Until the advent of GPS, sailors relied heavily on a navigational technique called ‘dead reckoning’ (DR) to keep track of their position when out of sight of land. (NB: Scientists prefer the term ‘path integration’ but it’s essentially the same thing.)

Though the origins of this ancient term are mysterious, DR is simple - at least in principle. You record your ‘course’ (the direction in which you’re headed according to a magnetic compass) together with the distance you have travelled (estimated with the help of a ‘log’ and a clock - or even just an hour glass). You can then - in theory at least - reconstruct the course you have followed and work out where you are in relation to your point of departure. And if you have a chart you can plot your position on it.

Unfortunately DR is in practice quite unreliable. Not only is it hard to steer an arrow-straight course, but the open ocean is very far from being a stationary mass of water. There are currents, which are sometimes strong and often unpredictable. And it’s difficult to detect them unless you have an independent means of fixing your position. Old-fashioned logs were also dodgy. They consisted of small triangles of wood attached to a long rope marked at regular intervals with ‘knots’. The ‘log’ was dropped over the side and the rope allowed to unspool for a fixed period of time. The number of ‘knots’ that ran out indicated your speed through the water. But this technique was not very accurate, especially in heavy weather. As a result, the longer you were at sea, the more uncertain you were about your exact position.

But over short distances DR can work quite well. And lots of animals make successful use of it.

The best-studied example is the desert ant of the Sahara (Cataglyphis fortis).

The great German scientist, Rüdiger Wehner, has spent the best part of 50 years exploring the astonishing navigational tools used by Cataglyphis. At the heart of these is a DR system based on the ant’s ability to detect patterns of polarised sunlight in the sky. These patterns (invisible to us) enable the ant to keep track of the course it is following as it canters over the baking, featureless salt flats in search of its prey - usually dead insects. In effect it has a built-in celestial compass.

But a compass alone is not enough for DR. You also need some way of measuring how far you have gone. In other words, an odometer. And, as I explain in Incredible Journeys, Cataglyphis has an odometer based (in part at least) on its ability to keep track of how many steps it has taken!

The desert ant’s impressive DR system allows it to head straight for its nest even after wandering erratically over hundreds of meters. And bear in mind that its nest entrance is no more than a tiny hole in the ground. It is quite invisible to the ant until it gets very close to it. Quite a feat!

Lots of other insects - including honey bees - also use a celestial compass based on polarised sunlight. Some nocturnal ones even make use of polarised moonlight or the orientation of the Milky Way! Obviously flying insects don’t count their steps. Their odometers work differently. I’ll come back to this intriguing subject on another occasion.

Let me finish this post by mentioning another, very different animal that makes use of DR: the fiddler crab (Uca rapax).

These animals are a common sight on the coasts of Central America and the Caribbean where they live underground and usually only go out to find food. The crabs need to get back to the shelter of their burrows quickly when they perceive a threat and they do this very well.

Fiddler crabs are really remarkable. As they wander around, and no matter how complicated the route they follow, they always point their bodies (sideways) towards their burrows. And, by playing various tricks on them, scientists have shown that - unlike desert ants - they don’t make use of visual information to maintain this homeward orientation.

First the researchers placed crabs on discs and spun them around. The crabs didn’t much like that and tried hard to counteract the rotation. But they didn’t always succeed and sometimes, for all their efforts, they ended up pointing in the wrong direction. The extent of their homing error depended exactly on how far they had failed to compensate for the rotation.

Now if the crabs’ homing ability was based on some external cue - like polarised light or the position of landmarks - they would have been able to correct their orientation after being rotated.

The same researchers next tried putting a patch of slippery plastic in the path of the crabs as they headed for home.

Some crabs had difficulty getting traction on the plastic and scrabbled to get across it. This meant that they took a number of steps without making much progress. These ones ended up stopping short of their burrows - suggesting that they had overestimated how far they had travelled. By contrast, those that crossed the obstacle without losing their grip, stopped in just the right place - as did controls that didn’t have to cross the plastic sheet at all.

So what does all this mean?

On the basis of these findings it looked very much as if fiddler crabs performed DR by paying attention primarily to the movements of their own bodies - specifically how many steps they had taken and the angles through which they had turned. That’s a pretty sophisticated system and one that we humans certainly can’t match - at least not without the help of technology.

More recent research has confirmed that the fiddler crab’s DR skill does indeed depend on their ability to count their steps.

More soon…