I shined my flashlight into an angled hollow log and caught the eyeshine of this pair of amplexing frogs. I had to take the photo from a distance and couldn't illuminate it well, so it's basically just a crappy picture. However, it became more interesting when I realized from the distinctive pointy snouts that these frogs must be Kalphrynus baluensis, because at the time little was known about the mating habits of this species. To quote A Field Guide to the Frogs of Borneo, third edition: “Males call from the entrances of burrows in the ground. Not much else is known about its reproduction.”

However, after I had returned from Borneo, I received the March 2018 issue of Herpetological Review, which included a Natural History note about the reproduction of this species. This note confirmed that they breed in tree holes, and that this had been known since 2010, though not published before. So it turns out I'm not the first person to ever see this species in amplexus in a water-filled hole in a tree (in my case, an ex-tree). Still, I am probably one of a very few such persons.

The next night I got to see one of these frogs up close on the forest floor.