Mexican Amphibians

By James Hanken

Mexico truly is a biological hotspot. What is mostly unappreciated, however, even by many professional biologists, is that much of the biological diversity of Mexico remains undocumented. It is virtually certain that many new species appear to await even initial discovery, while many known but unnamed species remain to be formally described. Much of my current research, as well as that of that of my students and colleagues, seeks to reveal the hidden diversity of Mexico as it pertains to one group in particular: amphibians. Over the years, I've specialized in salamanders especially one unique type that is among the tiniest vertebrates alive in the world today.

Mexico holds a special place for biologists, and for good reason. Straddling the bio-geographic boundary between the northern temperate zone and the New World tropics, and with terrestrial habitats ranging from sea level to elevations above 5700 meters, Mexico offers a range of habitats and climatic regimes that is matched by few other countries anywhere in the world. Responding to this tremendous ecological opportunity, animals, plants, and other organisms have, over millions of years, diversified to a spectacular degree.

I began my research on Mexican amphibians in the mid-1970s, as a second-year graduate student at the University of California, Berkeley. My major advisor suggested that I initiate a study of the biology of a unique group of salamanders of the genus Thorius, which is found in the pine-oak forests that define the southeastern edge of the Mexican plateau in the states of Veracruz, Puebla, Oaxaca, and Guerrero. What makes these salamanders unique is their size. Adults of some species measure less than an inch in length, and more than half of that is tail! In Thorius, miniaturization has promoted the evolution of several highly unusual anatomical features, allowing them to function and behave normally at such a small body size. The teeny animals were interesting and little-researched, offering what seemed to be a fertile field for a graduation research project. What I was also really looking for was a manageable and limited research topic that I could complete for a Masters  thesis, since I was very unsure as to whether I was interested and willing to pursue a Ph.D., let alone embark on a career as a professional biologist. Little did I or my major advisor realize at the time that, more than 25 years later, I would still be focused on these same critters.

What took me so long? Well, besides the predictable minor setbacks and blind alleys that typically slow research, the basic fact is that earlier biologists had grossly underestimated the diversity of even this one relatively small genus. When I began my studies, only ten species of Thorius had been formally described, although there were indications from isolated specimens in museum collections of a few additional, new species that needed to be named. Now it is clear that there are at least 25 distinct species, with several more on the way. Indeed, on each field trip that students, colleagues, and I make to southern Mexico to look for Thorius and these days we make about two trips a year we discover an average of one or two new species in need of formal description. In our work we employ sophisticated molecular tools, which can distinguish species that may differ in anatomy only slightly, if at all. This accounts in part for the differences between our results and those of earlier taxonomists. However, other new species look very different from those seen before. These new species went undetected for so long simply because they live in remote montane localities that have never been adequately inventoried or even visited by field biologists, if they've been visited at all. Some of these localities are almost a full day's drive from the nearest paved road; others can be reached only on foot or by horseback. Despite these efforts, it is likely that it will be another several years before we are even close to knowing how many species of Thorius there really are.

The above story, with its message of cryptic biological diversity, is being repeated in group after group of Mexican animals and plants. Tragically, at the same time that biologists are beginning to get a more realistic handle on the true size and richness of Mexico's flora and fauna, the diversity is being severely eroded as a consequence of large-scale human impacts on the natural environment. The recent decline of amphibian populations is a global phenomenon that impacts many countries, and Mexico is no exception.

Throughout much of its natural habitat, Thorius, which at one time was arguably the most abundant tropical salamander, is hard to find. Indeed, there already are examples of new species that appear to be going extinct even as they are being formally described. We still know relatively little about what exactly is causing these declines, and increasingly our field work is being used to monitor the status of once-healthy populations of these and other amphibians. One can only hope that such impacts are minimized quickly, and before they extract an even more severe toll on the spectacular biological heritage of Mexico.

James Hanken is Alexander Agassiz Professor of Zoology and Curator in Herpetology at Harvard University's Museum of Comparative Zoology. He received a 2000-2001 DRCLAS faculty research grant to further his investigation of Mexican salamanders.

See also: Mexico