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Learning Their Place

Dave Black


As Honey bee workers mature they undergo a behavioural development scientists call “temporal polyethism”, more commonly referred to as an age-related (not age-dependant!) division of labour. Younger bees for the first two to three weeks of adult life work inside the hive at tasks such as brood care and hive maintenance, and older individuals work outside the hive as foragers. The transition to foraging involves changes that cause many thousands of alterations in gene activity in the brain affecting metabolism, circadian clocks, hormone activity, and phototaxis. This is largely related to the effects of juvenile hormone (JH), and it seems the gradual increase in JH influences the hive bee to forager bee transition. A honey bee brain has about 950,000 neurons and occupies a volume of about 1 microlitre, tiny compared to the 86,000 million neurons and 1.2 litre capacity of a human brain. Venturing outside the nest creates an instant need to acquire and store information about how to return to it. Foraging is thus cognitively demanding, and besides learning the appearance and location of the hive, new foragers must learn to navigate in the environment and learn to harvest food from different floral types. Given the limits of their cognitive capacity it's not just about what they learn, but what they remember and what they forget.


Bees acquire information about the location of their nest-site and food sources in ways that are common to other hymenoptera, including the ones that can't fly! Foraging bees behaving as 'scouts' follow an innate fractal search pattern that is known as a Levy flight. Levy patterns have been found in just about every animal that's been looked at, terrestrial, avian, or aquatic. A Levy search consists of a repeated series of a single long step followed by a cluster of short local steps. This behaviour seems to be deeply buried in the sub-conscious 'psyche' of things that move, almost as fundamental as Brownian motion, a strategy beyond disruption.


The orientation fights of the bees and wasps have been recognised for a long time as essential to their 'homing' ability, and for 'fixing' a new food locality they need to return to. The most characteristic feature of this learning episode is the 'turn back and look' (TBL) behaviour. A bee will leave the hive at first almost hovering in reverse, then fly in a series of increasingly long arcs turning to look back and view the hive from the turning point on each arc. Eventually it will circle high above the hive before returning to rest. In doing this bees experience an image of their nest from different angles, seeing it from a distance in the context of the horizon, landmarks, and the sunlight polarized sky. Within a few flights they will be able to recognise the images they have stored and return to it directly. This suite of behaviours will not be repeated for this goal unless the insect has difficulty finding it, in which case it will perform some (shorter) version of the activity to 'refresh' it's memory. The same sequence will occur when it first leaves a food source location. This appears to 'reinforce' the memories it gained on its approach, particularly if the resource turns out to have some significant value. If the resource has little value they won't bother to reinforce their memory of it with TBL.


Bumble bees, and to an extent honey bees, are known to communicate and learn foraging behaviours by observing others. Honey bees are famous for learning from each other not just the mechanics of harvesting from a flower but communicating the distant location of the food source itself. This has an immense effect on their respective foraging abilities. Individual bumble bees and solitary bees are masters of the local search and develop very efficient routes (like 'trap-lines') gaining the maximum reward with minimum effort. Honey bees are not efficient in the same way, more often returning to a flower they have just visited for example. However, their ability to recruit nest-mates to exploit the same resource, and the ability to navigate effectively over large distances, ensures efficiency at the colony level.


Experiments observing and modelling the results of laboratory work have revealed some of the mechanisms involved in three types of memory-based guidance that bees and other hymenoptera use. Two of these are types of image-matching, sometimes called ‘snapshot memories’, of the surrounding panorama. 'Alignment image-matching' and 'positional image matching' are based on recalling simplified retinal views of its surroundings. Alignment image-matching allows an individual to recall a path that it has previously taken. Positional, unlike alignment, image-matching, can provide guidance from novel locations and in novel directions to match the known image of a goal. The third mechanism is known as 'path integration', by which an insect monitors and stores distance and compass metrics derived from its own movement. They obtain directional information principally from cues deriving from the sun and distance information from monitoring the optic 'flow' of patterns across the eye or sensory input generated by their body movements. Unlike image matching, path integration does not require prior experience of the visual environment.


In each case, the insect compares its current sensory experience with a memory of the desired sensory experience to derive a heading that encodes the direction to the goal. While positional image-matching is most reliable in the vicinity of the goal, path integration can be used over large distances. Together, these guidance mechanisms allow bees to forage far from their colonies, and with experience, to embark on, and return from, complicated journeys. While these strategies work together one may be prioritised over the others at any particular time to resolve conflicts or deficiencies, and importantly each can be used train or 'calibrate' the other guidance systems. The emerging consensus from researchers studying hymenoptera is that older hypotheses that invoke 'cognitive maps' as an explanation for successful navigation are unnecessary and do not fully explain observed behaviour. Rather than consulting a map, bees' sense of place might be thought of as constructed from a series of carefully archived photographs.


