How does a lost animal find its way home?


Scientists don’t know whether and how strongly mammals can sense magnetic north, but they have other orientation strategies. (iStockphoto)
February 18, 2013

A couple from West Palm Beach, Fla., lost their cat in Daytona Beach in November, only to have the animal show up two months later less than a mile from their house. The cat had apparently walked 200 miles. The amazing accomplishment isn’t unprecedented. An Australian cat reportedly traveled 1,000 miles over the course of a year to get home. Bobbie the Wonder Dog was a minor celebrity in the 1920s after walking home to Oregon from Indiana. A labrador named Buck traveled 500 miles from Winchester, Va., to Myrtle Beach, S.C., last year. How do animals find their way home from such great distances?

Let’s start with the basics. The simplest orientation strategy for animals, known as taxon navigation, barely seems like navigation. Taxon navigation is the ability to directly sense the place you want to be.

“Rats and dogs follow scent trails,” says Matthijs van der Meer, who studies the neuroscience of spatial decision-making at the University of Waterloo in Canada. “Worms follow chemical trails, moving in the direction that increases the concentration of the chemicals that attract them.”

Birds, perhaps the most famed animal navigators, can detect the magnetic field of the Earth. So, when a goose is migrating north for the summer, one of its strategies is to sense the planet’s magnetic north and go toward it. There are indications that other animals have this magnetic sense, but in most of them, it occurs in weaker form.

The task is much more complicated when an animal can’t directly sense its goal. Now, it needs a new set of capabilities. The most fundamental is memory: It has to remember something about the place it wants to be. Cats, dogs, even birds all have this sort of ability. Navigating without memories is like trying to find a restaurant on a map without knowing the address.

Another fundamental skill is the ability to relate one’s location to other places, what we humans refer to as a sense of direction.

“A pigeon can detect north using its magnetic sense,” van der Meer says, “but that doesn’t help if it needs to go east. It has to add the additional knowledge of knowing the relationship between north and east.”

Scientists don’t know whether and how strongly mammals can sense magnetic north, but they have other orientation strategies. Mammals have tiny hairs inside their ears that are pushed to one side or the other as they turn, telling the brain that the body is rotating. This so-called vestibular system is handy, but it’s rudimentary compared with birds’ internal compass. The hairs can’t detect slow turns, while spinning too quickly is disorienting (remember “Pin the Tail on the Donkey”?).

Visualizing location is another orientation strategy. One version of it that humans use is landmarking. When someone asks for directions, you might say, “Go right at the church” or “Turn left when you see the coffee shop.” Animals also rely on landmarks, it turns out. According to van der Meer’s tests, if a laboratory rat is spun to the point of disorientation, it will use visual cues, such as a human-made exit sign or a light bulb, as a pole star to find its way home.

There is also neurological evidence that other mammals share the human ability to construct a mental map. As a rat explores a maze, special mapmaking neurons called head cells spring into action. When a rat moves around, different cells turn off and on, possibly enabling it to identify its location in the maze based on the brain cells that are active. There is a separate set of cells, called grid cells, that identify a rat’s general environment — for example, whether it’s in Maze A or Maze B. (Think of head cells as city maps and grid cells as national highway maps: You need both for effective navigation.)

Enough neuroscience. Let’s get back to that cat. How does a feline get from Daytona to West Palm Beach? The ocean was probably a key. West Palm Beach is 200 miles straight down the coast from Daytona. Once the cat found the ocean, it had to make only one choice: Left or right?

“It’s possible cats have a weak magnetic sense or that it remembered which side the ocean was on during the northward journey,” van der Meer says. “But it may have been a lucky guess.”

That’s a bit harsh. Remember the number of capabilities this cat might have used: memory (home is near the ocean), landmarks (the ocean itself), taxon navigation (sensing the ocean) and possibly mental maps to avoid repeating mistakes.

As Louis Pasteur said, “Chance favors only the prepared mind.”

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