Some possible reasons for saying no:

• We aren't as good at being happy, but we might be better at improving the scope for future happiness. (E.g., maybe space colonization some day.)
  • This one becomes harder to justify if these critters are actually our intellectual superiors.
• Other things matter (to us) besides happiness, and these critters' lives don't provide those things as well as ours do.
  • This answer may be irrelevant if you're only interested in utilitarian arguments.
  • My moral values are approximately utilitarian, but I would say that what I care about when thinking in utilitarian terms isn't happiness as such but those things of which happiness is a measure. (This doesn't require me not to care about happiness; happiness is in fact one of the things that makes us happy. If I were suddenly made much less happy about everything then that fact itself would be a source of unhappiness for me from then on.)
  • It may be illuminating in this connection to read "Not for the sake of pleasure alone".