Who knows whether they can or not? Not me. lol
But in animation, the fun is in imbuing the characters with certain human traits and imperfections, especially anthropomorphised animals. Nestor's underbite turned out to perform the dual function of adding another touch of humor, and also to help make this character stand out in a very subtle way from a crowd of penguins. Usually in animation, distinction between similar characters is accomplished through clever use of relative proportion, or color variance. However, with the highly restrictive realism used for HAPPY FEET, the animators had to work within a convincingly natural scale, and a less distinctive color palette of mostly black and white aquatic birds in formal wear.
An underbite was a brilliant solution.