When you walk along streams in Madagascar, you frequently hear sounds like doors creaking and stomachs gurgling. These are the calls of little muddy-brown frogs in the Mantidactylus frog subgenus Brygoomantis. Today, my colleagues and I have published our magnum opus on these charming but secretive little frogs, describing 20 new species and four new subspecies! The paper is published open access in the journal Megataxa.
In the first edition of their Field Guide to the Amphibians and Reptiles of Madagascar, published in 1992, Frank Glaw and Miguel Vences wrote, ‘Species determination within this group is very difficult… evidence exists that calls differ between specimens from different localities, which can not be distinguished morphologically… This indicates that the taxonomy of this group is not sufficiently clarified. Since our data do not allow a substantial contribution to such a clarification, we do not undertake any taxonomic discussion here‘.
30 years later, we finally have enough data to undertake that taxonomic discussion.
Over the intervening decades, the team of researchers working on Malagasy amphibians has grown enormously, and we have amassed small but representative collections from across Madagascar over dozens of expeditions. We brought together the results of all that work, with DNA sequences from over 1300 frogs, and measurements from hundreds of specimens from numerous different museums.
To bring some more resolution into the picture, we also added phylogenomics, sequencing thousands of genetic markers using the FrogCap hybrid enrichment approach to get unprecedented resolution in the phylogenetic tree.
But the starting point, as with most taxonomic work, was to look at the names that were already available for this group. Until today, Brygoomantis had 14 recognised species, but 20 ‘available’ names. So, we had to examine a lot of the material associated with these old names, the oldest of which are over 140 years old!
This is always challenging in frogs. They are soft and often in quite poor condition in museum collections, meaning both traits and measurements may not be reliable. It is still more difficult in a group like Brygoomantis, where even identical-looking frogs might emit totally different advertisement calls, and represent separate species.
How, then, do you figure out which name belongs to which species?
The key was to use museomics, the sequencing of DNA from archival material. We used an approach called barcode fishing that we developed for Mantidactylus frogs a couple years ago. We were able to sequence mitochondrial DNA from 16 of the available Brygoomantis names. This allowed us to assign those names with confidence to genetic lineages we had fresh material from. We were able to recognise some old names as valid species (we revalidated two names), and confirm or establish some names as synonyms.
With all 20 old names assigned (15 being valid), we were still left with some 20 lineages that were genetically, morphologically, and/or bioacoustically clearly distinct at species level. So we described all of them, bringing the total species count to 35!
We also found a few curious cases where we had to employ subspecies to describe variation. Take for instance Mantidactylus ambohimitombi. This species has a ‘typical’ form that is streamlined with a few spots, a second form that is similar but more speckled from a different area, and a third form that is morphologically highly distinct, being a flattened stream-dwelling frog with a broad, short head and lacking spots.
Amazingly, these three lineages are extremely closely related, with genetic distances far less than typical among species. To recognise that these might represent lineages at the early stages of speciation, we have described them as separate subspecies (M. a. ambohimitombi, M. a. miloko, and M. a. marefo, respectively). Subspecies are a somewhat controversial topic in evolutionary biology at the moment, but we have largely followed the arguments of Kevin de Quieroz, which we think make it still a relevant and useful category.
This latest major step forward on the taxonomy of Madagascar is part of our ongoing effort to describe all the species that are known to science but not yet named. In 2009, there were 244 species of frog known from Madagascar, and estimates were between 373 and 465 species in total. That estimate was revised in 2014 to 533. Today, we have brought the number of described species to precisely 400. Many more species await description (with numerous manuscripts already underway), and we have also discovered many lineages since 2014 that were not included in previous estimates. But given that we have described some 155 species in the last 13 years, it is feeling increasingly possible that we may close the taxonomic gap in Madagascar’s frogs over the next 30 years!
The full citation of our new paper is here:
Scherz, M.D., Crottini, A., Hutter, C.R., Hildenbrand, A., Andreone, F., Fulgence, T.R., Köhler, G., Ndraintsoa, S.H., Ohler, A., Preick, M., Rakotoarison, A., Rancilhac, L., Raselimanana, A.P., Riemann, J.C., Rödel, M.-O., Rosa, G.M., Streicher, J.W., Vieites, D.R., Köhler, J., Hofreiter, M., Glaw, F. & Vences, M. (2022) An inordinate fondness for inconspicuous brown frogs: integration of phylogenomics, archival DNA analysis, morphology, and bioacoustics yields 24 new taxa in the subgenus Brygoomantis (genus Mantidactylus) from Madagascar. Megataxa, 7(2):113–311. DOI: 10.11646/megataxa.7.2.1 [high-resolution pdf, 336.8 MB; low-resolution pdf, 14.1 MB]