TOEFL Speaking Task 3 Practice 001Science

Narrator:Now listen to part of a lecture in a biology class.

Professor:Ok. So for example, there are these two different types of ants in Africa that share space in this way. Living inside a hollow tree branch. And one of these types of ants is very small, and the other type is much bigger, so they're different. And what's interesting is that the bigger ants collect food and bring it back to the hollow tree branch and eat it there. But some of the pieces of food end up scattered around small bits of waste that the bigger ants don't eat. And the tiny ants go around and eat these leftover scraps of food. So the tiny ants get their food this way from the scraps left behind by the bigger ants, and they don't have to go out to collect their own food. Scientists think that this arrangement works out nicely for the bigger ants too. See when the small ants eat up all the little crumbs and scraps left behind, they're actually helping to keep the home inside the tree branch clean. And keeping the inside clean like this prevents harmful bacteria from growing. Bacteria which could cause sickness and threaten the survival of the bigger ants.