Poohsticks , the game in which Piglet and Winnie the Pooh throw sticks into the river from one side of a bridge, and then rush over to the other side to see whose stick appears first, is all about current flow. Disappointingly, neither Piglet nor Pooh mention fluid dynamics despite its pivotal importance in determining who won.
Authors
- Rory Wilson
Professor of Aquatic Biology and Sustainable Aquaculture, Swansea University
- Richard Michael Gunner
Postdoctoral Researcher in Wildlife Biology, Max Planck Institute of Animal Behavior
Unlike sticks, though, animals can respond to those flows. The movement of water and air - with their winds and currents - can affect flying and swimming animals profoundly. And as we recently discovered , penguins are far more tuned in to these dynamics than anyone realised.
Anyone who's ever swum in the sea will know how cross-currents can drag you along the coast, even when you're trying to swim straight in. Magellanic penguins , a South American penguin, face this challenge daily, but they appear to have found a clever solution.
Penguins can swim far from land but seem to know exactly where they are. More importantly, they seem to know how to get back to their breeding colonies, whether currents are confounding them or not.
To understand how they do this, our team - which included researchers from Argentina, Germany, Japan and the UK - fitted high-tech tracking tags to Magellanic penguins breeding in Argentina. These birds often forage up to 43 miles offshore, far beyond the range of visual landmarks. And it's unlikely they're using the seafloor as a map, as Magellanics rarely dive that deep.
The tech we placed on the penguins recorded some pivotal information. Global positioning systems (GPS) gave the birds' positions when they were at the surface between dives. And trajectories underwater could be calculated using dead reckoning . This is what a car navigation system does when it goes into a tunnel - it starts with the last GPS position and uses vectors on the car heading and speed to work out the path.
Our team did this with the penguins' data, calculating the underwater pathways for every second of their one to three day trips. We then integrated this with the currents. This was no simple undertaking because currents change dramatically over the tidal cycle and vary with position.
So what could the penguins do in such a dynamic environment? One option (assuming they somehow knew both where they were and where home was) would be to head straight for the colony. But doing this would often have meant swimming against strong currents, sometimes of up to 2 metres per second (around 4.5mph). That's about the same speed as an Olympic swimmer.
Although penguins can cruise at that speed, going faster to beat the current would cost them a lot of extra energy.
Interestingly, we found that during slack water, when the currents were trivial, the penguins headed directly home. So, somehow they knew where they were in relation to the colony. Theories about how animals might do this include them using magnetic field sensing, celestial cues, or even using smell to find their way but it's a mysterious and hotly debated topic among experts.
When the current was strong, the penguins generally aimed in the right direction to return home. But they often combined this with swimming in the same general direction as the current, which typically flowed across the direct line to the nest. So, some birds appeared set to overshoot the colony, probably landing further down the coast.
However, the yin and yang of tidal currents means that what flows one way on the rising tide reverses on the ebb. The penguins seemed to understand this. They swam roughly equivalent, but mirror-imaged, trajectories on both incoming and outgoing tides, according to the direction of the current.
This strategy effectively cancels out potential overshoots over the course of a tidal cycle. Once they were close enough to the colony, the penguins launched into a final burst of power and made a direct line for home. This strategy increases the length of the path to get home. But it's easy travelling since much of the work to move is done by the current and the increased distance gives the penguins opportunities to find prey.
Navigational experts
This suggests that Magellanic penguins can detect both the direction and speed of ocean currents. While some theories propose that animals sense small-scale turbulence to gauge flow, the mechanisms remain poorly understood.
Still, what these penguins manage is remarkable. It's a kind of navigational party trick that helps ensure they return reliably to feed their chicks, seemingly untroubled by shifting currents.
Ocean and air circulation patterns are becoming more chaotic with climate change. If penguins, and other marine animals, can keep navigating our waters with skill and instinct, it's one small piece of good news in a rapidly changing world.
The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
