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Journey to the Amazonian Rainforest

One of my particular interests is the reproductive behavior of frogs, especially of certain small poison frogs that occur in tropical regions of Central and South America. On one expedition in the late 1990s, my field party, including Laurie Vitt, Teresa C. Ávila-Pires, and Verônica R. L. Oliveira, and I worked in the state of Acre in western Brazil, near the small village of Porto Walter. This remote region of Brazil has been relatively unexplored and was known to have a high diversity of amphibians and reptiles, including poison frogs.

To reach Porto Walter, we first flew from the United States to Belém in eastern Brazil to meet our Brazilian collaborators. From there, we flew to the small town of Cruzeiro do Sul. The lack of roads in this area required us to hire a small wooden riverboat to take us to Porto Walter by way of the Rio Juruá. The boat was about 15 ft in length and had two lengthwise wooden benches in the back where the four of us sat, in addition to the bench used by the driver. Fortunately, the boat had a wooden top, although the sides were open and periodically the driving rain left us soaked.

Travel to Porto Walter required two days on the river. Initially, we traveled upstream, and because the river was flooded, as are all Amazonian rivers during the rainy season, the trip was exciting and terrifying at the same time. Our boat was constantly battered about by large branches and entire trees floating downriver. The boat motor was attached to the end of a long pole that enabled the driver to lift the motor out of the water as we glided up and over large trees. At times it seemed like the boat couldn’t possibly make it over the next huge tree coming right for us, but, as we all held our breath, we made it over. When not watching for logs, we marveled at the spectacular rainforest on both sides of the river. Many of the large rivers have been heavily logged to the edge of the water, but in this remote area, large trees covered with lianas and dense vegetation were common. We spotted birds, basking caimans, and other wildlife, including several troops of noisy monkeys.

We could not travel at night because of the danger of unseen logs. Toward evening, we were beginning to get anxious because we had seen relatively few people in this remote area. Fortunately, just as it was getting dark, we spotted a small house. We met the family that lived there and explained that we were looking for a place to spend the night. They were very gracious and invited us to hang our hammocks inside their home. Throughout most of the Amazon region, the small wooden houses have large, uncovered windows, so we were lucky to have nets that covered our hammocks to keep out mosquitoes and large spiders. The next morning, we were on our way again, and after another day of wondering if we could make it over the never-ending logs, we arrived at Porto Walter toward evening.

Porto Walter was a small settlement with no vehicles or commercial buildings. As we pulled up to the dock, a few people stopped to stare at us. The mayor soon heard that strangers had arrived in town. We felt very important when the mayor himself came to greet us. Arrangements were made for us to stay in the local Catholic church, the largest building in the village. The church had many long hallways and empty rooms that gave it an eerie feeling. We were directed to a sparsely furnished room where we hung our hammocks. We questioned our guide about the long streak of blood down one of the hallways and learned that a recent murder had occurred!

The following morning, we began searching for a suitable area to set up camp for several months. We hired a local person who knew the area to help us locate a place. Ultimately, we trekked about 3 mi through swamps and fields to find undisturbed forest. The weather was hot and humid, and by noon we had finished all the water we had with us. We felt weak and exhausted when we stumbled upon a house where the family was using a hand-operated machine to squeeze sugar cane juice. Seeing the state we were in, they kindly offered us cups of juice, which tasted better than anything I have ever had to drink in my life! Eventually we located a suitable area for our camp, and the following day we returned by boat to Cruzeiro do Sul to collect our field gear and to buy enough food, mostly rice and beans, that would be our diet for the next few months.

After several days of shopping and organizing our gear, we hired the same riverboat to return to Porto Walter. During our return trip, we planned to stop for the night at the same house. We arrived in the evening and were surprised to find no one there. Eventually an older man appeared and told us the whole family had contracted malaria and was in the hospital in Cruzeiro do Sul. We were again invited to spend the night in the house, and this time we were even happier we had our mosquito-proof hammock nets!

Once we arrived in Porto Walter, we hired several local men to help transport our supplies and equipment and set up a camp at our chosen site. We began to survey the area, determine what species of amphibians and reptiles were present, and develop our individual and group research projects.

Verônica, at the time a Brazilian graduate student, shared my interest in poison frogs and was eager to participate in the research. Although it was one of her first field trips, she was enthusiastic about spending long days watching tiny frogs while not moving and fighting off the abundant, hungry mosquitoes. Her positive attitude and adventurous personality made our work much easier!

We chose to focus on a particularly common species, the Brazilian Poison Frog (Ranitomeya vanzolinii). Several characteristics of these frogs made it possible to study their behavior. They were active during the day, and they were black with a few bright yellow irregular spots, making it possible to sketch the spotting pattern and follow and record the behavior of the same individuals day after day. The frogs occurred in the forest understory, generally not going more than 6 ft above the forest floor. Verônica and I had no idea about the amazing behaviors that we were going to observe. Our suffering with the mosquitoes in the hot, humid weather was rewarded with almost daily exciting revelations.

