Research just published shows that migratory Painted Lady butterflies (Vanessa cardui) found on the coast of South America had crossed the Atlantic from West Africa - and might even have started their journeys in Europe. That means they may have travelled - in total - 7000 km or more! Astonishing, even by comparison with the mighty Monarch butterfly, though the Painted Ladies would have been helped on their way by the Trade Winds.
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.
