Is it possible for a ferret to be retarded?

I think my male ferret is mentally stupid. Compared to his sister, he doesn't notice things like she does, his sense of smell doesn't appear as sharp, he's not clever at getting out of boxes or climbing over things. He's strong, a brute, but that's it. His sister, on the other hand, I can see her thinking through problems and situations - when I hold up the can of their favorite treat she runs to her food dish where she knows I'm going to place the treat. Her idiot brother does nothing until I physically put him in front of the treat where he gobbles it right up. Is he stupid? Or is this typical male ferret behavior?