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A Rainy Evening in the Pantanal

One of the wonders of being a field biologist is witnessing events no one has ever seen. In late spring of 2000, something special happened during my fieldwork in the Brazilian Pantanal, an event that would guide my academic career.

With approximately 210,000 km², the Pantanal is one of the world’s largest floodplains. It is located in central South America, mostly in Brazil but extending into Bolivia and Paraguay. The flat and open landscape, dominated by native grasses and some patches of forest, resembles a savanna with a patchwork of rivers and seasonal ponds. It is a place full of life, and of extreme contrasts. Temperature often exceeds 40 °C in spring/summer, but during the winter it may suddenly drop to zero or below. Floods are seasonal and may last four to five months. At the peak of the dry season, in September or October, the landscape becomes xeric, green leaves turn shades of gray and brown, and depending on the year, most of the ponds dry out, the rivers barely flow, and animals perish. But this cycle of floods and drought sustains the Pantanal’s rich biodiversity and enormous animal abundance.

I fell in love with the Pantanal and frogs when I was an undergraduate student at the Federal University of Mato Grosso do Sul. During my first year in the biology course, one of our professors organized a trip to study the embryonic development of amphibians in the field. The Pantanal was completely flooded, and our bus could not reach the university´s research station. Instead, we rode in boats to the station. I was very excited with that adventure and the opportunity to see all the caimans, capybaras, and birds up close. Of course, not everything was so pleasant; it was extremely hot and humid, and there were tons of mosquitoes around us. At night, we finally went to the ponds to collect frog clutches with our professor and a veteran colleague who knew the species. I was really impressed by her ability to listen and identify the frog species only by their calls, and I thought to myself: I want to be like her! I can say that I was successful.

Between 1992 and 1999 I conducted fieldwork around the research station at the margins of the Miranda River, about 370 km from the main campus. During six consecutive years, I visited the field at least once a month to collect data for my undergraduate and master’s work. In 2000 I began my doctorate and moved to São Paulo state. I decided to continue my study of the frog reproductive strategies in the Pantanal, as I had collected enough data for most of the species except the frogs that reproduce only following heavy rains and then disappear. Contrary to what many people think, rainfall is low in the Pantanal and heavy rains are rare. The floods are mainly due to rains in the surrounding highlands; water of the tributaries flows down to the plain, causing the main Pantanal rivers to overflow. I needed to live in the Pantanal. For two rainy seasons, from October 2000 to March 2001, and the same period the following year, my home was the Pantanal research station. Those were the happiest days of my life. No internet, no cell phone signal, no TV or radio. Most of the time I had little contact with people. The mornings were welcomed by the sound of many different birds and the sun rising over the river, with Jabirus flying to cross to the other side.

I had a borrowed laptop, and I took along two boxes—one full of scientific papers and another with my favorite CDs and a CD player. My doctoral advisor, Célio Haddad, lent me a Canon film camera and a Uher sound recorder. I arrived at the Pantanal research station at the peak of the dry season, in early October 2000. Everything was dry and extremely hot. There were no ponds nearby, and the river level was very low. After a month of searching the sky for clouds, finally a heavy rain arrived in mid-November. It rained the whole night, and a pond formed beneath the research station, which had been constructed on stilts as protection from floods. The next day remained cloudy and the rain turned to drizzle. In the afternoon, I heard a frog call I had never heard before, even after all the years collecting in this area. I changed into my field clothes and rubber boots, grabbed my field equipment, and began exploring the recently formed pond.

Suddenly, everything changed after the rain. There were at least 15 different frog species calling at the same time in that recently formed pond. In the middle of that explosion, I listened to that unknown low-frequency call again. After searching for a while, I finally found the frog. I was surprised that the frogs calling were males of the Miranda’s White-Lipped Frog (Leptodactylus macrosternum, formerly L. chaquensis), a very common species in that region. Males can attain 8 cm in length; they have hypertrophied arms and spines in their thumbs that are used in male–male combat. I already knew that the species was a foam-nesting breeder and that females cared for eggs and tadpole schools, easily observed in spring/summer. However, during my previous visits to the field, I had not observed a breeding event because the species is very explosive, reproducing only for one or two days. After many years of fieldwork, I was listening to their calls and observing their spawning behavior for the first time. Amazing!

It was midafternoon, on 13 November 2000, still cloudy and drizzling, when I observed a pair of L. macrosternum spawning, but they jumped out from the foam and hid in the vegetation when I approached. At dusk, I returned to the same site, and there were two males calling and fighting in the middle of the foam nest previously initiated. An hour later, there were four males calling and fighting. The resident male was large, and the other males were visibly smaller in size. I was by myself and so excited that for a moment I could not believe I was observing this behavior. I did not know what to do at first, as I did not have a video recorder with me, so I started taking pictures and tried to record their aggressive calls. The frogs continued wrestling, and the resident male eventually clasped another male with the forelimbs, both coming out from the foam chest-by-chest pressed together. Additional males showed up, and when there were eight males wrestling, a hidden female jumped into the nest, and the resident male grasped her and immediately they started spawning. At the same time the other seven males began to kick the foam nest with their legs in synchrony with the amplectant male. At that exact moment, the film of my camera finished. Fortunately, based on some of the pictures I took, a technician from our department could draw the multimale spawning behavior, and I was able to publish this observation, which my advisor, Célio, and I interpreted as an alternative reproductive behavior, with sneaker males trying to fertilize some of the eggs released.

I remained at the research station until March, going to the field almost every night, and also the following summer, but I never observed that behavior again. At the time, although there were some reports of simultaneous polyandry for a dozen frog species, this was the first record of this behavior for a South American frog. I am very happy I could witness that behavioral event on that rainy evening in the magical Pantanal floodplain.