If it looks like there have been mice in your filament, it may be because your printer is having a hard time pushing the filament through the hot end.
If your printer has been running for a long time without maintenance, a plug of dust may have built up in the nozzle which keeps the plastic from coming out and the extruder from pushing to the filament. Usually a plug like this will result in the extruder starting to "click" (make a click-sound) because it does not have the power to overcome the pressure required. If your extruder is not fully tensioned, the gear can slide on the filament and quickly dig into it. This leaves marks like these.
Tighten the extrusion
Many extruders have a way of tightening and releasing the pressure applied to the filament, often by compressing a spring with a screw. If you have a plug as described above, your extruder will start clicking after you have tightened it up and you can now try the next solution.
It's a good idea to clean the nozzle with a cold pull which is performed by heating the nozzle up to the print temperature of the material mounted on the printer. When hot, push some filament through before setting the printer to cool. When it reaches 50 degrees, the heat can be turned on again and then pull on the filament until it releases from the hotend and takes impurities out of there. Do the same 2-3 times more to be safe. For best results, use filament such as nylon or PETG for this as they are a bit more slippery than standard PLA.
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