Tag Archives: harmonic radar

Making a beeline

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Synopsis : Honey bees use a range of navigation skills including path integration – to shorten return flights – combined with map-like spatial memories to relocate the hive.

Introduction

Regular readers will be aware that I’m interested in the origins of words. The Oxford English Dictionary (OED) is a fantastic source of information and produces a free Word of the Day email 1. This includes both the meaning and etymology of one word each day.

Since the complete dictionary includes over 600,000 words it will take a few years to collate the 20 volumes that comprise the entire dictionary 2.

At the beginning of this week the word of the day was beeline.

The word beeline of course means:

A straight line or course, such as a bee follows in returning to its hive after having collected a full load of nectar; (occasionally) the course taken by a bee.

The word originated in the US almost 200 years ago. It was first recorded in the American Quarterly Review in June 1828. Anyone who has read Tom Seeley’s Following the Wild Bees will appreciate the context in which the word beeline was used:

The bee-hunter..encloses them [sc. bees] in a tube, and letting one fly, marks its course, by a pocket compass. Departing to some distance, at right angles to the bee-line just ascertained, he liberates another, observes its course, and thus determines the position of the hive, which lies in the angle made by the intersection of the bee-lines.

Beelining is the art of finding feral or wild colonies by following the returning flight of bees. The book has a companion website with some interesting videos if you’d like to know more.

Find and tell

Beelining ‘works’ because bees fly in a straight line back to the nest 3.

The basics of beelining

Assume the blue flowers above are nectar-rich and favoured by the bees. You capture a couple of bees feeding on the blue flowers and give them some additional syrup so that they are replete and need to return to the colony to unload.

When you release the bees at ‘A’ they fly at a particular bearing back to the colony. However, if you instead release them at ‘B’ they fly at a different compass bearing back to the colony.4  .

How did the bees find the nectar-rich blue flowers in the first place?

Perhaps they observed another worker in the colony performing the waggle dance which informed them of the angle (from the sun) and distance to the blue flowers?

Alternatively, they might have just been searching around and chanced upon the blue flowers … they didn’t know they were there in the first place.

If they found the blue flowers by interpreting the waggle dance then you should be thinking how the waggle dancing bee found the blue flowers.

Alternatively, if they found the blue flowers by chance then you should be wondering how they will communicate their location to other foragers in the colony.

Transient nectar sources

Nectar sources are transient. They yield at particular times of the year … and of the day. The nectar may be dependent on recent rainfall or a variety of other environmental conditions.

All this means is that foragers may have to search widely to find a good source of nectar. If the source is really good – ample sugar-rich nectar and with lots of flowers producing it – then it’s important that the forager that found it tells her half-sisters how to also quickly find the same source.

Foraging and finding

On the left the blue flowers have been yielding for days. The workers fly there in a straight line and return along the same path. Newly orientated workers observe the returning foragers waggle dancing and follow the same route to quickly and efficiently exploit the source.

But all good things come to an end …

On the right is what happens when blue flowers stop yielding. The foragers that arrive at the blue flowers find slim pickings and start casting about looking for a better source of nectar. They first find the marginally better yellow flowers, then the similar (but far from outstanding) purple flowers … so they keep looking.

And eventually, they find the red flowers. Lots of nectar and lots of flowers. They load up and return directly to the colony (black dotted line).

There are two striking things about this return flight. The first is that it does not follow (in reverse) the route by which they reached the red flowers. The second is that when these returning foragers perform the waggle dance they ‘instruct’ the observing bees to fly in the direction of the red dotted line … rather than to the blue, then yellow, then purple and then red flowers.

Path integration

The foragers who find the red flowers perform a process termed path integration to return:

Path integration is the process by which an animal, when moving away from a start point, often its nest, cumulatively sums its path, generating an internal vector that specifies the line from the animal’s current position back to the start point, however circuitous the outward trip (Collet, 2019).

This is a skill I singularly lack when trying to relocate my vehicle in the multi-storey car park.

