Study Area

The Guaporé River flows through an area rich in biodiversity and natural attractiveness, in the transition of the 2 important Brazilian ecotones, the Pantanal and the Amazon. It starts at Chapada dos Parecis (Mato Grosso State), 630 m above sea level and discharges into the Mamoré River, a tributary of the Madeira River. In its whole extension of approximate 1400 km, 1150 km are navigable from Vila Bela da Santíssima Trindade (Diegues 1994). The Guaporé River, in its whole extension in Rondônia State represents the border between Brazil and Bolivia (Fig. 1). It is a black-water river, and there are many lakes, known locally as baías, that connect with the river in the flooded season. At the beginning of the falling water level season (September to November), known locally as vazante, P. geoffroanus is the first turtle species to nest on high ground on the banks of the Guaporé River. At this time, high sand beaches (known locally as tabuleiros) are exposed where Podocnemis expansa and P. unifilis nest. Podocnemis expansa specifically nests on coarse sand beaches where the incubation temperatures are the highest for any known turtle species, while P. unifilis is less restrictive in its nest site selection, laying up to 2 clutches of eggs per year, in clay soil river banks, in fine sand near the river's edge, or on coarse sand beaches. Since incubation temperature controls the sex in these 2 species they are more particular about where they nest, and wait for a time when temperatures are higher.

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Monitoring

This study is a compilation of data collected from June to September 1989 and July 2008. Incubation experiments in the laboratory were conducted to confirm what species was collected. We described nesting sites, nests, clutch size, egg size, and survivorship of nests from lizard predation.

In both years we started monitoring the banks of the Guaporé River searching for P. geoffroanus nests in the morning along the margins of the river between 0600 and 1000 hours. Fresh nests were identified by the small humid mound of soil, which had been recently moistened by the viscous fluid from the female's cloaca, a peculiar characteristic of P. geoffroanus nests. In older nests this fluid in combination with the nest substrate becomes rock hard, and possibly functions to inhibit small egg predators such as the small lizard Ameiva.

Every morning we chose a different transect between the Divisa Sector in Brazil (lat 12°29′599″S; long 63°59′074″W;World Geodetic system 84) and Versalles village in Bolivia (lat 12°41′334″S; long 63°15′241″W). We did not repeat transects navigating 12 km/h in order to spot nests. Once we found a nest, we recorded the point using a Garmin model 50 GPS. In 1989 we counted and measured egg diameters and weights. In 2008, we also noted if the nest was depredated by lizards or trampled by cattle. In addition we measured nest diameter, maximum nest depth, and the distance of the nest from the river. All eggs were weighed with an electronic digital balance accurate to 0.1 g and were measured in length and width with digital callipers accurate to 0.1 mm.

Eggs from 1989 were measured in the field and taken to Costa Marques to be incubated at the Brazilian Institute of Environment and Renewable Resources (IBAMA) lab. Eggs from 2008 were placed in styrofoam boxes filled with humid sand and taken to Manaus where they were measured. The data on controlled temperature incubation experiments will be published elsewhere.

Statistical Analysis

We compared the clutch size between 1989 and 2008 conducting an independent samples test. We also tested to see if there was a significant difference in egg length between 1989 and 2008 by conducting a mixed model of nested analysis of variance (ANOVA) with 2 factors: year as a fixed factor and nest within year as a random factor. We used a mixed model to test the relationship between egg length and nest diameter (covariate) and separately egg length and nest depth (covariate). We carried out 3 logistic regressions to analyze the correlation among physical features and status of predation. For the logistic analysis, we considered the predation status (1  =  depredated; 2  =  not depredated) as dependent variable and distance from the water, depth, and diameter of the nest as independent variables. All analyses were calculated using PASW statistic 17.

Deforestation Rate Analysis

Since 2000, the Instituto Nacional de Pesquisas Espaciais (INPE) has been processing LANDSAT satellite imagery and using geographic information systems (GIS) to assess changes in forest cover. We used these spatial data layers to calculate the percent deforestation rate from 2000 to 2008 by dividing the current loss in forest cover by the total forest cover in 2000. The area used for the deforestation rate calculation was set as the 60 km distance area from the edge of the river on the Brazilian side of the study.

