Think about the article. Why are many middens located at the edge of a group's territory?

They send a friendly invitation to potential mates that live in the territory.
They send a message to other large social groups that food can be found nearby.
They send a stern warning to other animals to stay away from the territory.
They send a message that predators in other large social groups cannot detect.

which one

The most likely answer is: They send a message to other large social groups that food can be found nearby.