Path integration is seen in other insects … Drosophila fruit flies can do it (over a range of centimetres), walking ants can do it over a range of hundreds of metres, and honey bees can do it over at least 5 kilometres (and probably more).

Path integration requires two pieces of information – the direction and the distance of travel.

Path integration – individual parts of the flight are in different directions and of different lengths

Clearly, the very existence of the waggle dance provides compelling evidence that bees are aware of both. The dancing forager reports the angle (relative to the sun) of the nectar source and the distance at that angle that must be covered before the nectar source is located.

But for path integration, not only must the angle and distances be determined, they must also be cumulatively summed.

Neurophysiology and evolutionary conservation

Detailed neurophysiological experiments – recording the firing of individual neurones in the bee’s brain – have identified that these events occur in a region called the central complex (CX).

Two types of neurones are involved; the first is a set of polarised-light-based compass neurones and the second are optic-flow-based speed neurones. The former use celestial cues to create a visual compass. The latter provide a visual odometer (Stone et al., 2017).

Together – and there are additional integrator cells that link these functions – this relatively simple 5 neuronal circuitry allows path integration, enabling the bee to return ‘home’ directly after a convoluted outward flight.

Many of these studies were conducted on the nocturnal sweat bee Megalopta genalis. This forages at night when polarised skylight provides the directional cues in its rainforest habitat.

Importantly, similar neuronal organisation is found in the CX’s of locusts, some butterflies and dung beetles. The visual odometer neurones were analysed in Megalopta genalis, but are physically and likely functionally similar to structures found in Bombus terrestris (a bumble bee).

You may have noticed that none of these studies used our favourite, Apis mellifera, the honey bee.

The evolution of termites, ants, wasps and bees

Nevertheless, there’s every reason to think that honey bee path integration involves very similar neuronal activity. Megalopta (belonging to the family Halictidae) and Bombus (a member of the Apidae family) are very distantly related and evolved from a common ancestor over 100 million years ago (Cardinal and Danforth, 2011). It’s therefore likely that all bees derived from this common ancestor – including honey bees – share similar neuronal activity underpinning their path integration ability.

Food vectors

Before considering another point about honey bee flight I wanted to to briefly mention features of the outbound trip back to the high quality food source (the red dotted line in diagram above). This is termed the food vector and is essentially the reverse of the path integrated return flight back to the colony i.e. the same length, but pointing in the opposite direction.

The waggle dance communicates this food vector to nest mates of the successful returning forager.

But what happens if bees are displaced when starting, or while following this food vector?

For example, if a huge gust of wind blew them off course by tens or hundreds of metres, or an evil eager scientist captured them as they left the hive and transported them in a dark box across a couple of fields and then released them?

Where do displaced foragers go?

Do the bees fly a corrected route to the food source (the blue dotted arrow), or do they continue flying the same vector (angle and distance – the green dotted arrow) they would have done when they left the hive?

I’m not sure this exact experiment has been done with bees (but see below), but it has been done with ants (Cataglyphis fortis). In these studies the ’displaced’ ants did alter their direction of travel (Collett et al., 1999). The food vector is more than just an angle and distance, it also points to a position relative to the nest. The redirection exhibited by the ants was not perfect, but it clearly showed they were able to integrate the path to a location other than the nest after displacement.

Gusts of wind are not the same as eager scientists

However, back to the bees.

The gust of wind and eager scientist are not equivalent. Bees cope with gusts of wind every day. It always amazes me how well bees cope on windy days.

When blown off course they will get lots of visual cues – not least changes in optic flow and their angle to the sun – both of which should be readily corrected. If they didn’t then foragers would be lost in droves on windy days … or fail to find the food source.

In contrast, the eager scientist took care to place the bees in a darkened box, thereby immediately removing visual cues such as the angle of the sun and the optic flow.

In the studies conducted with the ants the scientists made sure the ants could see the sky but not the surrounding landscape (they trained them in open topped channels). This is because ants can also use landmarks in the surrounding landscape for orientation 6.

