Chipmunks are an anthropomorphic species of rodent that encompasses a majority of the main characters in the Alvin and the Chipmunks franchise.
The Chipmunks' very first physical appearance were as wooden-headed, soft-bodied puppets on The Ed Sullivan Show in 1958. Their facial structure closely resembled the structure of a real chipmunk's, though their tails were a little larger and bushier.
In 1959, Richard Scarry's designs of The Chipmunks were perhaps the third closest resemblance to a real life chipmunk. Their posture was of a bipedal stance. Their tails were thin and not-so bushy, like how a real chipmunk's tail would be. Their facial structure resembled that of a real chipmunk's too. Their ears were more rounded and smaller, and they had the two stripes over the eye. They each had a different shade of fur; Theodore being a grey, while Alvin and Simon had lighter and darker shades of brown. It's also noted that these designs marked The Chipmunks' first look as real chipmunks.
Then came the designs from the 1959 records and The Three Chipmunks comic. Both designs took on the bipedal stance, and the close resemblance of a real life chipmunk, though the facial structure of the designs from the records were closer to the animal whereas the comic versions were more cartoon-like. Their tails were small and bushy and both carried the same stripe patterning. The ears of the comic versions were more smaller and rounded and their faces lacked stripes, whereas the record versions were taller and pointier and had the stripes of a real chipmunk.
60s and 80s Series
Males typically have tufts of hair on their heads that are the same color as the fur on their bodies, while females, like The Chipettes, have more human-like hairstyles and lighter fur on their bodies. They have a bipedal stance and are about half as tall as an adult human.
The CGI film version of chipmunks are more designed to look like their real life counterparts. As such, they have shorter hind legs so they can scamper on all four paws. This also makes them the first version of chipmunks not to be entirely bipedal. Another feature that reflects this realism is the characteristic lateral stripe that runs up from the nose all the way down their back and to their tail.
Like their CGI film version, the chipmunks in the CGI series have a lateral stripe in the same color as their hair and tail. They stand bipedally and are taller than the CGI film chipmunks but shorter than both the 60s and 80s series chipmunks.
Their origins as a species are mostly unknown, however it is suggested in the 80s series that they have existed as far back as the Aztec empire, as evident from the Twee-Twee Tribe. According to Simon in the first CGI film, chipmunk parents only take care of their young for one week and then leave them. This would suggest that in the CGI film canon, they have a very fast maturity rate after birth.