The image provided by INPE was classified into deforestation areas, forest, and nonforest. The deforestation area represented the destruction and eventual removal of the forest cover considered by the Prodes Project for raising cattle and the cultivation of agricultural crops. The forest area represented the Amazon forest formation considered by the Prodes Project. Nonforest area represented cerrado and woody savannas, characterized by a variety of forest areas with a tree height of up to 20 m to nearly treeless grassland areas with only few or no shrubs. We did not consider values for nonforest areas in our calculations since INPE has not monitored deforestation rates in this area. We did not calculate deforestation rates for the Bolivian side because the study area corresponds to a protected area (Reserva Forestal Itenez) and has no deforestation detected along our transect area (Fig. 2). The deforested area calculation was done by using the polygon select and summary statistics functions of ArcGIS 9.2.

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Nest and Egg Data

In 1989 we found 10 nests. The mean clutch size was 9.2, SD  =  1.7 (range 7–14, n  =  10); the mean egg length was 3.2 cm, SD  =  0.15 (range 2.9–3.5 cm, n  =  92); the mean egg weight was 17.81 g, SD  =  2.18 (15.2–22 g, n  =  92). There were depredated nests as well, but they were not counted.

In 2008 we found a total of 46 nests, of which 38 were depredated, 3 partially depredated, and 5 intact, for a total of 89.1% of nests depredated, all by Tupinambis teguxin lizards. The confirmation of lizard predation of the eggs is that they leave different shaped holes from their teeth in the egg shells than mammals. We also saw lizards eating the eggs, and there were also lizard tracks in the substrate around the depredated nests.

Mean clutch size in 2008 was 12.8, SD  =  2.9 (range 10–16, n  =  5); mean egg length was 3.2 cm, SD  =  0.050 (2.9–3.4 cm, n  =  68); mean egg weight was 18.56 g, SD  =  2.23 (15.4–22.5 g, n  =  92). Mean nest opening diameter was 9.6 cm, SD  =  1.8 (range 5.3–12.3 cm, n  =  46), mean depth  =  12.6 cm, SD  =  2.1 (9–15.9 cm, n  =  46) and mean distance of the nest from the water was 5.3 m, SD  =  4.0 (1–20 m; n  =  46).

The Levene's test for equality of variances gave a significant result between mean clutch size in 1989 and 2008 (p  =  0.049) (Fig. 3). By conducting the ANOVA nested analysis we found no significant difference in egg size between 1989 and 2008 (p  =  0.827). Also, there was no significant relationship between the size of the eggs and size of the nest (p  =  0.550 for diameter and p  =  0.912 for depth). We also found no correlation of distance from the water, depth, and diameter of the nest with predation status of the nests (p  =  0.080; p  =  2.68; p  =  4.04, respectively).

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Characteristics of the Nest

The openings of the nests were narrower than the nest chamber, with a small mound in the posterior region. In the chamber, eggs had little or no space between them. All the nests were found in river banks with hard-packed soil. When a female had recently ovoposited, we observed a clear viscous fluid in the nest, which perhaps helped to maintain the shape of the nest cavity as well as covering the nest. After nests were covered, there was a mound of mud that was useful for us to locate them.

Characteristics of the Eggs

Eggs of P. geoffroanus are nearly round and strongly calcified. In 2008 we transported all 74 eggs in styrofoam boxes with moist sand, first by boat, then bus, truck, taxi, and finally airplane to Manaus. Only 3 eggs did not develop; this was a high rate of survivorship considering all of the traveling done. All the nests had eggs develop, and we were able to confirm that all the eggs collected were P. geoffroanus. Data from the 1989 analysis of female reproductive tracts, by measuring diameters of corpora lutea and ovarian follicles, demonstrated that this species nests 2 to 4 times during the June–August nesting season in the Guapore River.

Deforestation Rate

Since 2000, ca. 40% of the study area in the Brazilian site has been deforested, corresponding to a total of about 2092 km² of forest, with 60% (3213 km²) remaining as intact forest.

Nest and Egg Data

Our clutch size data for 2008 ranged from 10 to 16 eggs and corroborates Medem's (1960, 1969) data of 10 to 20 eggs per clutch. Wied (cited in Bokermann 1957) registered 12 to 18 eggs per clutch and Souza and Abe (2001) noted a range of 8 to 20 eggs. The records from captive colonies are more variable; 13 nests ranging from 8 to 25 eggs per clutch (Guix et al. 1989) and 30 nests with 8 to 25 eggs per clutch (Molina 1998). The species most closely related to P. geoffroanus are P. hilarrii and P. williamsi (Rhodin and Mittermeier 1983), and they lay 8 to 32 eggs per clutch (Astort 1984).

Our mean clutch size recorded from 1989 was significantly smaller than from 2008, but still comparable to published records. Since the difference between 1989 and 2008 in this study refers to the same area, this increase in the clutch size could possibly represent clutches of eggs from the same females that increased in size over this 19-year period, with little recruitment due to high predation of the eggs; however, this is only speculative.

