And after?

“We were very happy to see that our model supports current hypotheses on how the greater aggressiveness of African bees could have evolved. One of the next steps will be to collect empirical data on real bees in Africa to verify the results, ”says Nouvian. .

Another step for the future is to model a more diverse bee population. As mentioned earlier, there are at least two different types of bees involved in attacking a real hive defense: guards and recruits. “In the current model, each bee in the collective followed the same decision-making process. Training a model with two different types of agents and comparing it with experimental data will be very interesting,” adds Müller. In general, the modeling approach is very versatile and can be applied to other tasks and species, providing a valuable new tool for studying the evolution of collective behavior.

It’s bee season. To avoid getting stung, stay calm and don’t hit

More information:
Andrea López-Incera et al, Honeybee communication during collective defense is shaped by predation, BMC Biology (2021). DOI: 10.1186 / s12915-021-01028-x

Provided by the University of Constance

Quote: To prick or not to prick? How Bees Organize Defense Behaviors (2021, May 26) Retrieved May 26, 2021 from

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