Invisible Giraffes

In 2001, a rare chemical was sprayed throughout the trees of the San Diego Zoo. Because of this, all the giraffes who ate from those trees gained the ability to become invisible. Some giraffes used this ability to their advantage by escaping from the zoo. Why was it that only the giraffes could become invisible instead of all the animals that ate from the trees? We're still not sure, but thanks to otherwise unexplainable crushed cars, we know that there are still some invisible giraffes in the San Diego area.