If you have a function that requires both the species and a speak message from an animal, it would make sense for the interface to have a function that returns both the species and speak strings. I think that this [0] does what you would want.

If you want to guarantee that two structs contain a set of the same required fields, I believe the best practice would be to include an anonymous field for a struct with the fields across both. [1]

[0] http://play.golang.org/p/0zMqPUicgq [1] http://play.golang.org/p/cM3XPfitxH