This spectacular - and very beautiful series - is now available on Disney + and if you’re interested in animal journeys you’ll really want to watch it!
I was asked to help the production team at Plimsoll Productions and had fun trying to answer their questions about the navigational tools employed by a pretty diverse set of animals.
It was very nice to be given an on-screen credit as a ‘series consultant’!
When I last wrote on this subject (see blog #4) I focussed on the possible role of geomagnetism in helping humpback whales to navigate across oceans with remarkable precision.
Recently I came across an article (dating from 2020) that raises another possibility.
Perhaps migratory fish - which are known to be sensitive to infrasound (very low frequency sound below the threshold of human hearing) - make use of it to orient themselves?
The idea is that turbulence caused by ocean currents or seismic activity in the ocean floor (eg along mid-ocean ridges or around submarine volcanoes) gives rise to characteristic infrasound signals that are capable of travelling over great distances. Breaking waves along a coast are another infrasound source.
Any marine animal that can detect such sounds could in principle extract useful navigational information from them.
We know that many animals other than fish (including eg birds and elephants) can detect infrasound, so the possibility that whales may do so too seems plausible.
To the best of my knowledge, this idea remains speculative (it’s hard to experiment on whales!) but it’s not a big leap to imagine that whales (as well as migratory fish) may navigate with the help of infrasound.
Of course that wouldn’t conflict with the possibility that they may use geomagnetism. The two systems would complement each other well.
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!
Common seal
Part of the charm of a seal is its whiskery face, but those whiskers are not just decorative. A fascinating new article reveals that they are astonishingly sensitive and can be used to track prey over long distances.
It’s long been known that seals (Phocidae) use their whiskers to detect things underwater - a useful skill, especially when visibility is reduced to zero. But until recently nobody knew just how sensitive they were. It now appears that they can pick up the turbulence left in the wake of a herring swimming 180 meters away!
A seal’s whiskers are obviously exquisitely sensitive - and are therefore well supplied with sensory nerves. But that’s not the whole story. The shape of their whiskers also seems to be significant. Rather than being simple, tubular structures like a human hair, they have curious undulating contours along their length. These increase their ability to detect the minute vortices that persist for a long time after a fish has swum by.
Scientists are interested in harnessing these discoveries to develop ‘autonomous underwater vehicles’ (i.e. robotic submarines) that can mimic the seal’s amazing tracking skills. No doubt the military are paying attention too.
Off the coast of Portugal a week or two ago…
I’ve been away for a while, on the high seas, doing some real navigation in the Mediterranean and Atlantic - and tasting a bit of freedom after all those months of lock-down. Good to see some wildlife too: lots of Cory’s shearwaters, dolphins, green turtles (a few), a humpback whale breaching, swordfish leaping…even an errant hoopoe heading south towards the distant shore of Africa. I hope it made it!
Now I’m home again, I’ve been catching up on the latest research. Here are a few highlights that caught my eye.
First, there was this paper about magnetoreception from Jingjing Xu et al. This strengthens the case for cryptochrome molecules in the retina of night migratory birds being the basis of their magnetic compass sense. This theory - which I also discuss in Supernavigators/Incredible Journeys - is the subject of enormous research interest at present. Eric Warrant has provided a really good commentary that explores the significance of this piece of research. As he explains, Xu et al. have made a major advance by showing that ‘the version of CRY4… in the migratory European robin has a crucial property needed to sense Earth’s magnetic field: the ability to form radical pairs that have high magnetic sensitivity’. Moreover, they have demonstrated that CRY4 is more sensitive in the robin than it is in other, non-migratory birds.
Then there was this fascinating article by Eliav et al. about how bats navigate. Up until now, most of what we know about how the brains of mammals support their ability to navigate has come from laboratory-based studies of rats and mice (and humans). Nachum Ulanovsky and his colleagues in Tel Aviv have however developed wireless techniques for recording the activity of single cells in the brains of bats on the wing! This latest paper shows that single place cells (which ‘represent’ the positiion of an animal in space) in bats flying through a long tunnel can operate on a range of scales - from about 1m to 30m. This multiscale coding provides a really efficient way of representing large environments of the kind that bats inhabit. It will be very interesting to see what emerges when this kind of research is extended to other animals that navigate over long distances - like birds.
