The toads of Mexico

Recently I completed a complete draft of a manuscript that, essentially, I had been working on since 1989. It’s a few hundred pages long and reviews everything that is known about the toads (Family Bufonidae) of the amazing nation of Mexico. eventually it will be a component chapter of a broader effort entitled The Amphibians and Reptiles of Mexico which will likely fill an entire bookshelf, a testament to the mega biodiversity of that country.

I really like toads and, by extension, all types of frogs. I grew up in San Diego, California, where there we two native species of frog that I could find, plus the ever-present invasive American bullfrog (Lithobates catesbeianus), and a couple of species of leopard frog (Rana spp.) that disappeared before I ever saw them. Nevertheless, I grew up primarily fascinated by snakes—there was an amazing diversity of native snakes I could find there. I left California to pursue graduate studies in herpetology, and that landed me in Texas. On my first visit to Texas, I found a toad in the sand dunes on a barrier island while I was at a beach party. It was a Gulf Coast toad (Incilius nebulifer) and I was immediately enamored of it. It had these cool bony crests atop its head that reminded me of a crown.

In Texas, I also was impressed by the diversity of frogs and toads to be found. In 1989, my graduate advisor arranged for me to conduct a solo expedition to survey amphibians and reptiles in a remote corner of Guatemala. This was my first foray into the tropics and, in doing my pre-trip homework, I realized that this same species of toad likely would occur there. That turned out to be true. But on 29 July 1989, I found a similar toad that just did not look quite right to me. Skipping ahead many months and after reviewing museum collections from eastern Guatemala, I found a few specimens that matched my unusual toad. For the first time in my career, I had discovered a species new to science. I was thrilled and, truthfully, rather stunned that a tropical neophyte could discover new species. I was honored to formally name the species a Campbell’s forest toad (Incilius campbelli) in honor of my graduate advisor Jonathan Campbell, who was raised in Guatemala and devoted his career to documenting and conserving the biodiversity of the country.

After finishing my master’s thesis—on tropical snakes—I started my doctoral program at the legendary Museum of Natural History at the University of Kansas, renowned for its history of neotropical herpetology. I couldn’t get that toad discovery out of my mind and, in doing the research involved in naming the new species, I became aware that there may be additional unknown species, mostly in Mexico. So, I decided to make a review of these odd crested toads of Texas, Mexico, and Central America the focus of my dissertation.

Whew! Over 30 years of lab and field work later, I *think* I have them all sorted out and their distributions mapped. Completing my small monograph on the species in Mexico feels like a career capstone to me that is both rewarding and bittersweet. For those of you just dying to know the details, I’ll tell you that there are 34 species of toads in Mexico, most of which are these crested toads, genus Incilius. My work over the years resulted in the discovery of five of these species and also in resurrecting three ancient species names from previous centuries that had been proposed but that had been overlooked or improperly dismissed. I know you are waiting to read the entire piece yourselves—it’s a real page-turner of toad nerd-dom, I’ll assure you! But I learned a few things in the process.

Exciting discoveries of new species aside, I also clarified a couple of species that do not occur in Mexico. One such species, Incilius luetkenii, has always been listed as an inhabitant of Mexico. It turns out that this listing was the result of a single museum specimen that was misidentified. That species is abundant in Guatemala and southward into Central America, but does not actually occur in Mexico. Another species, I. gemmifer¸ had been named in 1940 and confused everybody who cared since that time. Along my journey, I decided that the toad apparently was extinct. Sadly, I’m no stranger to considering recently extinct species of amphibians, but from what I could piece together, this toad did not at all fit the pattern of other regional amphibian extinctions caused by the terribly pathogenic amphibian chytrid fungus or even habitat destruction. It had occurred on the coastal lowlands near the easily accessible city of Acapulco. But I certainly could not find anything there that seemed to fit the description, and nor could any of my colleagues. There were plenty of toads outside Acapulco, but not this one. Finally, I decided to start from scratch, so I studied the original specimens collected in 1940 that are housed in the Field Museum of Natural History in Chicago. I was in Chicago to give a lecture at the Shedd Aquarium and stayed an extra day to spend at Field Museum. I recognized them immediately as the regionally abundant Sinaloa toad (I. mazatlanensis). The herpetologist in 1940 had made a taxonomic mistake and so the toad was not extinct at all, and a generation of herpetologists afterward had been summarily confused. Oops. The same herpetologist also named I. mazatlanensis in the same 1940 publication, and that coincidence contributed to the confusion. But what wonderful news to discover that a species was not, in fact, extinct! 

In contrast, however, the Sierra Madre del Sur toad (I. cycladen) from that mountain range in the states of Oaxaca and Guerrero had been named in 1966 and dismissed by herpetologists as a bogus species. I was not convinced that it was bogus, and I set out to find it. During a very long trip in 2000, my colleagues and I found and photographed a single individual.

A group of colleagues visited the region a couple of years later and did not find the toad. I returned there, with a large group of excellent field herpetologists, in 2013 and we could not find it. It has never been seen since 2000, and only one image of it exists. On that same trip in 2000, we also found a species of treefrog and a streamside frog that also have never been seen again. While the timing seems about 15 years too late to fit our understanding of the epidemic of amphibian chytrid fungus that ravaged Mexican frogs, the general pattern certainly fits the scenario of yet additional pathogen-driven extinctions. I certainly hope that I am wrong about that.

I also had looked diligently for another species, the crested forest toad (I. cristatus), in the Sierra Madre Oriental, on the other side of the country, in the states of Puebla and Veracruz, but I never could find a population. I had personally decided that it must be extinct, presumably because of the fungus. But some colleagues managed to find three populations in places I had not managed to survey. Great news! Critically endangered is much better than Extinct!

Another enigmatic toad deserved some special attention. The Tacana toad (I. tacanensis) was named in 1952 from Tacana Volcano, which straddles the Mexico–Guatemala international border. I have only spent a tiny amount of time within the known range of the species, on the Guatemala side of the border and, unsurprisingly, I never found this odd little toad that has no eardrums. During my reviews, however, I realized that this toad has only been sighted by scientists a few times. I found one photograph circulating on the internet of it and realized immediately that the photographed individual was a misidentified specimen of a different, and regionally abundant, toad. To this day, we have no photograph of this species in life. As far as I can tell, it was last seen by a herpetologist from University of California Berkeley in 1983. That was the year that we believe the chytrid fungus spread through southern Mexico, so this apparent disappearance completely fits the sad scenario of extinction. Literally, on the last day I was working on my manuscript, I became aware of a few records in a small museum collection in Chiapas, Mexico, as recent as 2003. I am working now to see if I can examine these records—in hopes that they were correctly identified.

So, after 30 years of chasing mystery toads through Mexico and Central America, I think I am now done. I have a suspicion that there remains one more as-yet-unnamed species in the lowlands of Veracruz State, Mexico, on the Caribbean side of the country. I saw it one time. It currently goes by the name of marbled toad (I. marmoreus) which otherwise is known to be abundant along the Pacific Coast of Mexico. That distribution does not make any biogeographical sense to me. Maybe one day I’ll sort that out, or maybe I’ll leave it to someone else. I mention this suspicion in the manuscript, in hopes that it will pique the interest of someone.

What shall I do with my career now? It’s been largely framed by the biology and conservation of these toads. For the moment, I’m finding myself drawn back to my original passion of snakes. I really like snakes too.

(photo courtesy Joe Mendelson)

Joe Mendelson, PhD
Director of Research