Secrets Of The Flooded Forest

by John Thorbjarnarson, Ronis Da Silveira

Where do Amazonia's top aquatic predators nest? Scientists find the hidden nurseries of black caiman.

"It is scarcely exaggerating to say that the waters of the Solimoens are as well stocked with large alligators in the dry season as a ditch in England is in summer with tadpoles," wrote Henry Walter Bates in 1863. A British naturalist, Bates was astonished by the abundance of "alligators"--or black caiman--particularly in western Brazil, where the Amazon River is referred to as the Solimoes (Bates's Solimoens). For more than four years, Bates lived in the sleepy village of Ega, now called Tefe, and observed the natural history of the region, including the habits of the area's top predator, the black caiman.

During the mid-twentieth century, the black caiman became the target of intensive commercial skin hunting and in the 1970s was declared endangered. With the cessation of hunting, black caiman have made a comeback in some areas. Today the largest known population resides in the Mamiraua Reserve, just upstream from Bates's old haunts. A complex of islands at the confluence of the Amazon and Japura Rivers, the reserve encompasses 2.7 million acres of forest, lakes, marshes, streams, and river channels. Yet every year the distinct features of the landscape are largely submerged, for this is the land of the flooded forest, or varzea, that borders the white-water rivers of the Amazon basin. Water ebbs in September and October and begins to rise in November and December, reaching its annual peak in June. During an average year, the water level in the reserve can rise thirty to forty feet.

In the varzea and in other Amazon aquatic lowlands where they are numerous, black caiman have probably long played an important ecological role. Yet little was known about the natural history of this large reptile--and in particular about its nesting habits in the varzea--when we began to study it in 1993. We have found the Mamiraua Reserve, which was first established in 1990 (see p. 75), to be the ideal location for our work. Here the eminently practical solution both for staying dry and for doing research is to live in a floating house, or flutuante, built on massive logs from the spiny-trunked assacu trees common in the varzea. Our project's buoyant house, named the Cauacu, after a local palm, is situated in Lake Mamiraua, one of the largest lakes in the reserve. We rope the house to trees along the shore to prevent it from drifting away when we are buffeted by strong winds during the area's frequent tempests.

When the Amazon is high, the reserve is a water world where the trunks of trees disappear into dark depths that are home to freshwater dolphins, more than a hundred species of fish, and the occasional Amazon manatee, as well as black caiman. Caiman belong to the group known as crocodilians, which also includes alligators and crocodiles. Black caiman are slightly larger than American alligators--adult males reach lengths of seventeen feet--but they are not easy to spot from our floating station at this time of year, being widely dispersed throughout the flooded forest. Using an old hunter's trick, however, we imitate the caiman's calls and find that they reveal themselves by bellowing in return, at times in an impressive chorus of voices that emanate from the surrounding forest. Later, during the height of the dry season, the tree trunks emerge from the water, and the lakes and streams take shape. Caiman from nearby areas concentrate in the upper end of Lake Mamiraua, which takes on a primeval look as thousands of black caiman rub shoulders. In some areas the eyeshines that reflect our headlamps are so numerous that they resemble the distant lights of a modern metropolis--a startling sight in the black Amazonian night.

We knew that black caiman needed dry land near the water's edge for nesting and that, like alligators, they laid their eggs in mounds of decaying vegetation scraped together by the female. So during our first year in Mamiraua, we began looking for nests, during the dry season, in the most logical place we could think of--the forested banks that line many streams in the reserve. These are some of the park's most elevated areas and are flooded for only one to two months at a time. We thought they would offer female caiman the best chance of keeping eggs safe from rising water during the two to three months of incubation.

But the nests we found along the stream banks were those of a smaller species, the spectacled caiman. Situated in the dense shade of the trees, the nests were occasionally as much as several hundred yams from any permanent water source. We would sometimes also discover a four- to five-foot-long female not far from the nest, hidden under leaves or wedged beneath a fallen log. Many of the nest mounds had been opened by predators, and in some areas, half the nests had been lost to egg-raiding animals. We were surprised by this finding; the entire reserve constitutes a group of islands even when not flooded and is thus an inhospitable place for the usual caiman-egg predators, such as raccoons and foxes. Some nests had clearly been raided by tegus--large black-and-white lizards common in the reserve--and jaguar footprints and scratch marks on trees near nests implicated the big cats as well. Typical jaguar prey (including agouti, deer, peccary, and tapirs) are not found on the varzea islands, so during at least part of the year, the jaguars turn to caiman eggs and, in some cases, we discovered, to the nesting females themselves.

Black caiman nests, however, remained a mystery. It was not until we started working with local fishermen who knew their way into some of the most inaccessible lakes that we hit the mother lode of black caiman nesting sites. To reach some of these lakes, we spent hours paddling or poling up vegetation-choked streams, or dragging a canoe through the forest that separates the lakes from the main rivers and channels. Some of the lakes are full of the floating mats common in the varzea. These range from flimsy "floating meadows" of intertwined grass to thick rafts of peatlike material that often support rooted trees and are likely to support nests as well. While we found some black caiman nests on the mats, most were on land, within two or three feet of the lake's edge.

Black caiman appear to be more selective about where they nest than their spectacled cousins are. The edges of Lago Sarapaio, a small forest lake, were home to many nests, while the perimeter of Lago Samaumeirinha, a very similar lake in many respects, had none. On our first excursion to the lakes, while paddling and portaging between the two, we noticed that the water level of Lago Sarapaio, judging by vegetation and water marks, appeared to have been static over the previous few weeks. On Lago Samaumeirinha, in contrast, we found evidence that the water level had dropped about two and a half feet just in the preceding few days.

While much of the Amazon floodplain appears to be a flat, monotonous landscape, we found that by virtue of slight topographical differences, the water levels of certain lakes are decoupled from that of the Amazon River, with its great yearly oscillations. These lakes are to some degree raised or "perched," and their water levels are determined by the height of the surrounding land and by the streams that lead from the lakes into the main Amazon. From October through December, as the river falls and then rises, water levels in perched lakes can remain high--up to thirteen feet higher than the Amazon at its lowest--and also stay relatively constant. The perched lakes make ideal nesting areas for black caiman because they allow females to build their nests right along the edge of the water. Brief periods of intense rainfall can temporarily raise water levels, but they usually return to normal after a few days, and caiman eggs appear to be highly resistant to occasional flooding.

Since 1993 we have found many more of the small, hard-to-reach, vegetation-clogged lakes that serve as the breeding nuclei of black caiman in the Mamiraua area. They are always full of the reptiles. Once we understood the hydrology of the preferred nesting sites, we set out to gather information on black caiman numbers and habits. While mothers defend the nests against animal predators, only the most aggressive 2 or 3 percent dare take a stand against humans. Even so, counting the eggs in a caiman nest does require a bit of caution. Nest predators are the same as for spectacled caiman, the top three being tegu lizards, jaguars, and humans (who collect the eggs for consumption). Jaguars, however, appear not to prey upon black caiman females, which can grow to almost double the length of their spectacled counterparts. And while spectacled mothers hide out near nests on land, black ' caiman females station themselves near nests but in the water.