Finally I want to mention a piece of research that touches on something that really bothers me: the scourge of light pollution. In this paper, James Foster et al. have shown how light pollution can seriously interfere with the navigation of nocturnal dung beetles - which rely on polarised moonlight and the orientation of the Milky Way to maintain a straight course as they roll their balls of dung. They found that ‘that light pollution…induces dramatic changes in dung beetle orientation behavior, forcing them to rely on bright earthbound beacons in place of their celestial compass’. The beetles are attracted toward artificial lights, and this results in more competition between individuals and reduced dispersal efficiency. As the authors rightly observe, ‘for the many other species of insect, bird, and mammal that rely on the night sky for orientation and migration,
these effects could dramatically hinder their vital night-time journeys’.
This important article by Van Doren et al. also relates to light pollution. Based on historical records at a site in Chicago, it suggests that bird deaths caused by collisions with lighted buildings could be substantially cut by reducing lighting to more sensible levels.
Here also is a link to good BBC article about the problem of light pollution.
That’s all for now.
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.
Sandhopper (Talitrus saltator) - photo courtesy Arnold Paul / edited by Waugsberg and Buchling / CC BY-SA 2.0 DE (https://creativecommons.org/licenses/by-sa/2.0/de/deed.en)
We’re only just starting to realise that our love affair with artificial light at night (ALAN) is bad not just for us, but also for many of the creatures with which we share this planet. Including, for example, the sandhopper.
But before I talk about the problems facing this remarkable crustacean, let me sketch in a bit of background about light pollution.
Paris was the first European city to introduce street lighting. That was in 1667, during the reign of Louis XIV - The Sun King. Very soon other major cities across Europe followed suit. (I’m not sure about Asia - I suspect that Edo was pretty well lit too and maybe even earlier than Paris. Does anyone know?)
Streetlights in those days were pretty dim - just candles or oil lamps. While they made nocturnal city life easier and perhaps a bit safer, they didn’t have much effect on the visibility of the night sky. In the countryside, the nights remained as dark as they had always been.
Things changed with the arrival of gas and electric lights in the 19th century. Cities could now be brilliantly illuminated. More light seemed to be an unqualified good, and few resisted its encroachments. Today of course we have many other kinds of lighting - like LEDs and fluorescent tubes.
ALAN is spreading rapidly across the face of the earth. Everywhere you go, there are brilliantly lit office buildings, factories, airports, docks, military installations, car parks, apartment blocks and even private houses.
But ALAN is not restricted to built-up areas - it extends outwards along the roads that connect the cities and towns where we live. You can’t even escape it completely on the open ocean, where oil rigs, ships and fishing boats are often lit up like Christmas trees.
The problem is not just the direct glare of the lights themselves.
There is also the phenomenon of ‘skyglow’ caused by the scattering of light from clouds and dust in the atmosphere.
One of the sad consequences of ALAN is that most people on earth can no longer enjoy the wonder of a dark night sky with its thousands of stars - and they will never see the Milky Way.
Big cities generate so much light that the skyglow above them can sometimes be detected a hundred miles away or more. It is the most widespread form of light pollution and apparently now affects 23% of the earth’s surface between the latitudes of 75 degrees North and 60 degrees South. That includes many of the most important areas of biodiversity around the world. And it continues to spread rapidly.
Of all the many serious environmental problems we face, ALAN is actually the easiest to solve.
Better designed lights, properly shielded, pointing in the right direction, and only as bright as they need to be, would make a huge difference. Steps like these would not be expensive to implement. They would also save energy and reduce costs in the long run. (For more information about light pollution and how to combat it, visit the excellent website of the International Dark-Sky Association.)
In recent years scientists have begun to pay attention to the effects of ALAN, both on us humans and on other animals, as well as plants.
ALAN certainly has a disruptive effect on the internal clocks that govern many natural processes - including our own sleeping patterns.
It is a well-known menace to turtle hatchlings, as well as migratory birds. It may also be implicated in the decline of many insect species.
And now it looks as if ALAN is also making life difficult for the poor little sandhopper.
