Icy humor

Alongside its immaculate blending of captivating vocal characterizations by a group of superbly talented actors, Ice Age triumphs by eschewing the inside adult humor that infiltrates so many other animated children's movies. Where movies like Shrek attempt to cater to adult audiences with not-so-subtle sexual innuendo and overloaded pop-culture cross referencing, Ice Age stays the course of its genre's Bugs Bunny slapstick humor, and in so doing proves a welcome point about the limited appeal of blue humor in children’s movies.
Director Chris Wedge (Oscar winner for animated short Bunny), and co-director Carlos Saldanha go one better than Disney's and Pixar's stilted style of animation, giving their characters a more fluid three-dimensional depth of personality that coincides with their voices in a way that animation seldom achieves. 
That being said, the first creature we see is a voiceless cross between a squirrel and a rat appropriately named "Scrat." This hapless little bug-eyed creature desires only to bury and protect his precious acorn from the quickly approaching Ice Age that threatens to obliterate his world's delicate ecosystem. Scrat's humbling experiences with his acorn serve as microcosms for the main story in which an unlikely trio of animals returns a lost human baby to its tribe.
Manfred, or Manny as he's called (Ray Romano, Everybody Loves Raymond), is a giant woolly mammoth who chooses to avoid the vast migration of animals away from the advancing cold by going in the opposite direction. Manny's stubborn individualist nature comes as a lucky circumstance for the hyperactive sloth Sid (John Leguizamo, Empire) whom Manny rescues from two very angry rhinos. Sid is an unintentional troublemaker with a heart of gold, while Manny's a terminal bachelor concerned primarily with being left alone. Things change quickly for the newfound buddies when Sid discovers an abandoned human baby and resolves to return it to its spear-carrying family members. Manny grudgingly admires Sid's determination and offers his reluctant assistance.
But already a clan of saber-toothed tigers has assigned one of their own to capture the poor little infant for a saber-toothed feast. Diego (Dennis Leary, The Job) is a wily feline predator who will stop at nothing to get the baby in his pointy clutches. By promising to guide Sid and Manny through the mountainous tundra, Diego joins the uphill pilgrimage. The role is well suited to Dennis Leary's crafty sense of deadpan humor and makes for some toothpick-rolling moments of villainous fun. Leary, who also voiced the ladybug in A Bug's Life, takes an evident joy in entertaining children from behind the mask of a cartoon character.
But it's John Leguizamo's lisping Sid who leverages the humor farthest with surprising rhythms of vocal delivery that function like a Morse code of rapid-fire comic barbs. Leguizamo's infectious energy is given full reign as the feisty Sid, and it's his character who links the main narrative to Scrat's recurring sub-plot by similarity of type. In one especially silly battle with a bunch of Tae Kwon Do-practicing Dodo birds, Sid transforms into a pro-football receiver attempting to capture a watermelon to feed the hungry baby. Sid's comical movements feed right into an audience visualizing the movements of the actor himself. Although the actors recorded their parts separately, the chemistry between the cartoon characters plays like a symphony of toy instruments delivering a well-rehearsed Duke Ellington tune.