Vol 7 Test 8 Part 4

Section 4
(21:08 - 23:05)
Now turn to section 4 on page 8. Section 4. You'll hear part of a student presentation about a type of frog called the African Clawed Frog. First, you have some time to look at questions 31 to 40 on page 7. Now listen carefully and answer questions 31 to 40. The aim of our study was to look at how naturally occurring substances can be used to treat infections.

And we carried out a laboratory study using a frog called the African Clawed Frog, which has the scientific name Xenopus laevis. So, before I describe our study in detail, let me give you a general description about this frog. Like most other frogs, the African Clawed Frogs have a smooth skin.

(23:06 - 24:13)
This is one way you can distinguish them from toads, whose skin is rough. And they can change the color of their skin to blend in with their surroundings. The surface of their skin also has another feature.

It's covered with a layer of mucus. This makes it hard for other creatures to pick them up, and so it provides protection against predators. How big are they? Well, it depends on whether they're male or female, because the females, in this case, are a whole lot bigger than the males.

A large-sized female may be 8 inches long, while the males are about half that size. So they're pretty small in comparison. What about their natural habitat? As their name suggests, these frogs are native to Africa, and they're found in most parts of Africa, south of the Sahara Desert.

(24:13 - 26:14)
They make their houses in fresh water, but they're rarely found in running streams. They like still water. As far as the temperature of the water goes, they're not too fussy.

It can be warm or cool. They don't need to be in water where there are plants growing, and they can live in water that has quite a lot of salt in it. So they can live in brackish areas, such as marshes and estuaries.

However, although they can live in a wide range of conditions, one thing that they are unable to tolerate is even very small traces of metal. That's immediately fatal to them. And that's important to remember when they're being kept in captivity.

You've got to be careful what kind of tank you use and what you put into the water. In their natural habitat, the frogs normally live in water the whole time. But in unfavorable conditions, such as drought, they can burrow into the mud and survive there for up to a year until conditions improve.

As far as their diet is concerned, they're carnivorous. They eat other creatures they find in the water. And they're scavengers, which means they'll eat almost anything, living or dead.

They've got huge appetites. Because the water they live in is often quite dirty and visibility is limited, they can't locate their food using their sense of sight. Instead, they use smell and also the sensitive fingers on their front legs.

(26:15 - 26:42)
And they also have what's called a lateral line system on the sides of their bodies, which is sensitive to vibrations in the water. They use their front legs to get the food into their mouths, together with a type of pump arrangement that sucks the food into the mouth and down into their stomach. They don't have any teeth, so everything just goes straight down.

(26:44 - 27:32)
Okay, well, although Xenopus started off in Africa, they're now quite widespread. One of the reasons for this is that a lot of people in the U.S. like keeping them at home as a pet. They're quite easy to look after and they can live for up to 20 years.

But in some states of the U.S., such as California and Virginia, they have escaped from captivity and are reproducing in the wild and threatening native species there. So, they've been classified as a pest in these areas. But another very important use for the frogs is for research in laboratories.

(27:32 - 28:04)
Because the females lay large numbers of eggs and the embryos are transparent, it's easy to see how they are developing, and so they're used for research into genetics and development. But also, the slippery mucus that covers their skin acts as an antibiotic, killing parasites and bacteria. And this is why we chose this frog for our research.

(28:04 - 28:58)
So, let me describe the aim of our study. That is the end of Section 4. You now have half a minute to check your answers. That is the end of the listening test.

You now have 10 minutes to transfer your answers to the listening answer sheet.