A merry saga of death, extinction, and migratory depletion, this sequel may be the grimmest entertainment for kids since Uncle Walt killed Old Yeller. Underneath the intermittently funny slapstick, there's a worldview in which everything exists to eat, be eaten, and die. Struggling to stay off the menu are Diego the saber-tooth tiger (voice of Denis Leary), Sid the sloppily sibilant sloth (John Leguizamo), and Manny the lonely mammoth (Ray Romano), who head for higher ground to escape a flood. (That's as compelling as the story gets -- a long walk.) A romantic subplot about a possum-raised mammoth (Queen Latifah!) tries to put the warm in global warming, but the unappealing character designs, incessant celebrity-voice chatter, and slickly inexpressive 3-D animation thwart any emotional pull. The only feeling that registers is hunger -- that of show-tune-singing vultures, of unfrozen sea monsters, and especially of the starving scene-stealer called the Scrat. Watching this zoo of long-vanished quadrupeds, you're left with an eerie message for our own warming times: They fought the thaw, and the thaw won.