Honey, evolution shrank the frogs! 14

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Hot off the press today in PLoS ONE, my colleagues and I have published a paper describing five new species of frogs, including a new genus, Mini, containing three species that we have dubbed with punning names: Mini mum, Mini scule, and Mini ature. The largest of these is 14 mm long, the smallest 8–11, making them not just among the smallest frogs in the world, but even among the smallest vertebrates—there are only a handful of frog and fish species that are smaller.

Mini mum, one of our new species, is among the smallest species in the world. Several fo them could fit comfortably side-by-side on your thumbnail. Photo by Andolalao Rakotoarison.

There are about 8000 species of amphibians recognised today, the vast majority (>7000) of which are frogs. The largest frog, Conraua goliath of west-central Africa, reaches sizes of over 30 cm, and is considerably larger than the next largest frogs (potentially the South American toad Rhaebo blombergi, which reaches just 25 cm—still a huge frog, but not nearly the size of C. goliath). At the other end of the scale, everything is much more bunched up. Paedophryne amauensis is currently considered the smallest frog in the world, and is not known to exceed 9 mm in adult body length, but there are actually a slew of species that reach adult body sizes between 8 and 10 mm from a wide variety of branches of the frog evolutionary tree.

No group is more prone to miniaturisation than the narrow-mouthed frogs, family Microhylidae, of which Paedophryne is a member. This family is widespread, occurring on all continents except Antarctica (and Europe, if you consider it a continent, which most non-English speaking countries do not), and enormously diverse, with over 650 recognised species; roughly one in every ten frog species is a microhylid. Of the 50 smallest frogs in the world, 35 are microhylids. This family contains a large number of subfamilies, and the smallest frogs in the Microhylidae are distributed among these subfamilies.

Currently, about 115 species of microhylid frogs are known from Madagascar, but the number is rapidly rising. Since 2014, 45 species of microhylids have been named from Madagascar. In 2017, my colleagues and I published a monograph describing 26 new species of Stumpffia, including S. contumelia, which reaches just 8.0–8.9 mm adult body size. This is Madagascar’s smallest frog, and unless I am much mistaken, is the second smallest frog in the world after P. amauensis. Two species of the new genus Mini sit resoundingly among the top 50 smallest frogs in the world, and Mini scule is even in the top 20, reaching a maximum adult size of just 10.8 mm.

Mini mum in situ. Photo by Andolalao Rakotoarison.
Mini mum on a finger tip. Photo by Andolalao Rakotoarison.
Mini scule in situ. Photo by Sam Hyde Roberts.
Mini scule on a fingertip. Photo by Sam Hyde Roberts.
Anodonthyla eximia in situ, another tiny species of independent origin. Photo by Miguel Vences.

What’s interesting here is that Mini are not closely related to Stumpffia, although they are both members of the subfamily Cophylinae. In fact, Mini is the sister genus of Plethodontohyla, which includes, among others, the largest microhylid frog in the world, P. inguinalis! What is more, the two other species described in our paper, Anodonthyla eximia and Rhombophryne proportionalis, are also among the top 50 smallest frogs in the world. That means that four separate genera of cophyline microhylid frogs in Madagascar have convergently evolved to be among the smallest frogs in the world!

As you can imagine, this provides an awesome setting in which to look at the effect of miniaturisation in driving morphological convergence. Does getting small (a) force you to evolve certain features (so-called ‘determinism’), or (b) is the feature set dependent on what the ancestor had before the group got small and a little bit of chance in how the miniaturisation proceeds (so-called ‘contingency’)?

The answer to this question is still in the works, really, but we argue in the new paper that there is a little bit of column a, a little bit of column b. When you get small, there seem to be physiological limitations that result in similar features (reduced fingers, larger eyes, relatively larger brain case, etc.), while at the same time some things are lost or retained based on what you had before or based on luck of the draw (miniaturisation can but need not result in the loss of teeth, the loss of the collarbone, and a generally ‘paedomorphic’ [juvenile-like] appearance). A cool contrarian example comes from the new species Rhombophryne proportionalis, which is also tiny at just 11.0–12.3 mm, but which retains more or less the proportions of a larger frog, with small eyes, short limbs, and teeth on its palate (these are lost in almost all other miniaturised frogs).

Rhombophryne proportionalis, one of our new species, is a ‘proportional dwarf’, retaining the proportions of larger-bodied species at a tiny size. Photo by Miguel Vences.

I’m thrilled that this paper is finally published. It’s one of the chapters of my PhD thesis, which I just submitted last week. It is a thrill to get to name a new genus, and I have long wanted there to be pun-named reptile or amphibian species in Madagascar. It’s great that we were able to find a funny name that is also informative; Mini is not just amusing, but also an accurate descriptor. People who know me personally know that wordplay is about 70% of my personality. Yes, this is a bit whimsical, and frivolity is often frowned upon in science, but it can also help to make that science more accessible, interesting, and engaging. That is our hope here.

Read the whole paper here!!

Update: Like for Geckolepis, I will try to collate media coverage of these frogs as it happens.

Edit: some more great news:

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