Miniaturization to the extreme: discovery of world’s smallest frog and vertebrate

At the beginning of this year a paper came out in PLoS One describing the discovery of the world’s smallest vertebrate, a frog that has an average body size of only 7.7 mm, Paedophryne amauensis (Rittmeyer et al. 2012). The same team discovered another diminutive frog, Paedophryne swiftorum, which reaches an average body size of around 8.25-8.90 mm. The news was picked up by several media including the Guardian, that had an interview with one of the authors, Christopher Austin.

Apart from feeding the structure in our brain that loves to see another record being shattered and has a fascination for everything with Guinness-Book-of –Record-qualities, there are other interesting lessons to be learned from this discovery. From a conservation point of view, it comes to show that we are still not coming close to uncovering the full potential of (amphibian) biodiversity, seeing how this discovery follows not long after a previous discovery of the world’s smallest frog from the same mycrohylid genus. Especially the island of New Guinea appears to be have been a blind spot in the eye of the scientific world, although that has been partly made up by a 10 year-expedition of WWF that reported over a 1000 new species in this island.  And the importance of the discovery for amphibian conservation, is worded by  Austin: “The discovery of two new frog species comes as great news against the background of more prevalent accounts of tropical amphibian extinction”.

The new species Paedophryne amauensis is the world's smallest vertebrate. Photo from: Rittmeyer EN et al. (2012) Ecological Guild Evolution and the Discovery of the World's Smallest Vertebrate. PLoS ONE 7(1): e29797.

From a scientific point of view, Austin says it helps scientists “better understand the advantages and disadvantages of extreme small size and how such extremes evolve. Fundamentally, these tiny vertebrates provide a window on the principles that constrain animal design.” Before these tiny frogs were discovered, the world’s smallest vertebrate was a bony fish and the world’s biggest vertebrate is still the blue whale, leading to the hypothesis that extreme sizes are linked to life in water. However, these frogs hatch out of the eggs as ‘hoppers’ which means they do not spend their youth as a tadpole in the water and moreover as an adult they live on the forest floor, also outside of the water. This means the old hypothesis needs adjustment.

The discovery also made me think about evolution of small size and the advantages that drive this evolution into the microworld. I can imagine how a smaller creature can step into a vacant ecological niche and how it can survive better in areas where resources are scarce because of its small size. At the same time, such a tiny frog appears to have the disadvantage that more animals can predate on it and will have to fear more in general from the world around it. It reminds me of footage of the BBC Life-series of a tiny gecko during a rain shower in which it looks like a boat adrift on a stormy ocean, while in reality it is only trying to escape from single rain drops. So being small makes the world a bit more dangerous, and so what kind of evolutionary advantage do they experience?

A quick literature search lead me to an article by Zimkus et al. (2012) who looked at, among other things, miniaturization in African puddle frogs. Miniaturization is regarded as a ‘key innovation’, meaning “Trait shifts that enable extension into and colonization of new environments, or ecospace, and subsequent diversification in ways that were not previously possible” . Such a key-innovation increases rates of speciation, because, as the definition makes clear, allows for the occupancy of new environments . The answer to my question which advantages are associated with miniaturization is given by them: “…predator avoidance, exploitation of alternate food sources, utilization of physically smaller niches and attainment of reproductive maturity at an earlier age.” This brings me back to the original discovery of the world’s smallest frogs.

The other diminutive species that was discovered Paedophryne swiftorum. Photo from: Rittmeyer EN et al. (2012) Ecological Guild Evolution and the Discovery of the World's Smallest Vertebrate. PLoS ONE 7(1): e29797.

In anurans, miniaturization has occurred frequently with currently 29 species known to science. I do not want to go into detail on how miniaturization can arise and its morphological consequences, but   I can recommend the article by Hanken and Wake (1993) to anyone who is interested in these issues as well as the evolutionary significance of miniaturization of this feature. The ecology of these diminutive frogs is partly determined by their small size; since frogs have a skin that is permeable to water and a smaller size means a high surface to volume ratio making them more vulnerable to desiccation, these small species live in a highly wet environment, mostly in the leaf litter. Of course, living the aquatic life would be the most adequate response to the imminent threat of desiccation, however the high incidence of invertebrate predators in this ecosystem, probably kept them out of the water.

It is proposed by Rittmeyer et al. that these tiny species actually form a specific ecological guild: “These discoveries […] highlight ecological similarities among the most diminutive anurans, suggesting that these species are not merely curiosities, but represent a previously unrecognized ecological guild.” Such an ecological guild inhabits a specific niche, in this case the leaf litter of tropical rainforests and predate on small invertebrates, fulfilling a specific ecological role.

These frogs were discovered at night due to their strange, high-pitched and insect-like call. Clearly spotting such a tiny creature only by sight is a sheer impossible task and makes it difficult to provide a good estimation of the population size and the viability of these species.  It is very probable that in the future more of these amazing miniature frogs will be discovered, because we have to keep in mind that we are still living in a biologically speaking unexplored world. Let’s be very happy about every species that can be added to life’s catalogue, because to know something is to love it.

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