And bees can do the same, which is the final sub-topic for this post on bee flight and orientation.

The map-like spatial memory of bees

Path integration is both useful and necessary. It means that foragers can return – fully laden – with minimum delay to the hive. They can therefore tell other foragers (via waggle dancing) promptly, and – in the case of elite foragers – they can set off again on another trip.

By reducing the distance flown – by integrating the path – they save not only time but ‘fuel’ as well i.e. path integration allows bees to maximise the nectar returned at the end of the foraging trip.

But, if all flights were a combination of random searches and path-integrated returns, why do bees go on orientation flights?

Orientation flights are short range (10’s to 100’s of metres) flights around the hive. These are taken by workers around 3 weeks after emergence as they transition form hive bees to foragers. They are also taken by older foragers if the hive is moved.

The very existence of orientation flights is compelling evidence that honey bees also use learned environmental landmarks for route finding, or at least for mapping the area around the hive to aid efficient return trips.

What evidence is there that these landmarks are used for this purpose?

Harmonic radar tracking of displaced foragers

I’ve previously discussed the use of short range harmonic radar to track bees ‘tagged’ with a small transponder. The key point is that it allows relatively accurate mapping of the entire flight of a bee up to 900 metres away. The resolution is, at best, about 3 metres.

Menzel and colleagues (Menzel et al., 2004) tracked the flights of three types of ‘displaced’ foragers:

  • SF-bees trained to a stationary feeder a few hundred metres from the hive; these have ‘route memory’ and have traversed the route from the hive to the feeder multiple times
  • VF-bees trained to a regularly moved feeder within 10 metres of the hive; these bees have no route memory
  • R-bees which were recruited by a waggle dancing forager and have only secondhand route information of the position of the feeder i.e. they have never made the trip themselves

These are not trivial experiments. To ensure the environment was as uniform as possible they conducted the experiments in a large, flat mown field approximately 800 metres square. There was no forage within the field other than the experimental feeders. The field was surrounded on all sides by uniform coniferous woodland with insufficient variation in elevation (<1.5°) above the horizontal to provide any visual clues to the bees.

The field itself was not uniform. There were differences due to different mowing times and soil conditions. In addition, the scientists erected a number of radar-transparent coloured tents around the hive to provide additional landmarks.

Common features of flight paths determined by harmonic radar studies

Bees were allowed to orientate to the new hive position and then SF- and VF-bees were collected at a feeder and R-bees were captured as they left the observation hive (having ‘watched’ a waggle dance). The bees were fitted with a transponder, released some distance away from the feeder or the hive and then tracked by radar.

SF- and VF-bees were stuffed full of syrup and so – although they could fly for a long time – were motivated to return to the hive to unload their cargo. R-bees, whilst ‘primed’ to seek the feeder, had limited range and so would have to return to the hive to refuel.

Return flights of SF-, VF- and R-bees show some common features.

The SF- and R-bees exhibited three broadly conserved flight patterns during their return trip to the hive:

  1. A fast (20 m/s) straight line flight in the direction they would have taken back to the hive (for the SF-bees) or out to the feeder (for the R-bees). The length of this part of the flight was approximately the distance between the hive and the feeder.
  2. A slow (13 m/s) curved search flight.
  3. A fast homing flight back to the hive.

The VF-bees only exhibited the slow curved search flight and the final fast homing flight. This was unsurprising as they had never learned (or been told) to follow the route between the hive and distant feeder.

Food vectors and von Frisch

We therefore have the answer to the question I posed earlier (in the Food vector section above). A bee displaced when about to embark for the first time on a trip to a distant feeder – learnt from following a waggle dance – initially flies at the angle and to the approximate distance they would have taken from the hive (stage 1 of the flight).

Remember, unlike the ants, these foragers are ‘in the dark’ while being displaced, so have no visual clues about the displacement.