These animals live on sandy beaches and, as I explained in Incredible Journeys (Chapter 10), they are remarkable navigators.
In the evening they make their way to water’s edge to chomp on seaweed. In the morning they retreat up the beach and bury themselves in the damp sand to avoid getting dried out by the sun. Since they can easily get drowned, it’s important that they don’t go the wrong way.
Amazingly, sandhoppers have not one but two celestial compass systems that help them steer in the right direction - whether up or down the beach. One is based on the light of the moon and the other on the sun.
A new study (unfortunately locked behind a paywall) from D. Torres et al. looks at how skyglow affects the navigational ability of sandhoppers on the coast of the island of Anglesea, Wales.
They report that skyglow - though much weaker than direct light pollution - nevertheless disrupts the sandhoppers’ lunar compass system, making it harder for them to navigate safely at night.
Since many other animals use the stars and the moon to help them orient at night, it is a sure bet that ALAN is not just causing trouble for sandhoppers.
Torres et al. conclude that ‘a renewed focus on the ecological impacts of artificial skyglow is urgently needed’ in order to understand how it is affecting the world around us.
Yes indeed.
And in the meantime, please do think about how you can reduce your own light footprint!
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!
Dogs may be interesting, but whales are really fascinating!
In Incredible Journeys I wrote about the mysteries that surround the extraordinary navigational feats of humpback whales.
And I mentioned the possibility that whales might be making use of gravitational information.
How could that possibly work?
Well, roughly speaking, the idea is that these huge beasts may be able to track changes in their buoyancy resulting from alterations in the gravitational forces to which they are subject.
These changes may result from anomalies in the bedrock beneath the ocean, changes in latitude, or astronomical influences - like tidal forces and the varying distance between the earth and moon.
An ability to detect gravitational patterns in the submarine landscape would obviously be navigationally useful. And if the whales are also aware of changes in the earth-moon gravitational system, those could perhaps help them time the start of their migratory journeys.
Now some new research from Travis Horton and his colleagues has shed more light on these intriguing ideas.
Horton and his team have been sifting evidence about the migratory behaviour of humpbacks travelling between their calving grounds on the Abrolhos Bank (in the tropical seas off the coast of Brazil) and their feeding grounds far to the south, in the chilly waters of the South Georgia Basin.
The data come from old Soviet-era records of whale-hunting activities (bizarrely hidden for many years in a potato cellar!), and more recent satellite-based tracking studies.
These records, which stretch back 50 years or more, show that the whales have continued faithfully to follow the same migratory ‘corridor’ - despite the fact that the relevant geomagnetic and oceanographic conditions (eg currents and food availability) have varied greatly over that period.
As Horton et al. put it, ‘humpback whale migrations do not change in a changing ocean’.
After carefully analysing the tracking data, the team conclude that the whales follow ‘richly patterned and highly reproducible trajectories’. Moreover, these trajectories do indeed appear to reflect the varying gravitational forces to which the animals are subject.
They also found that the tracked whales fell roughly into two groups: those that tended to travel relatively fast and those that travelled at a more sedate pace.
Interestingly, eight of the nine ‘slow’ whales started their southbound migrations just before or during the first or last quarter of the moon. By contrast, eleven ‘fast’ whales head south near the full or new moon. Curiously, the whales that ‘sang’ swam more slowly than the non-singing ones. I wonder why?
While lending weight to the theory that the whales make use of gravitational information, this research certainly doesn’t rule out the possibility that the whales also make use of geomagnetic information - as several earlier studies have proposed. It does however suggest that they don’t rely exclusively on magnetic cues. And this would certainly make good sense if, as in this case, those cues are constantly changing.
Mantis shrimp
New research reveals that mantis shrimps make use of a sun compass to find their way home. It seems to be based either on the sun's azimuth or on polarisation patterns in the sky (‘e-vectors’) - both of which are often visible beneath the surface, at least in shallow water.
When clouds conceal the sky, however, they fall back on DR or path integration.
They’re the first marine animals in which these abilities have been demonstrated, though many insects can do similar things.
Congratulations to the authors, Rickesh N. Patel and Thomas W. Cronin, of the University of Maryland.