The hidden history of the humming frogs

  • In the depths of Brazil’s Atlantic Forest, researchers are battling to identify species before they are lost for good. Dr Rafael O de Sá, Professor of Biology at the University of Richmond, Virginia, USA, specialises in a group known as the ‘narrow-mouthed frogs’ and has already found four species new to science.
  • The amphibians (frogs, toads, newts, salamanders and caecilians) are one of the least-understood groups of vertebrates on earth, and one of the most vulnerable. In the Neotropics — a poorly-explored region extending from Mexico and the Caribbean to the southern tip of South America — urgent conservation efforts are required to protect amphibians and their rapidly disappearing habitats. However, the species cannot be protected unless we know that they exist, how to identify them, and where to find them.
  • Dr de Sá’s research aims to fill the gaps in our knowledge surrounding one particularly enigmatic group of amphibians — the ‘humming frogs,’ technically known as Chiasmocleis. Chiasmocleis is the largest genus of the narrow-mouthed frog family found in the Neotropics. The family Microhylidae has a worldwide distribution, except Europe and Antarctica, and currently consists of over 600 known species.
Species cannot be protected unless we know that they exist,
how to identify them, and where to find them
  • Like all Neotropical narrow-mouthed frogs, Chiasmocleis live in burrows, emerging from their underground hideaways only during the breeding season. According to Dr de Sá, this is the best time to catch and study the elusive little creatures. He listens for the males breeding calls, follows the sound to its source and then catches the culprit.
  • A frog family tree
    The foundation of Dr de Sá’s current research will be a ‘family tree’ — known as a ‘phylogeny’ — of all the frogs considered to belong in the Chiasmocleis group, built using genetic data. By examining the similarities and differences in the DNA of individual frogs from right across the range of the group, Dr de Sá can determine which are most closely related and which are more distantly related.
  • Genetic data provides a clearer understanding of relationships than phylogenies built using morphological characteristics, which provide conflicting evidence as similar features seem to have evolved repeatedly in unrelated species as a result of their similar burrowing lifestyles in this group of frogs. The final phylogeny will track the evolutionary history of Chiasmocleis, providing a framework to understand the evolution of humming frogs both genetically and morphologically.
  • Previously, Dr de Sá built a family tree for the narrow-mouthed frogs as a whole, to work out where within this extended family Chiasmocleis fits. Already, that tree has shown that not all species thought to belong to Chiasmocleis actually fit together in the same part of the family tree: three species posing as Chiasmocleiswere really members of a nearby group called Syncope, and one further imposter belongs to another distantly-related group, Elachistocleis.
  • Dr de Sá has officially renamed these species according to their correct affiliations, leaving him with a neat collection of 33 currently known species of Chiasmocleis, all descendants of a single common ancestor. He will now focus in on the relationships within this group, producing a revisionary up-to-date description of them all, known as a ‘monograph’.
  • Cryptic creatures
    One of the first things Dr de Sá noticed while working on Chiasmocleis was that there were certain clusters of individuals with DNA similar to each other and distinctly different to the DNA of other groups. These clusters would suggest new species, yet they had not previously been identified as such based on their appearance. Dr de Sá calls these ‘cryptic species.’
  • Remarkably, once the cryptic species had been identified from their DNA, a careful analysis of morphological characteristics did reveal some distinguishing features, which had been overlooked in previous studies. This included a small difference in size, foot webbing and skin pigmentation. As Dr de Sá says: “The molecular data guided us to reexamine … the variation of external morphology … and identified the characteristics that would allow us to recognise those populations as new species.” This means that the new species can be recognised, not only in the lab, but in the field — a crucial factor for finding out more about them and ultimately saving them from extinction.
The new species can be recognised, not only in the lab, but in the field — a crucial factor for finding out more about them.
  • The yellow line indicates the border of Brazil’s Atlantic Forest By NASA and Miguelrangeljr
  • Taxonomists are the scientists that describe and name new species; this is how species become “known” and is key to all future biodiversity research. So far, Dr de Sá’s team have formally described four new species of Chiasmocleis but Dr de Sá says his team have identified other potential new cryptic species, almost indistinguishable from one another to the naked eye. For Dr de Sá, this highlights the importance of using genetic data in biological studies: “The diversity [i.e., number of species] of Neotropical frogs goes beyond what we can identify through traditional morphological approaches.” It is highly likely that the number of species of humming frogs will continue to grow.
  • A foundation for conservation
    Dr de Sá’s monograph will not only list the species of Chiasmocleis — cryptic or otherwise — it will also provide crucial information for their conservation. Firstly, a fully diagnostic description of each species and how it differs from the others, will enable future researchers to identify the frogs they come across.
  • Secondly, based on records of all the places the frogs have been found over the years, Dr de Sá will be able to estimate the range of each species. The humming frogs can be found throughout the lowlands of South America to the east of the Andes Mountains. However, like most animals and plants of the Neotropics, little is known about the area each individual species extends to cover. Combined with information on the extent of suitable habitat, this provides essential data for estimating the level of threat each species may face. At present, almost a third of species of humming frog are recorded as simply ‘data deficient’ on the IUCN’s Red List of Threatened Species — because not enough is known about the species and their distribution.
  • Given the rate of habitat destruction in the Neotropics, Dr de Sá fears that even where frog species have been assigned a Red List status, this may be dangerously out of date. Many of the newly-described Chiasmocleis species are restricted to the Atlantic Forest of Brazil — an ecosystem under severe threat from habitat degradation and loss caused mainly by agriculture, such as cattle ranches and soybean plantations. Some estimates suggest that only 3.5% of the natural vegetation of the Atlantic Forest now remains. Dr de Sá’s new monograph will provide invaluable information for researchers seeking to conserve these fascinating animals. Hopefully for the humming frogs, it will not come too late.