Microhyla annectens — Larut Hills Rice Frog
Some other names for this species:
Boulenger’s Chorus Frog
My guide Kurt "Orion" G spotted these two tiny frogs calling in a culvert, and warned me to get photos before they hopped into the water and disappeared. One of them did so pretty quickly anyway; I chased the other one around for a little while to try to get better photos. This species is known only from a few highland spots in Peninsular Malaysia.