Mad monkey or (very) happy monkey?

Apparently, this picture of a monkey was taken down because someone (i.e. someone who actually knows a great deal about monkeys) pointed out that, according to his face/stance, he must either be angry or aroused.

This has naturally generated some discussion in the office as to which it might be.

(download)

If anyone out there is a monkey doctor and could offer some insight, that'd be real cool.