>>41110219
It is a stress recovery response that discharges the trapped energy from stress. It naturally occurs during or after extreme stress situations.

Look at these videos. Do you feel like the shaking is harming these animals in any way? It's the opposite, the shaking reanimates the animals.
https://www.youtube.com/watch?v=-QgglTik6G4
https://www.youtube.com/watch?v=xDlR-wl7iFI

When this response is disrupted or repressed, it is extremely harmful to the organism. Adult humans repress their shaking and trembling out of fear for social repercussions.

>>41110219
Monkeys masturbating is actually quite an interesting example. In my opinion, they are purposefully overstimulating themselves and numbing themselves out. Similar to a PTSD victim that gets addicted to drugs. The captivity feels so dangerous to the monkeys that they never feel the opportunity to discharge the stress, instead they stay on alert and cope with short-term rewarding behavior. It is a horrible thing.