Honey bee brain diagram. By C. Wietholt, Wikimedia Commons
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Bee brains are hard working organs. They are small, lobed, ovoid structures in a bee’s head that vary in size (depending on the species of bee being considered) from a sesame seed all the way up to a grain of rice. Each brain houses slightly fewer than one million neurons (compared to the 86 billion neurons in a human brain!), and each brain has significant areas dedicated to visual and olfactory senses.
Large, shaggy bee. Photo by A. Staverlokk. Wikimedia Commons
The large, shaggy bee (Panurgus banksianus) has a very large brain compared to its fellow bee species (I assume that it has one of the “grain of rice” sized brains!). The large shaggy bee is found in grasslands all across Europe. It makes its solitary burrow in sandy soil and carries out active pollen and nectar gathering with subsequent pollination benefits involving a very limited array of flowering plant species.
A research group at the University College, London wanted to see if bee brain sizes were influenced by evolutionary factors similar to those that have been observed in vertebrates. For example, in birds, species with more generalized diets tend to have bigger brains. It is thought that these larger brains are required in order to allow the bird to both find and recognize the many different foods that it is able to eat and also carry out the often complex tasks of food acquisition and processing needed before the food can actually be ingested. In primates and other mammals social behaviors are also accompanied by larger brains. These larger brains are needed for recognition of other individuals in the social group, communication with those individuals and to store patterns of behavior and behavioral recognition to allow the individual to “fit in” to the fabric of its society.
Interestingly, the University College group found that the evolutionary trends for larger brains in bees follows exactly the opposite pattern as those seen in vertebrates!
Ninety-three species of bees were evaluated for brain size, the nature of their diet and their degree of social behavior. The largest bee brain (in the large, shaggy bee mentioned above) was in species that only consumed pollen and nectar from yellow flowering aster species and which lived a solitary existence in its soil burrow. The other species examined also conformed to this general pattern. The more specialized the food source and the less social the species was, the larger the brain the species possessed.
Why are bees so different from vertebrates? One explanation involves visualizing what it is like to be a bee. If a bee consumed nectar and pollen from many different types of plants it would have very little need to find and recognize specific plant species. But, if a bee only consumed nectar and pollen from a very limited number of plants or even just a single plant species, that bee would have to have excellent visual and olfactory senses and memory capacities to search through a complex vegetational expanse in order to find its food source!
Photo by R. Moehring, USFWS, Flickr
Also, social behaviors in bees do not involve individual recognition and communication. Instead, each bee in a hive has specific tasks mostly hardwired into their genetic makeup. These behaviors can be modified by pheromones and other chemical communicators, but are not affected by learned or interactive behaviors. In fact, a large brain might be a detriment to an individual living in a genetically determined, chemically controlled social environment! A bee living solitarily, on the other hand, has to carry out all of the life-functions associated with food gathering, nest building and reproduction and needs significant brain power in order to accomplish all of these tasks.
How exactly honeybees (who have sesame seed sized brains) use their brains is a topic that has been the focus of many laboratory and field experiments. Honeybees can remember nectar locations in the field and can follow landmarks to a nectar source even after those landmarks have been moved! Many laboratory experiments on honeybees have shown that single exposure to a learning stimulus generally generates a short-term memory response, but that it takes multiple exposures to stimuli (at least three repetitions!) to generate a long-term memory response. A long-term memory response being defined as a memory that persists for a 24 hour period or longer.
Memory work with other insects (including fruit flies and ants), though, has shown that long-term memory responses are often generated after a single training/learning exposure to a stimulus. Researchers at the University of Toulouse in France wanted to test this prevailing assumption that honeybees are “slow learners’ (i.e. that they require a much longer and more intensive learning/training experience in order to generate long-term memory responses). The results of their experiments were published in Cell Reports on February 25, 2020.
Honeybee. Photo by C.J.Sharp, Wikimedia Commons
The learning experiments that the Toulouse team conducted took into account a factor that had been ignored in previous memory work on honeybees: not all honeybees in a hive are the same! There are three different castes of bees in every hive: worker bees, drones (the males), and the queen. The worker bees make up by far the most abundant type of bee, but there are also three different types of workers: nurse bees who tend to the eggs and larvae, guard bees that protect the hive, and forager bees that go out and gather nectar and pollen to feed the entire hive. Previous experiments on honeybee memory generation simply took an assortment of bees from a given hive for testing. Probably most of those bees were worker bees, but which type of worker and what proportion of the types of workers were included in each experiment was not determined.
The Toulouse team postulated that the forager bees would be most likely to have well developed learning/memory systems since their jobs of locating nectar sources, returning to the hive after filling up with nectar, and then re-locating those productive nectar sources to fill-up again (and again!) most acutely depends upon rapid learning and long-term retention of learned information.
So, using entirely forager bees for their experiments, the Toulouse team studied the number of stimuli exposures that were required to generate a long-term memory response. They found that for most forager honeybees a single exposure to a learning stimulus was sufficient to instill a long-term memory pattern. They also found that some of these long-term memories persisted for up to three days!
Photo by I.Tsukuba, Flickr
The Toulouse team also explored the physiology of memory in honeybees by inhibiting translation (protein synthesis) or transcription (mRNA synthesis from DNA) in bees undergoing different types of training. They found that short-term memory was not affected by the inhibition of either translation or transcription. Short-term memory, then, must utilize the already existing proteins in the honeybee’s brain.
“Mid-level memory,” though, (memory that lasts just four hours after training) was inhibited by the disruption of translation but not by the disruption of transcription. Mid-level memory, then, utilizes new proteins that are being synthesized in a honeybee’s brain by mRNA’s that have been previously synthesized.
Long-term memory, though, needed both active translation and transcription in the bee’s brain cells. This indicates that persisting memory patterns not only utilize proteins that were previously being synthesized in the honeybee’s brain cells but that new sections of the bee’s DNA must also be activated and allowed to make new mRNA’s in order to create even more types proteins that then can then accomplish the bee’s memory retention.