Small Brain, High Performance
Professor Martin Giufra works at the Center for Research on Animal Cognition in Toulouse, France. He is a specialist in neurobiology, with a particular focus on neurocognition in invertebrates. The research team he leads has investigated the remarkable learning abilities of the honey bee’s small brain.
The honey bee brain is plastic
The honey bee is a highly social insect that has developed complex communication strategies and also possesses memory capacities and behavioural plasticity related to foraging. The earliest observations were reported by Aristotle, who noted the bees’ ability to forage repeatedly on the same flowers, a phenomenon known as floral constancy. The honey bee therefore has a memory of both locations and floral species to be visited. It is capable of learning the colour, odour, and shape of a flower. In addition, it can be trained and displays a high degree of cooperativeness as long as it is rewarded with a sugar solution. Researchers quickly realised that this insect represents a valuable source of information for understanding the functioning of a small brain containing approximately 950,000 neurons per mm³ (by comparison, the human brain contains about 100 billion neurons).
Ingenious tests to understand brain function
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Memory can persist throughout the entire lifespan of the animal. Research has developed highly sophisticated protocols to investigate associative olfactory learning in the laboratory using immobilised insects endowed with the proboscis extension reflex. In a first step, an odour is presented to the insect, followed by the immediate association of a sugar reward with proboscis extension. After only two to three trials, the insect acquires and stores the long-term association “odour → proboscis extension”. By exposing the honey bee brain through ablation of the interocular cuticle, it becomes possible to study the neural processes induced by the experiment using electrophysiology, calcium imaging, or pharmacological blockade. |
The insect olfactory circuit consists of the antennae, which serve as olfactory sensory organs (analogous to the human nose), bearing approximately 60,000 olfactory receptors that transmit their signals to the antennal lobes. These lobes are composed of about 160 glomeruli, 800 projection neurons, and 4,000 local interneurons. The signals are processed locally and then partly transmitted to other regions (lateral horns), but predominantly to the mushroom bodies (approximately 170,000 neurons). These structures are considered the storage sites of olfactory memory or, more precisely, multimodal brain centres integrating multisensory convergence and combined multimodal output, coordinating exchanges between visual, mechanosensory, and gustatory modules in close association with attentional and reinforcement systems.
The mushroom bodies themselves contain upper (olfactory) and lower (visual) microglomeruli. These microglomeruli, measuring approximately 3 μm (10−6 m), consist of projection neurons originating from the antennal lobes at the centre, surrounded by Kenyon cells. The latter form genuine synaptic boutons endowed with plasticity and undergo structural modification (synaptogenesis) as a function of olfactory learning and memory formation. These experiments lead to the following conclusions:
- The honey bee brain is plastic and is therefore capable of learning and memory formation.
- The synaptic architecture of the upper lip, or olfactory region of the mushroom bodies, is modified following long-term olfactory memory formation.
- These structural modifications depend on protein synthesis.
- Increased olfactory activity following learning induces an increase in the number of connections between olfactory neurons and a greater number of microglomeruli.
- These microglomeruli are therefore capable of storing olfactory memory.
Are honey bees capable of solving high-level non-linear problems?
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The mushroom bodies are not limited to memory storage but constitute crucial structures for higher-order learning. They can be reversibly anaesthetised to investigate non-linear or ambiguous discrimination tasks involving inhibition of the reflex response (negative patterning: A+, B+ versus AB−). Experimental results show that anaesthesia of the mushroom bodies blocks the ability to acquire non-linear discriminations, whereas linear discrimination remains preserved. |
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In conclusion, the mushroom bodies are necessary for solving complex, non-linear problems but are dispensable for elementary, linear tasks. As in vertebrates, there exist neural structures dedicated to higher-order learning and others involved in simpler forms of learning, enabling the honey bee to solve problems independently of the type of stimuli employed. Honey bees are also capable of learning to make choices based on a concept of difference.
The impact of pesticide treatments on learning capacity
The relevant question is: what happens to the mushroom bodies when honey bees are exposed to sublethal doses of pesticides?
Experiments by Peng & Yang (2016) demonstrate that pesticides can affect long-term olfactory memory. The consequences for learning and memory are dramatic, as the honey bee is no longer able to memorise food sources.
In summary, the honey bee brain consists of a network of neurons and identifiable neural structures capable of producing both stereotyped behaviours and plastic behaviours that extend beyond elementary learning. Experience-dependent brain modifications can be demonstrated (plasticity). The honey bee brain is therefore highly efficient and remarkable, providing insight into the basic mechanisms underlying certain primitive cognitive processes. Unfortunately, exposure to pesticides can impair this plasticity and substantially weaken a colony.
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