Concept implies abstraction and the mark of abstract thought is language.

Karl Popper's four functions of language might help here. Language has an expressive function, a signaling function, a descriptive function and an argumentative function. No other animal besides Man shows any sign of the anything but the first two, but it's the second two that require abstract concepts. These animals may very well possess imagination, but imagination is essentially concrete. You don't imagine Triangularity, but rather specific triangles (though in human beings, by virtue of having an intellect, the image of a triangle is "accompanied by" the concept of Triangularity).