In the last ten years some of the neurological basics of how memories are established and lost are coming to the surface. The 'chemistry' of memory looks to be much the same whichever animal we look at, and given their relative simplicity, economy, and manageability honey bees are often the subject of this branch of scientific enquiry. In simple terms there are three phases of memory. Short Term Memory (STM) has a duration of seconds to minutes and is a feature of the connections made between neurons, of the transient chemical elicitors and receptors that pass signals from sense organs to muscles. Mid Term Memory (MTM), lasting hours, involves a 'cascade' of secondary messenger molecules caused by the repetition of a stimulus. If around for long enough, these secondary messengers eventually produce an enzyme (kinase) which acts on a family of proteins regulating phosphorylation or transcription of genes, creating a Long Term Memory (LTM) that can last days, weeks or more. STM is easily 'contradicted' and decays rapidly. MTM is not reversible, but by not involving protein synthesis needs to be constantly reinforced before it decays. The protein alteration characterising LTM is relatively fixed, causing a structural shape or biochemical change in the animal's proteins, but subject to decay, modification,  and not necessarily permanent. In a remarkable study in 2010 Jill Dolowich (remarkable because she was 16 at the time!) was able to show with respect to route and landmark learning that without regular reinforcement LTM is lost within 6 - 9 days. There is also a complex relationship between memory acquisition and extinction with physiological age. Older foragers are often more likely to show evidence of the progressive loss of some types of brain function, particularly spatial memory. On the other hand younger foragers (like bumble bees) are thought to have a shorter memory, which turns out to be a good thing. Forgetting what they knew yesterday encourages them to try new locations and food sources and so builds the experience they don't have. This might be particularly useful for bumble bees that forage on much more transient flora.


Honey bee memory is not simply a recording device, absorbing all experience. By staging memory acquisition in this way (and it is a little more complex than outlined above!) there is a sort of 'triage' operating on what will be remembered and what will be forgotten. Together with the behavioural response to 'forgetting' or 'not knowing' (for example - more repetition) bees are able to adjust their investment learning to suit changes in the need for information, or changes in the provision of information by their environment. Memory is constantly but dynamically accumulating or diminishing.


How animals retrieve stored memories still eludes us. Behavioural studies suggest that particular sets of memories might be 'activated' by associating them with a motivational state, a goal, or some relevant cue. For example, an insect will recall different memories depending on whether it is seeking food, or has gathered food. As its goal changes the image maps it has associated with its goals are exchanged for the more pertinent set of memories. A bee with a full crop of nectar heads for home if we move it to an unexpected site; if the crop is empty it heads off towards a feeding station. We seldom really appreciate that a crucial factor in the process of remembering some piece of information is context. We ought to know that from our own experience of struggling to name a familiar acquaintance encountered in a novel place! 


An example of one such memory cue can be scent, in nature perhaps provided by another forager after a recruitment dance. Training exercises with honey bees can accustom them to a scent in a sugar solution, and to different scents in two solutions each at separate sites distant from the hive. It is then possible to use scent alone to stimulate the foragers to leave the hive and visit a feeder even when it is empty. More intriguingly, they will go to one feeder site or another depending on which scent is blown into the hive, apparently associating the right scent with the correct site and recalling the appropriate set and sequence of 'images' for the journey. Landmarks are another plausible cue. Given the right motivation, a displaced ('lost') bee can recall the correct set of 'homeward' images after an accidental encounter with a familiar landmark. Also referred to as 'beacons' these features can be used to 'punctuate' a journey breaking it up into short memorable portions and cueing the appropriate recall. Bees that have swarmed to a new nest site do not simply forget the position of the old site. For a while, given the right motivation (like a lost queen) Worker bees can easily recall the right set of memories and travel back to the original site. In the opposite case Beekeepers moving a hive know to move it sufficiently far away so that a flying bee's experience is completely different, or they try to force re-orientation by blocking the entrance with leafy, twigs, a mirror, or some other device. By doing this foragers should realise landscape or image cues they are accustomed to using to recall the next appropriate guidance memories need to be revised. It is sometimes possible, given weather conditions that prevent the bees from flying, that their memory fades enough that they will need to re-orient themselves anyway. Hence the old beekeeper's rule of thumb for moving hives; three feet, three miles, or three days.


Honey bees have a robust system for navigating their environment. Their 'dead-reckoning', supported by celestial clues (path integration'), may be their primary source of guidance in distant novel landscapes, but it has been clearly demonstrated that using panoramic views of the skyline (image matching) is a significant (perhaps the most significant) strategy bees (and ants) use for finding their way around in landscapes they have some knowledge of. In orientation flights and when moving hives bees have been observed quickly spiralling upwards to 20-30m, presumably to gain an unencumbered view of the horizon. When navigational clues from the sky (the sun and polarised light) are absent or confusing bees use their memory of the sun's position with respect to the horizon and time of day, and if they can't see the horizon they make the best guess they can. Guidance from path integration becomes weaker as the goal is approached so that travel along routes tends to reflect route image memories more. As the insects move more precise or reliable cues have more successful outcomes. With increasing experience accuracy emerges automatically as information from the various strategies is evaluated and refined, so it's clear that memory is a crucial part of the system. While it makes the job of scientists trying to untangle and understand what goes on really complicated, by integrating these three components into a single, scalable, 'always-on' system honey bees are ready for anything Nature throws at them.

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