Unlike many species of frogs, poison frogs lay small clutches of eggs, usually fewer than 25, in leaf litter on the forest floor or in bromeliads or leaf axils in vegetation, as opposed to in ponds or other bodies of water. Once the eggs hatch into tadpoles, the male parent (but sometimes the female, depending on the species) carries the tadpoles on its back either singly or in a group to temporary pools, streams, or a water-holding plant such as a bromeliad, where the tadpoles complete their development and transform into frogs.

Initially, we found that we could locate the same male frogs in the same general areas every day, indicating that they maintained individual territories. We often observed pairs of males alternating calls, announcing the boundaries of their territories. As we got to know individual frogs from their yellow spots, we were surprised to find that the same males and females were always found together in the same territories.

It was easy to distinguish males from females. Females were heavier bodied because they were frequently gravid with developing eggs. Although males called nearly every day, their calls became more intense and more frequent when a female appeared. Further, when a female appeared in a male’s territory, we noticed that the female followed the male as he moved around.

At times, we found ourselves laughing at the frogs’ antics. As scientists, we avoid anthropomorphic terms when describing behavior in animals, but occasionally we couldn’t resist viewing their behavior in human terms. One male with a female following him took a huge leap from a large leaf to another leaf about 3 ft below. Instead of landing on the leaf, he bounced off the leaf high into the air and landed on the ground. He sat there for a long time, not moving, and appeared to us as if he wasn’t sure what had just happened. He eventually recovered and climbed the nearest tree. On another occasion, a fat, gravid female was following a male. The male jumped off a big leaf and landed on another leaf about 4 ft below. The female moved to the edge of the leaf and looked over. From our human point of view, she seemed to be thinking no way! as she backed off. Meanwhile, the male called frantically from below, trying to convince her to jump. After coming to the edge and backing off three more times, she finally got the courage to jump and landed safely.

Ultimately, after watching the interactions between males and females, we discovered that males were not leading females around randomly but were leading them to small holes in a sapling tree or a woody vine. Often the female, but not the male, would enter the hole, remain for a short period of time, then leave and not reappear for several days.

At first, these observations were puzzling to us. We had also seen several males, each with a single tadpole on its back, enter holes and exit without the tadpoles. We investigated the holes with a flashlight and discovered that many of them held a small amount of water and contained a tadpole. In some holes, two or three small eggs were attached to the side of the hole above the waterline.

Because the holes in which these eggs and tadpoles were deposited were very small, behavior of the frogs while in the holes was difficult to watch. Therefore, we placed several small plastic containers filled with water on a log a foot or so above the forest floor. After a few days, two tadpoles appeared in one of the pans. While watching the tadpoles, we observed a female poison frog approach and back into the water. As she did, the tadpoles approached her and began to nibble at her back legs. This activity stimulated the female to release an egg into the water. The tadpoles immediately attacked the egg, breaking through the outer membranes and consuming the yolk. Verônica and I were amazed at this observation and immediately began piecing together our observations.

We ultimately realized that males and females of these tiny frogs were cooperating to raise their young. It was truly a surprising revelation at the time because biparental care in frogs had not been described. (In recent years, a few more species of poison frogs have been found to have this behavior.) The male’s role, in addition to fertilizing the female’s clutch of two to three eggs, included transporting the tadpoles individually to a small vine or tree hole and later guiding the female to the holes so that she could provide nutritive eggs for the tadpoles. The small opening of the holes did not allow sunlight to enter, so the females’ eggs were the only food available to the tadpoles. A female frog was capable of producing eggs every four days; thus, the tadpoles were periodically fed until they metamorphosed into froglets.

Many questions remain unexplained about the behavior of these frogs. For example, as in birds, could a male be cooperating to raise young with more than one female at a time? We were shocked near the end of our time in the field when we discovered one of the males we were following was leading an unknown (to us) female to a small tree hole. We had seen this male previously leading a different female to a hole with a tadpole. Could this male have two clutches he was caring for with different females? We were unable to follow the frogs for more than one season, so it is unknown whether the same males and females bond to care for offspring in successive seasons. Indeed, we do not even know whether these tiny frogs have a lifespan of more than one year. Like almost all research, many additional questions arose as we continued to learn about the behavior of these frogs.

After several months of living in the forest, it was time to return to civilization. We were grateful to the people we met in Porto Walter for their kindness in helping us with so many different aspects of our trip. We visited many of them on our last day in the village to say farewell and express our gratitude. The trip back, again by riverboat, required one day, rather than two, because it was much faster to travel downstream. We were happy to see that fewer logs were floating down the river as the end of the rainy season approached!