Hmm.

[Nelliel pauses for a moment. Between what Grimmjow told her and what Shiro told her just now, she's starting to get a rough idea of what the motive behind this transformation was. This has to be a tremendous blow to Grimmjow's pride.]

I suppose animals can only handle high levels of spiritual pressure if they have the chance to build a resistance, like Akumi, or Aizen-sama's still nameless cat.