53. An introduction to the science of animal navigation
I recently enjoyed giving an online talk for students at the University of Kentucky. You might find it interesting if you want a very quick introduction to this fascinating subject!
Eurasian Reed warbler
47. How do migratory birds make use of the earth’s magnetic field? →
There is plenty of evidence that ‘experienced’ migratory birds (ones that have already made at least one migratory round trip) can cope with massive geographical displacements. They can, for example, successfully complete their journeys, even after being ‘kidnapped’ by scientists and taken to places that are totally unfamiliar - which may be thousands of kilometers away from their normal route.
This is a really astonishing feat. In the same circumstances we humans would be completely at a loss but for our maps, sextants and compasses - or nowadays, GPS.
Some experts believe that these birds are ‘true’ navigators, capable both of determining their present position and of selecting the right course to follow to reach their goal. I’m not so sure.
Well, how do they do perform this feat? It looks increasingly likely that their ability to ‘read’ the characteristics of the surrounding geomagnetic field is the key (though exactly how this sense actually works remains uncertain).
In my book, Incredible Journeys/Supernavigators I mentioned some intriguing evidence that reed warblers might be using changes in magnetic declination (the angular difference between the directions of true north and magnetic north) to estimate their longitude. Such a skill would help them compensate for large scale east/west displacements from their normal migratory routes, though it would not enable them to determine their position.
A fascinating new study from Dmitry Kishkinev and his colleagues now casts doubt on this hypothesis, but it sheds new light on the role that magnetoreception plays in avian navigation.
Kishkinev et al. have now shown that experienced Eurasian reed warblers exposed to altered magnetic fields (characteristic of completely different and unfamiliar locations far from their normal migratory routes) can successfully reorient themselves. However, they can do so only when all three components of the geomagnetic field (declination, inclination and intensity) are present and match a real place - not when declination alone is altered (as in the earlier experiment).
The crucial thing about the latest experiment (like earlier ones from the same team) is that the birds were not physically moved. The changes to which the birds were subjected related only to the surrounding magnetic field. All the other cues available to them (whether based on sight, sound, or smell etc.) remained unaltered.*
One possibility is that these birds have access to some kind of ‘cognitive map’ based on gradients in the three magnetic parameters. (Claims for such ‘maps’ are, I have found, seldom justified by the evidence presented though they make good headlines!)
Such a ‘map’ would presumably be based on the birds’ experience of how the magnetic field varies along their migratory route. Such a map might extend beyond the confines of their standard journey if - and this is a very big if - the birds could somehow infer their new position by extrapolating the spatial variation of the various magnetic cues from those they have observed along their normal migratory corridor.
There are plenty of experts who doubt whether that is even theoretically possible, and it is unlikely that birds could generate positional information from the geomagnetic field that is anything other than very coarse-grained.
However, as the authors of the new paper rightly point out, there is no need to postulate such an extravagant navigational system.
Rather than using their mysterious magnetic map to ‘fix’ their position and set a course to their destination, there is a much simpler and more plausible explanation.
Perhaps the detection of magnetic fields that differ markedly from those they are familiar with warns birds that they are off course. The size and nature of these differences may additionally tell them in which direction they have been displaced. They might then be able to make course adjustments that return them to the correct migratory corridor. Once they are back on track, they should have a good chance of reaching their destination.
A set of skills like that would certainly be very valuable as a way of dealing, for example, with the effects of strong cross-winds. Whether, as the authors seem to imply, it counts as ‘true navigation’ is debatable.
The authors also cautiously observe that their findings do not support ‘a strong role for other environmental cues’ in the navigational tool-kit of the reed warbler.
It will be very interesting to see whether their results can be replicated in other bird species.
*What the researchers actually observe is the direction in which the caged birds attempt to fly under the altered magnetic fields generated by the coil systems that surround them. This technique is widely believed to be a good proxy for actually tracking where the birds go, though some experts question whether it is reliable.