This is a really nice result and supports the contention made by von Frisch that the waggle dance communicates only distance and direction (relative to the sun) information, rather than positional information (von Frisch, 1967) 7.

Homeward bound

After a period of slow curved flights the returning forager switches to a direct, fast homing flight. These started at positions – starred in the figure above – from which the bee could not see the hive (based upon distance and the known resolution of honey bee vision).

Homing flights of displaced SF-, VF- and R-bees (A, B, C respectively). H indicates the position of the hive.

Individual bees were randomly displaced around the study field. The homing flights were in a straight(ish) line and bees approached the hive from a range of different points of the compass. This argues strongly against the bees following a particular feature on the ground that led them back to the hive.

Instead, the authors argue that, since all the bees exhibit these direct homing flights, it must be based upon previous exploratory memory i.e. from orientation flights.

The tents were not critical landmarks. If they were moved some distance away the bees still returned using the same three flight phases (in the case of SF- and R-bees) and with similar navigational performance. Clearly there was sufficient information in the ground structure alone (mowing patterns, soil differences) acquired during the orientation flights.

In support of this, some of the harmonic radar data showed bees flying along boundaries between mown areas (in a similar way to homing pigeons follow rivers or motorways; Guilford and Biro, 2014.).

These experiments indicate that during orientation flights the bee develops a local spatial memory of landmarks that provide a ‘memory map’. This enables the bee to return to the nest once it recognises some of these familiar landmarks.

Repeated displacement flights of the same bee further indicated that the landmarks recognised (whatever they were) could be approached from different angles.

Final inspections

My bees are still out foraging despite the large blocks of fondant most hives are now topped with. I’m not sure what they’re collecting but it’s clearly worth the trip … and going to the initial trouble of finding it and telling other foragers about it.

Returning foragers

We usually take the amazing navigational abilities of our bees for granted. Those returning foragers are using navigational skills that evolved at least 100 million years ago while dinosaurs roamed the earth.

100 million years is a long time to develop a range of skills and subtleties; it’s no wonder we still only partially understand honey bee navigation. Of course, we don’t have to understand it to still marvel at their ability to find the way back.

And it’s worth also remembering that these navigation skills – many of which are based upon the angle of travel relative to the direction of the sun – also operate on dull, overcast days. But that’s a topic for another post …

References

  • Cardinal, S. and Danforth, B.N. (2011) ‘The Antiquity and Evolutionary History of Social Behavior in Bees’, PLOS ONE, 6(6), p. e21086. Available at: https://doi.org/10.1371/journal.pone.0021086.
  • Collett, M., Collett, T.S. and Wehner, R. (1999) ‘Calibration of vector navigation in desert ants’, Current Biology, 9(18), pp. 1031–1034. Available at: https://doi.org/10.1016/S0960-9822(99)80451-5.
  • Guilford, T. and Biro, D. (2014) ‘Route following and the pigeon’s familiar area map’, Journal of Experimental Biology, 217(2), pp. 169–179. Available at: https://doi.org/10.1242/jeb.092908.
  • Menzel, R. et al. (2005) ‘Honey bees navigate according to a map-like spatial memory’, Proceedings of the National Academy of Sciences, 102(8), pp. 3040–3045. Available at: https://doi.org/10.1073/pnas.0408550102.
  • Stone, T. et al. (2017) ‘An Anatomically Constrained Model for Path Integration in the Bee Brain’, Current Biology, 27(20), pp. 3069-3085.e11. Available at: https://doi.org/10.1016/j.cub.2017.08.052.

Orientation flights

Part of the reason for the success of honey bees is the division of labour between workers of different ages. Young workers (hive bees) clean the cells, nurse larvae and look after the queen. Older workers are foragers, collecting pollen and nectar (and water) from across the landscape.

To be successful, foragers need to know where to look and how to return.

The ‘where to look’ is partly accounted for by the well-known waggle dance 1.

In this post I’m going to discuss the second component of successful foraging – the homing ability of foragers.

