Happy New Year to you as well! Honestly it’s a bit of both. Old world monkeys and apes will bite their nails when a nail becomes too long or damaged. Nails will also show some wear from normal use through the jungle / enclosure, and will generally chip / break before they grow too long. It’s all just another part of primate self-care / grooming!
While not a huge worry on its own, long nails can* serve as a warning sign and let keepers / caretakers know that this particular animal may not be taking care of themselves as expected. Long nails, like a poorly groomed coat, can indicate lethargy, depression, pain, or illness.
*Note: Occasionally long nails are simply a behavioral oddity due to individual preference like with the chimp Annie (hand) who is facial grooming [re. nose picking] Missy pictured below. Chimpanzee Sanctuary North West
On the opposite end of the spectrum, keepers & caretakers also are on the look out for over grooming (of nails or hair), which may be a sign of stress and anxiety. Although anxiety behaviors are a normal reaction, we keep an eye on these animals / situations since this behavior has the potential to develop into more concerning self-injurious behaviors.
TL;DR: If necessary, a primate’s nails can be trimmed with a standard nail clipper during a physical… but (in my experience) this is rarely needed and captive primates are generally left to maintain their own nail care preferences just like their wild counterparts.
EY FATTY BOOM BOOM 