【托福考满分ETS听力官方真题】Animal Navigation System

Listen to part of a lecture in an animal behavior class.

Before we leave the topic of animal navigation, let's look at one more species and how it finds its way through its environment.The species we'll look at is the blind mole rat.

Blind mole rats are interesting in terms of navigation because they live entirely in the dark, in elaborate underground tunnel systems.So how do blind mole rats find their way around in these complicated tunnels?

Well, for some time we've known that blind mole rats use some combination of two different navigation systems.Um, one system relies on their sense of time and their ability to remember underground landmarks.For example, let's say a mole rat wants to find its way through its tunnel system back to its nest, where it sleeps.Well, it goes along then feels some hard stones or a tree root under its feet, and it basically says to itself, "OK, here's where I took a left"And then it might remember how long it took to get to the next turn.And so forth.The mole rat goes straight or turns based on what it remembers having sensed along the way on previous trips and the time it took between turns.

Now, the other navigation system for the mole rat relies on Earth's magnetic field.Mole rats have the ability to sense the magnetic field and use it to orient themselves directionally.But until recently, scientists weren't sure about the role of these two different systems.

Recently, a team of researchers conducted an experiment to answer that question... do these two navigation systems play different roles?What they did was they designed a special structure that very closely resembles a blind mole rat's tunnel system... which looks like a bicycle wheel... a central hub with multiple spokes radiating outward.Now, this turned out to be quite important.Earlier experiments had used a more generic habitat- basically a wide-open circle- which was originally designed to test magnetic navigation in other animals.That design led to inconclusive results with mole rats.

Now, with their bicycle-wheel tunnel system, the researchers were able to vary the distance that the mole rats traveled between their "nest" and their food source... by creating long routes and short routes.In each trial, the mole rats started out in their nest, went to the food source, and then had to find their way back home, back to the nest.

To determine which navigation system they were using, the researchers tested the animals under two different conditions.First the mole rats were tested under conditions of normal magnetism... for both the trip to the food and the trip home.And under these normal conditions, they all followed their original route back home, regardless of the length of the trip to the food source.Then, in the second part of the experiment, the magnetic field was altered... but only for the trip home.A specially constructed set of magnets was used to shift the magnetic field around the habitat 90 degrees eastward.The researchers wanted to see how the mole rats responded to this shift.

Well...it turns out that the magnetic shift had no impact on the return route of the blind mole rats after a short trip.They returned to the point where they had started from.But after a longer trip, they took a totally different route... one that led them 90 degrees east of the nest.Essentially, they got lost.This was strong evidence that blind mole rats use magnetic navigation only for longer trips.

So, why two navigation systems- one for long trips and one for short trips?Well, for now we can only speculate, but we think that when mole rats rely on the first system- using underground landmarks or their sense of time- well they make small mistakes here and there.On a short trip, this doesn't matter much... the trip is short, so they can't make many mistakes.But on a long trip, well, there are lots of opportunities to make small mistakes and small mistakes can add up, leaving the mole rat far from where it wants to be.So on a long trip, a magnetic navigation system is better, more useful, since Earth's magnetic field is stable, constant, a more reliable indicator of direction.