According to an interview with Jay Leno on July 12th 2012, Denis Leary's character Diego originally died near the end of the film, which caused the test audience of children to burst into tears, so it was re-done.
The drawings of characters during the end credit roll were all done by the children of the animators. The same is true of the picture that Sid draws of himself on a cave wall. Sid's drawing was done by 3 year old Will Shefelman, son of a story artist Dan Shefelman. The story artist working on the scene was having difficulty drawing like a 3 year old so he consulted an expert.
John Leguizamo tried 30 different voices for Sid. After viewing a documentary about sloths, he learned that they store food in their mouths; this led to him wondering what he would sound like with food in his mouth. After attempting to speak as if he had food in his mouth, he decided that it was the perfect voice for Sid.
Despite the Dodo's less than brilliant survival instincts, they outlived all of the major character's species. Dodos were wiped out around the mid to late 17th century, whilst woolly mammoths (Manny) died out 10,000 years ago (a dwarf race existed until 1700 BC), ground sloths (Sid) are widely accepted to have become extinct some 2500 BC (although it has been proposed a small group survived till 1550 AD), and saber tooth tigers (Diego) were wiped out around 10,000 BC. Not even Neanderthals like Roshan and his father survived the Dodo.
Though Scrat was designed as a fictional animal, in 2009, scientists in Argentina unearthed the remains of a small, extinct mammal resembling a shrew with long fangs. This animal was given the scientific name of "Cronopio Dentiacutus", in tribute to Scrat.
Chris Wedge, director, is the voice of Scrat, but has no intelligible dialogue; the plan to have Scrat talk was quickly dropped, as he worked better as a silent character for comedic effect. The name 'Scrat' is a combination of the words 'squirrel' and 'rat', as Scrat has characteristics of both species; Wedge has also called him "saber-toothed squirrel." Scrat's opening adventure was inserted because, without it, the first real snow and ice sequence wouldn't take place until about 37 minutes into the film. This was the only role intended for Scrat, but he proved to be such a popular character with test audiences that he was given more scenes, and has appeared in other movies.
The first drawings seen in the cave (before entering the giant area with the mammoth drawings) are replicas of the earliest known cave drawings (found at Lascaux, in the south of France). They have been dated at somewhere between 15,000 and 17,000 years old.
Some additional character names: the baby is Roshan, his mother is Nadia, his father is Runar. The sabers are Soto, Zeke, Oscar ("haughty saber") and Lenny ("fat saber"). The female sloths in the pool are Jennifer ("He's not much to look at...") and Rachel ("All the sensitive ones get eaten."). The "rhinos" are Carl (the one with thick horns) and Frank (the one with thin horns). The source for this information is from an autographed original script purchased at auction. There is a signed COA from 20th Century Fox Archives, auction item number ICE0032.
The production team originally thought of turning the 20th Century Fox logo into ice when it appears before the film. Even though it can be seen in one of the trailers, they cut that out and placed the regular logo in the film.
The "rhinos" in the movie are actually titanotheres (or brontotheres), mammals from the Oligocene period and are no relation to modern rhinos as they went extinct, several million years before the Ice Age depicted in the film. Ice Age rhinos looked more like modern rhinos, but covered with thick fur coats just like the mammoths.
Originally, Sid was supposed to be a con-sloth and a hustler, and there were even two finished scenes of the character conning some aardvark kids and a very suggestive scene with two female sloths later in the movie. Sid was also supposed to have a female sloth named Sylvia chasing after him, whom he despised and kept ditching. All the removed scenes can be seen on the "Super Cool Edition" DVD.
All ground sloths in this film and its successors were designed with traits of both prehistoric ground sloths and modern tree sloths. Initially in design, Sid, as a ground sloth, resembled one of the more notable species of ground sloth, the Megatherium, which was the size of an elephant. The species of sloth that Sid and the others resemble most in the finished films is a species of Caribbean ground sloths that were more the size of pigs.
Writer Michael J. Wilson has stated on his blog that his daughter Flora came up with the idea for an animal that was a mixture of both squirrel and rat, naming it Scrat, and that the animal was obsessed with pursuing his acorn.
This was originally set to be directed by Don Bluth and Gary Goldman by Fox Animation Studios until the failure of Titan A.E. (2000) put paid to that. With the departure of Bluth and Goldman, the project migrated to Blue Sky instead.
Supposedly the reason Don Bluth refused to make the film is when 20th Century Fox said they wanted it to be CG after the failure of 2D animation, Bluth refused due to his personal hate for fully CG animation and angrily walked away from the project.
On the 2-Disc Special Edition DVD of the film, Ray Romano calls the movie a "dramedy" and explains why the movie is both drama and comedy, and this is corroborated by director Chris Wedge in his own interview on the special features.
Fox wanted a children's comedy, so Blue Sky took that direction, but kept the drama aspect a notable element of the movie, hence why the cast and crew refer to it as a "dramedy."
This is the only film in the Ice Age series to show human characters, and the only Blue Sky Studios film to have any human characters for almost a decade. Blue Sky wouldn't feature any human characters again until the 2011 film Rio.
Movie signage and the book version include a female sloth named Sylvia, who is not seen in the released movie. In the book, Sid manages to avoid accompanying her on the migration south, because she is interested in a commitment.
Blue Sky Animation has been developing improvements in computer-generated characters for over 15 years now. It was responsible for some of the aliens in Alien: Resurrection (1997) and the talking cockroaches in Joe's Apartment (1996).
The only stipulation that consulting anthropologists from the American Museum of Natural History in New York insisted on with the production was that there should be no dinosaurs. They would have been long gone 20,000 years ago, though a great deal of the animals shown in the film didn't live in the same eras or locations as the main characters. Dinosaurs were finally included in the second sequel, Ice Age: Dawn of the Dinosaurs (2009), the explanation being that they survived extinction in underground caves.