The world was a very different place 20 000 years ago. Humans and animals survived off each other and the land in a fend-for-themselves world; the chances of two species coming together for any purpose other than the hunt was unlikely. Thus the camaraderie between Manfred the Mammoth (voiced by Ray Romano) Sid the Sloth (voiced by John Leguizamo) and Diego the Saber-Tooth Tiger (voiced by Denis Leary) sounds odd but it proves it's possible to stumble upon future lifelong friends in the most improbable ways. In this case it's the caretaking of a human baby. When the baby washes up on the shore of a riverbank the three strangers become begrudging partners as they try to return the babe to its human family for their own reasons: Manny because he lacks a family of his own Sid out of the kindness of his heart and Diego to avenge his pride. The trek isn't easy of course; there are ice caves to navigate lava pits to jump over and secrets to unearth about each of them.
It doesn't take long to recognize comedian Ray Romano's voice as Manny. Although the screenplay credits belong to Michael Berg Michael J. Wilson and Peter Ackerman Romano really adopts the subtle jokes as his own. Admittedly it took a while to forget about Romano and realize there was a woolly mammoth on the screen. John Leguizamo's Sid though is by far the star of the show. His lisp and "slothful" way of speaking captures the kind of goofy talk kids love. Denis Leary gives Diego a strong and soothing voice one caught between loyalty to tradition and newfound friendship. Director Chris Wedge puts his voice to work too through the no-so-dialogue-intensive squirrel Scrat (and a few other minor characters) the acorn-chasing entrée act. His voice perfectly matches the squirrel's quick and erratic behavior.
Director Chris Wedge equally distributes the time spent on character development and the setting-up of family unit boundaries within a pack. Manny's naturally monstrous proportions make him the ultimate father figure and protector while his slow yet constant demeanor also makes him the decision maker and mediator. Also a parental figure Diego's inborn reflexes and hunting senses help him to be the better tracker of the group navigating the threesome and their tiny charge along the humans' path. Sid's long hook-like claws help him adapt to the ice-laden landscape skating across frozen lakes with ease but his small size and lack of maturity make him more an older brother to the baby than a parent. The major drawback to this familial cycle of life however is that it's unusually male dominated. Only four females appear in the entire script and each very briefly: one is the baby's mother one is the last known female dodo bird and the other two are skanky sloths whom Sid tries to scam in a mud bath.