44. Where the Wild Things Go: New Yorker article
The latest edition of The New Yorker magazine (5 April 2021) carries an excellent article by Kathryn Schultz. It’s a review of the science of animal navigation based on her careful reading of several recent books on the subject, including my own Supernavigators. As you might expect from a Pulitzer Prize-winning writer, it’s lucid and elegant - and well worth a read.
29.Bats with maps?
Bats suffer from big PR problems. They are traditionally associated with all kinds of bad things - from vampires and the devil, to rabies and COVID-19. And many people think they’re ugly too, though I think this Egyptian fruit bat is actually very cute.
But one thing is beyond doubt: bats are very clever.
Many of them fly by night and have astonishing powers of echolocation, while others are day-flyers that rely on their excellent eyesight. Some bats migrate over large distances. And they’re all very good at navigation.
A new study from Lee Harten et al. at Tel Aviv University sheds light on the Egyptian fruit bat’s impressive navigational skills - and in particular, whether they make use of a ‘cognitive map’.
As I explain in Incredible Journeys, the cognitive map has been the Holy Grail of animal navigation studies ever since it was first proposed back in the 1940s by the great Berkeley psychologist, Edward Tolman.
Because rats sometimes use shortcuts when searching for food rewards in mazes - entirely novel routes that they have never used before - Tolman thought they might be making use some of some kind of ‘cognitive map’ on which they stored information about the layout of their surroundings.
This was a very controversial idea because rats were then thought capable only of learning fixed routes based on trial-and-error (otherwise known as ‘stimulus-response’ or ‘S-R’ learning). Acquiring map-like knowledge couldn’t easily be explained in S-R terms, so it seemed impossible - according to the prevailing behaviourist orthodoxy.
Tolman was attacked on all sides and his opponents came up with all sorts of alternative explanations for the behaviour of the rats - some of which were pretty bizarre. But when cognitive neuroscience took off in the 1960s, it became possible to monitor what was going on in the brains of living experimental animals. And many discoveries made since the 1970s have born out Tolman’s hypothesis.
To cut a long and complicated story short, there’s now abundant evidence that various mammals - including rats, mice and human beings - have brain circuitry that allows them to form cognitive maps of their surroundings, though there’s still plenty of room for debate about how these work in practice.
What is less clear is whether any other animals have the same abilities, though there are some scientists who would make that claim - even for insects like honey bees.
Bats of course are mammals and it would be quite surprising if they were any less gifted navigationally than, say, rats or mice. After all, they navigate over much larger distances - and they are obliged to do so in three dimensions!
The new study is interesting because it provides pretty solid evidence for the first time that fruit bats really do use cognitive maps.
Using GPS trackers, Harten et al. mapped every single journey taken by 22 young bats (‘pups’) starting with their first excursion outside the nest and continuing every night thereafter for five months.
The pups gradually extended their home ranges until these reached a mean area of about 60 square km. Sometimes they undertook exploratory flights which took them beyond into unfamiliar territory. When they detected new sources of food, they would return to them later on to feed.
The striking finding was that the pups - like Tolman’s rats - often performed impressive shortcuts. These were defined as routes in which at least 50% of the animal’s journey was novel (i.e. the pup passed no closer th100m to any location it had previously visited).
You may wonder whether these shortcuts were just chance events. Well, they certainly looked intentional.
The paths the pups followed were almost as straight as their regular ‘commuting’ routes to familiar sources of food, and they were much straighter than their exploratory flights. Moreover, the pups headed straight for their targets right from the outset of these flights. Some of the lengthier shortcuts (described by the researchers as ‘long-cuts’) occurred at the end of exploratory flights. These sometimes involved navigating for many kilometers over unfamiliar territory.
And out of the 246 short- and long-cuts that were observed, hardly any was predictable on the basis of a randomised (‘random-walk’) movement strategy.
Could the pups have been following a scent trail?
There is plenty of evidence that homing pigeons make use of olfactory cues to find their way back to their roosts - especially when taken to distant and unfamiliar locations (though this is still the subject of some debate). But Harten et al. found no correlation between wind direction and navigational straightness. On this basis they judged it unlikely that the bats were relying on olfaction, though they could not rule it out completely.
