Surely it couldn't have been planned this way, but along with ''Shrek'' and ''Monsters, Inc.,'' Ice Age forms a trilogy of thematically related computer-animated cartoons. Like Mike Myers' green ogre, woolly mammoth Manfred (voiced by the aptly nasal Ray Romano) is an antisocial behemoth who acquires a pain-in-the-ass sidekick -- in this case, a lisping sloth named Sid (John Leguizamo). And like ''Monsters, Inc.'''s Mike and Sulley, these creatures have their hearts melted by a lost baby human (here, a primitive tribesman's son) whom they attempt to return home.
But ''Ice Age'' wisely throws in a wildcat: Diego (the menacing Denis Leary), a saber-toothed tiger who may be leading the makeshift ''herd'' into an ambush. Like their digital images, all these characters are pleasingly three-dimensional. The comedy's real secret weapon, though, is Scrat, a squirrel-rat hybrid whose hysterically Sisyphean quest to bury a single acorn continues in ''Gone Nutty,'' a short (included on both the VHS and DVD) that, like ''Ice Age,'' boasts some of the most inspired animated slapstick since ''Looney Tunes''' heyday.