I thought it would be difficult to identify this skink, but it turns out there are only three skinks in Costa Rica. Only two of those occur in the area I was in. And only one, Mabuya unimarginata, is bronzed like this one.
Chiefly arboreal, this one was nonetheless basking on a log near the ground.
See more images of this species here.
by Twan Leenders