Can monkeys who have learned sign language speak to each other?

The answer is not really. While some monkeys have learned to use sign language to communicate with humans, they don't use it to communicate with each other naturally. Here's why:

* Sign language is a human invention: Sign language is a complex system developed by humans for communication. It requires a level of understanding and cognitive ability that likely differs from that of monkeys.

* Monkeys have their own communication systems: Monkeys communicate through a variety of vocalizations, facial expressions, body language, and scent marking. These systems are complex and nuanced within their own species, but they don't rely on the same structures as human language.

* Limited evidence of spontaneous sign usage: While some monkeys who learned sign language have shown limited signs of using it with each other, this is rare and doesn't necessarily indicate a full understanding of the language.

The bottom line: Monkeys can learn sign language, but they don't use it to communicate with each other in the same way that humans do.