In this film & in the series the original main characters: Alex, Melman, Marty, Gloria do not appear even in a glance (their shadows are vaguely shown once).
I wonder what the reason for this is? It seems like some copyright/ownership issue.
I think it was because this was a spin off and not a sequel.
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Maybe they were too cheap to sign the voice actors just for a few lines.
shareIt's most likely they just didn't think it was necessary. The whole reason the penguins got the spin offs was because people liked them more than the main zoo animals. And the idea of copyright problems doesn't make sense. They're all owned by Dreamworks.
Also, there was a special longer episode of the Penguins series where Alex showed up. He was a hallucination, but still.