What about echolocation then?
The researchers note that the bats’ echolocation system has quite a limited range - for example, they can detect a large tree only when it is within 50 m.
Harten et al. also found no evidence that the bats were making the kind of errors that would be expected if they were relying on ‘path integration’ or DR to make their shortcuts.
So they concluded that the bats must have been relying primarily on their acute vision to perform these navigational feats.
But of course good eyesight alone isn’t enough.
Harten et al. believe that, before setting off, the bats look around them and use the spatial arrangements of distant landmarks (such as high-rise buildings) to judge where they are and the heading they need to follow to reach their goal.
And since the bats are not following the same routes again and again, they must have some way of storing the different locations of these landmarks (as well as their varied appearances) in a form that doesn’t depend on any fixed point of view.
So, it really does look as if they must have some kind of cognitive map, though it will be interesting to see how other experts react to the new study.
28.Green turtles aren’t perfect navigators - but they don’t have to be
Charles Darwin was one of the first scientists to speculate about how sea turtles navigate on the open ocean. It’s a fascinating question and two whole chapters of Incredible Journeys are devoted to it.
One of the experts I interviewed was Paolo Luschi whom I visited at the University of Pisa. Paolo has spent many years doing experiments on turtles in the field, and he warned me to be sceptical of claims that these animals - remarkable though they are - are brilliant navigators.
A fascinating new piece of research reinforces his point.
Graeme Hays and his colleagues (including Paolo) tracked 33 green turtles migrating from their nesting beaches on the remote Indian Ocean island of Diego Garcia to their habitual feeding grounds dotted around the western part of that vast expanse of sea. The turtles travelled anything from a few tens of kilometers to more than 4,000.
Several interesting things emerge from the analysis of their data.
Firstly, the animals very seldom went directly to their destination - sometimes they massively overshot and they often strayed wildly off track. Nevertheless, they were still able eventually to locate their targets.
Secondly, the turtles were mostly swimming in such deep water that they had no chance of seeing the seafloor beneath them (they don’t normally dive deeper than 50m). In these circumstances it would be hard for them to make use of underwater topography to guide them. However, when they got close to their destinations and entered shallower water, they were able to head fairly directly towards them. This suggests that they were making use of ‘landmark’ information - possibly acquired on previous trips.
This study also compared what the real turtles did with what they might have done based two different assumptions about how they navigate.
The first, stringent assumption was that the turtles are true ‘map and compass’ navigators - able, that is, both to work out where they currently are and where they need to go to reach their goal.
The second, much simpler one was that the turtles only had access to a compass of some kind that would enable them to maintain a steady course. This would of course give them no positional information.
When the actual tracks followed by the turtles were compared with the ‘virtual tracks’ that emerged from the simulations, it became clear that the turtles were not perfect map and compass navigators. They lacked ‘the ability to always locate small isolated targets with pinpoint accuracy’.
But, since they were still able to find their targets, it looks as if the turtles must have access to some kind of ‘map’, though plainly not a very detailed or precise one.
Such a map is very likely to involve geomagnetic cues, though other factors might also be involved. (Ken Lohmann’s studies of captive loggerhead turtle hatchlings have already shown their acute sensitivity to geomagnetic information - see Incredible Journeys chapter 22 for a summary.)
It’s also clear that the turtles don’t rely on following a single, fixed compass course. This makes very good sense as such a crude mechanism would make them vulnerable to the disturbing effects of ocean currents that deflected them from their proper course..
The researchers found no evidence that, in the final stages of their journeys, turtles were making use of olfactory information - either smells in the air or tastes in the water - carried to them from their target. This is quite surprising, especially as hints of such an ability have emerged from earlier research.
This new study illustrates an important principle. Evolution doesn’t favour the emergence of perfect systems of navigation (or anything else), when merely adequate ones will enable animals to survive and reproduce successfully.
Good enough is good enough!