More specifically, I’m going to discuss how the bee first learns about the location of the hive. 

Orientation flights

Bees do not instinctively know where the hive (or the tree they are nesting in for a wild colony) is located. They have to learn this before embarking on foraging trips to collect nectar or pollen.

This learning takes the form of one (or usually several – as we shall see) orientation flights. These enable the bee to memorise the precise location of the hive with relation to geographic landmarks. On subsequent foraging flights the bees use these landmarks to return to the hive.

Orientation flight have a characteristic appearance …

… and are very nicely described in the introduction to a paper by Capaldi and Dyer 2:

An orientation flight at the nest entrance begins as a departing bee turns and hovers back and forth, turning in short arcs, apparently looking at the hive entrance. Then, the bee increases the size of the arcs until, after a few seconds, she flies in circles while ascending to heights of 5–10 metres above the ground. This spiraling flight takes the bee out of sight of human observers. She returns a few minutes later, always without nectar or pollen.

Which I couldn’t have written any better, so have reproduced verbatim.

There are a number of features of the orientation flight that are immediately obvious from this description (which all beekeepers will recognize). These include:

  • A ‘local’ component, in the immediate vicinity of the hive
  • Wider ranging flight at a greater altitude and a longer distance
  • Direct observation does not allow the location, duration or track of these distant flights to be monitored
  • The bee returning from the orientation flight does not bring pollen or nectar with her

Do orientation flights allow orientation?

How do we know that these flights enable the bee to learn where the hive is located?

Early studies conducted by Becker (1958) showed that bees captured after a single orientation flight and then re-released up to 700 metres away from the hive could find their way ‘home’. In contrast, bees that had not gone on an orientation flight before were, by definition, disoriented and did not return to the hive.

However, the percentage that returned after undertaking a single orientation flight was related to the distance of the release point, and was never more than ~60% (at 200 metres). 

In contrast, reorienting older foragers (for example, as happens after moving a hive to a new location) were much better (~90%) at returning to the hive after a single reorientation flight. 

Capaldi and Dyer extended these early studies by Becker to investigate the impact of the visibility of local landmarks on orientation and reorientation, and also measured the speed with which bees returned after being displaced.

Landmarks

These studies showed that a single orientation flight allowed bees to identify the landmarks in the immediate vicinity (100 – 200 m) of the hive. When released from more distant locations, returning flights were faster and more successful (i.e. fewer lost bees) when the bees had sight of the landmarks in the vicinity of the hive.

The hive itself was effectively invisible except at very short ranges. This makes sense for a tree-nesting animal. One tree looks much the same as another 3, but if you learn that the nest is in the tree between the very tall conifer and the long straight hedgerow – two features visible from hundreds of metres distant – then orientation is straightforward.

This suggests that apiaries located near distinctive landscape features may be preferable in terms of increased returning forager rates.

“Distinctive” as far as an orienting worker bee is concerned, which may not be the same as distinctive to the beekeeper of course 😉

Reorienting bees (compared to first flight bees) took longer to explore the environment and were better at returning. Either these bees learn differently (a distinct possibility) or their prior experience in the wider landscape gives them an advantage when the hive is relocated.

Where do you go to my lovely?

The early studies by Becker and those by Dyer and colleagues defined many of the parameters that characterise orientation flights. What they did not do is show where the bees actually go during the orientation flights?

Do they just zoom around randomly?

Do they fly ever-increasing spirals?

Perhaps they perform some sort of grid search, exploring individual landscape features carefully for future reference?

Recent developments with harmonic radar have allowed tracking of individual bees during orientation flights over hundreds of metres. These have provided further insights into the process.

Because harmonic radar is also relevant to other studies of honey bee flight – for example, the impact of neonicotinoids on foraging ability – I’ll digress slightly from orientation flights to describe the technology.

Harmonic radar

Harmonic radar has revolutionised tracking studies of insects in much the same way as GPS tags have provided unique insights into bird migration (or, for that matter, shark migration).

