I am sitting here eating Cap'n Crunch and I noticed the Cap'n only has 4 fingers. So does homer simpson, mickey mouse and many many more. It got me thinking, when did losing one finger for a cartoon character become normal?
Pretty sure I wrote a lengthy answer to this a while back that pinpointed exactly when this happened. googles
Ah, here it is. https://www.reddit.com/r/AskHistorians/comments/266el2/when_did_cartoons_start_to_depict_characters_with/
The TL;DR of it is 1928±2, or around 15 years after "animation" became something you could actually start to call call "an industry".
I have some experience in studying Western Animation History, and while I can't speak to exactly when this practice began, it is an artistic choice that predates film and dates back to at least many early comic strips that were the basis for the first animated films. Look at Felix the Cat, Little Nemo, and other contemporary comics for examples.
For most artists, hands are notoriously difficult to draw. For more "cartoony" characters that are designed to be simplified caricatures, hands are usually just circles with "sausage" fingers attached. If you try to draw four fingers and a thumb in this style, the construction looks rather unnatural and becomes even stranger looking when you try to animate it.
As a general rule, cartoon characters that have a construction that more closely resembles real-world anatomy and proportions tend to have five fingers. Look at cartoons like Johnny Quest and the humans in Scooby Doo for some prime examples.
A technical reason which is not mentioned in egypturnachs otherwise excellent answer: In animation, characters typically are drawn adhering a "line of action", a single line and direction which the whole figure is structured around in order to give a clear dynamic in the movement. If the line of action extends through the arm you will have it follow a single finger. This is simplest to express if you have an uneven number of fingers, (disregarding the thumb which will be hidden or at an angle to the other fingers) which you can see on the last figure on the linked page. Adding a fourth (non-thumb) finger will not only be more work, but also make the animation harder to read, since the line of action will be less clear.
Newspaper cartoons and comic books have different dynamics than animation and are not necessary drawn with a clear line of action. Often comics which do use "line of action" are by artists with a background in animation (like Walt Kelly or Carl Barks).
