I spent a long time chasing the first of these frogs around in a shallow brook. The other two were much more cooperative. Initially I thought that the smaller frog was a different species from the larger one, perhaps Mantidactylus biporus, which according to Glaw and Vences is often found living together with M. betsileanus. But upon further inspection of the photos, I'm reasonably confident that the smaller one is just a youngster of the same species. Also, M. biporus isn't known from anywhere near the Masoala peninsula.

The 3rd edition of Glaw and Vences say that this species has been found "only in highly degraded areas where it occurs in flooded swamp areas (near Toamasina) or a small source (Maroantsetra) flowing through forest remnants". The ones photographed here are from small streams in an area of mostly undegraded forest in Masoala National Park, so maybe this was something of a minor discovery. The white spots on the snout are diagnostic of Mantidactylus betsileanus and its close relatives, so I was pretty certain that I had gotten the identification correct.

After Mark Scherz et al described M. jonasi along with many other new species in this genus in 2022, Dr. Scherz identified one of my photos as M. jonasi on iNaturalist. So now I'm even more certain that I had the right ID.

Here is a list of all the reptiles and frogs I saw on this 2007 trip to Madagascar.