The radar system has two components. The insect is tagged with a tiny antenna attached to a Schottky diode (together termed the transponder). The transmitter/detector is a ground-based scanner that transmits the radar signal. This is used as the energy source by the diode which re-emits a harmonic of the original signal which can then be detected.

Tagged bumble bee (left) and harmonic radar detector (right)

The transponder weighs less than a normal pollen load, though presumably there is some wind resistance from the antenna. In studies of bees with and without transponders fitted the orientation flights were of a similar duration, suggesting any wind resistance didn’t appreciably impact the flying ability of the bee.

Orientation (a-c) and foraging (d) flights monitored by harmonic radar.

 

Orientation flights 4 were taken by bees between 3 and 14 days post-emergence, with the mean onset of foraging being 14 days post-emergence.

Bees took between 1 and 18 orientation flights, though there wasn’t a direct relationship between the number of flights and the age of the bee, suggesting they may learn at different rates.

Initial orientation flights were generally in the immediate vicinity of the hive. Older workers – pre-foraging – ventured further afield. More recent studies have addressed this in greater detail (see below).

Orientation flights were distinctly different from foraging flights. The former were slower and less direct. The ground speed of orienting bees was ~3.6 m/s in contrast to foragers who flew at ~5.6 m/s and, as shown in D above, foraging flights were very much ‘there and back’ straight lines.

Venturing forth …

A more recent study 5 has used harmonic radar to investigate multiple orientation flights by individual bees, effectively analysing how the bee explores the landscape as it ages towards a forager.

This was a remarkable study. It involved addition and subsequent removal of the transponder from 115 individual bees during 184 orientation flights. When the orienting bee returned they recaptured it, removed the transponder and allowed it to reenter the hive. When it reappeared for another orientation flight they reattached the transponder.

Anyone complaining about their inability to mark queens 6 should do this as a training exercise 😉

The scientists also recorded several foraging flights of a smaller number of the same bees, to allow comparison with their behaviour during orientation flights.

Orientation and foraging flights of five individual bees.

Flights were defined as short or long range, but long range orientation flights were still significantly shorter than foraging flights.

  • Short range flights were made in poor weather and familiarised the bees with the immediate vicinity of the hive.
  • Consecutive long range flights reduced in duration as the bees learnt about the immediate hive vicinity i.e. the long range flights included some local exploring at first as well.
  • Orientation flights explored different areas of the landscape, rather than focusing on one sector.
  • Subsequent foraging flights involved areas that the bees may have never visited during orientation flights.
  • Some very long duration foraging flights may involve a degree of exploration, though it’s not clear whether this is truly orientation, or actual scouting activity.

Not all bees performed short and long range flights though early long range lights did involve local exploration as well. 

Ground clues and conclusions

The final part of this study investigated the influence of visual landscape features on orientation flights. This deserves a post in its own right as the techniques are quite involved.

Essentially they generated heat maps of the flights overlaid onto the geography. Using this approach they determined that some features visible from the air e.g. borders between grassland and a track, influenced the direction of flight and hence the orientation flights. 

There are additional studies of the influence of visible landmarks on bee flight which I’ll return to at some point in the future.

Again, like the comment made above about visible landmarks, it suggests to me that apiaries situated near such distinctive features may aid orientation and subsequent homing flights by honey bees.

When you next stand by your hive entrance on a warm, sunny afternoon and watch young workers flying to and fro across the entrance before spiralling up and away out of sight, you’ll know that it is an essential component of their training to be effective foragers.

They don’t forage for long – perhaps three weeks at most – but they are very effective, partly because they know where to return to.

Notes

Where do you go to my lovely? was the title of a rather syrupy (my blog, my opinion! … Apologies if it’s a favourite of yours 😉 ) song by Peter Sarstedt in 1969. It’s notable for some quite clever rhyming lyrics and a particularly dodgy mustache he sports in the YouTube video. (I’m just linking it, rather than embedding just because of the mustache).

It has nothing to do with bees.