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.
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.
A recent article in The Atlantic discusses the fascinating possibility that whales make use of the Earth’s magnetic field to help them navigate the oceans.
As I explain in Incredible Journeys, many great whales (including notably the humpbacks that migrate annually between the Antarctic and equatorial waters) regularly travel for thousands of miles across the apparently featureless open ocean following remarkably straight courses. Since they seem to be able to do this even when the sky is obscured, it seems unlikely that they rely on celestial cues (like the sun or stars) to perform these feats - though it’s conceivable that they can do that too in fine weather. The omnipresent geomagnetic field (see my earlier post ‘The great magnetic mystery’) would however be available to them at all times and places - and all depths.
The theory that whales may be magnetic navigators (like many other animals including some birds and insects) has been around for a long time. But Jesse Granger and her colleagues at Duke University have recently published some interesting work based on a review of data gathered over a 31-year period. They wanted to find out whether solar storms that disrupt the Earth’s magnetic field are correlated in any way with strandings of gray whales.
They looked at 186 strandings involving apparently healthy whales (injured or sick animals might get stranded for other reasons) and parallel data relating to levels of solar activity. They found that the two were indeed closely correlated.
So what’s going on?
As Granger et al. explain, ‘Solar storms could have two impacts on magnetic orientation. They could alter the geomagnetic field, leading to false information, or disrupt the animal’s receptor itself, leading to an inability to orient.’ But the key factor influencing the strandings appeared to be the increase in radio frequency ‘noise’ associated with the solar storms rather than any displacement of the Earth’s magnetic field.
Granger et al. conclude: ‘These results are consistent with the hypothesis of magnetoreception in this species, and tentatively suggest that the mechanism for the relationship between solar activity and live strandings is a disruption of the magnetoreception sense, rather than distortion of the geomagnetic field itself.‘
There is evidence from studies of migratory robins that RF noise (from AM radio transmitters) can disrupt the magnetic compass sense of these birds. It’s possible that the hypothetical cryptochrome magnetoreception mechanism may be disrupted by such transmissions. So, as Granger et al. acknowledge, it’s conceivable that whales may also rely on a cryptochrome-based magnetoreceptor. But it’s far too soon to start laying bets!
One thing that puzzles me is that gray whales - which tend to follow the coast on their migratory journeys rather than crossing the open ocean - should be so heavily reliant on magnetic information that they would get stranded if it was disrupted. Could they not simply use landmarks or the contours of the sea floor to help them find their way? And even if they did rely on their magnetic sense of direction, you’d think that they would notice when the water began to shoal and turn back before they went aground. It’s odd.
The humpbacks by contrast are prodigious oceanic navigators and it makes sense that they would rely on geomagnetic cues when out of sight of land. Interestingly their migratory journeys often seem to feature visits to underwater features called seamounts. Could it be that these act as waymarks? Maybe they even have unusual magnetic properties that the whales can detect?
Apparently Granger is now looking at 12 years-worth of data relating to humpback whale voyages. She wants to see whether they have more difficulty maintaining straight courses when solar storms hit the Earth.
That should be interesting!