Interface in a dynamic language?

Question

Interface (or an abstract class with all the methods abstract) is a powerful weapon in a static-typed language such as C#, JAVA. It allows different derived types to be used in a uniformed way. Design patterns encourage us to use interface as much as possible.

However, in a dynamic-typed language, all objects are not checked for their type at compile time. They don't have to implement an interface to be used in a specific way. You just need to make sure that they have some methods (attributes) defined. This makes interface not necessary, or at least not as useful as it is in a static language.

Does a typical dynamic language (e.g. ruby) have interface? If it does, then what are the benefits of having it? If it doesn't, then are we losing many of the beautiful design patterns that require an interface?

Thanks.