You will now listen to part of a lecture. You will then be asked a question abou

Today we’re going to talk about how animals defend their food once they find it. I mean, often they can’t eat it all at once. One common tactic adopted by animals involves the use of a barrier. There are two barriers animals use to protect their food. I’d like to use ants as an example.
The first strategy is known as a chemical barrier, which means secreting certain kinds of chemical substances to keep other animals away from the food. For example, ants create a chemical barrier around the food they’ve found. The food, after all, might be too big for them to carry back to the hill without help. So, before the ants head back home to get help, they secrete a chemical onto the food. This chemical is highly unpleasant to most other insects and small animals, so the food will not be taken while the ants are gone.
The other type of barrier that ants use is a little more obvious. They make an ordinary physical barrier to block the food. For instance, ants may pile small stones over the food, creating an obstacle that most other insects cannot overpass. Meanwhile, the ants have gone back to their home to enlist more help.