In relation to the size of the eggs and clutch size, we expected a relation between them as reported earlier for P. geoffroanus (Souza et al. 2006). However, in our study there was no relationship between egg and clutch size.

The mean nest opening diameter of 9.6 cm in the Guaporé River site is similar to that in the São Paulo Zoo, with a mean diameter of 8.7 cm (Molina 1998). As well, our mean nest depth of 12.6 cm was similar to the data recorded in São Paulo Zoo of 15.1 cm (Molina 1998). There are no other studies recording nest measurements of this species in nature.

This is the first study recording nest predation by Tupinambis teguxin lizards. This rate of 89.1% is high, and local ranchers remarked that more nests are now being depredated with the forest having been cut down since 20 years ago. We expected that the predation rate would be correlated with the distance of the nest from river, as found for Podocnemis unifilis (Escalona and Fa 1998) and P. erythrocephala (Batistella and Vogt 2008) regarding the distance from vegetation. However, the number of nests not predated was low, and we did not leave these nests intact to observe predation. We cannot document if predation has increased with deforestation since this is the first known study regarding predation of P. geoffroanus, but further studies are needed to check deforestation impact on the nesting survivorship of this turtle species.

Characteristics of the Nests

Our data regarding the nesting sites chosen by the turtles corroborates Guix et al. (1989) and Molina (1998) describing areas covered by low shrubs and clay soil. However, Wied (cited in Bokermann 1957) and Medem (1960) described P. geoffroanus nests constructed in sand banks. In the Guaporé River of the Amazon, P. geoffroanus nests on the clay river banks probably because this is the first land available to nest—by the time the sandy nesting beaches where Podocnemis nest are exposed, P. geoffroanus has finished nesting. By nesting in the first exposed areas they are also ensuring that these will be the last areas to become flooded, providing their eggs with sufficient time to incubate. The shape of the nest has the opening narrower and the deeper portion wider as an incubation chamber; this is typical among chelonian species (Cagle 1950; Vestjens 1969; Moll and Legler 1971; Foote 1978; Guix et al. 1989; Molina and Gomes 1998a, 1998b). The secretion of a viscous liquid during the deposition of eggs was also described for P. geoffroanus by Molina (1998) as a function to lubricate the eggs, as well as to help the female maintain the open nest cavity and to cover the nest. There are many explanations for this liquid (Patterson 1971), but Molina (1998) surmised that this liquid seems to help moisten the soil and help in nest construction. We documented that the substrate in the nest cavities in which eggs were recently laid, only a few minutes before, were indeed moist. Two to 4 days after the females laid the eggs, the mud nest plug had become hard, perhaps preventing the entrance of small lizard predators.

Characteristic of the Eggs

Our description of the hard, spherical eggs of P. geoffroanus is much like the description by Medem (1960, 1969) in Colombia, Wied in Bahia (cited in Bokermann 1957), and Molina (1998) in São Paulo. Molina (1998) described eggs with a calcification problem, but this was not recorded in our study in nature. The eggs we found were strong and not very sensitive to movements, since they traveled long distances and withstood many bumpy situations by bus and by airplane. Further descriptions of the eggs and their development are being prepared for a separate publication.

Period of Nesting and Predation Threats

The period of incubation differs from other studies in different parts of South America. Eggs from the Guaporé River incubated under controlled laboratory temperatures of 25°C and 30°C took up to 150 days to hatch (Vogt 2008), while eggs from females of unknown origin in the Sao Paulo Zoo hatched in 149–331 days (Molina 1989). Wied (cited in Bokermann 1957) recorded that the nesting season was from December to February in nature in Bahia. In Colombia, the nesting season occurs from November to February (Medem 1960). In captivity, P. geoffroanus nested in the São Paulo Zoo from March to November (Molina 1998). Similarly, P. geoffroanus nested from March to September in captivity at Ribeirão Preto, São Paulo (Guix et al. 1989).

The high predation rate of P. geoffroanus recorded in this study can be related to its nesting period. After the flood waters recede, P. geoffroanus is the first species to lay eggs along the first river banks to emerge. In this part of the Amazon, the deforestation has increased recently due to clearing land for cattle ranching and sale of illegal forest products, particularly lumber. In the study area, ca. 40% of the forest has been cut down since 2000. Consequently, the teiid lizard Tupinambis teguxin is losing forest habitat and primary prey. During the high water season food availability for a carnivorous lizard is scarce; however, when the flood waters begin to recede, P. geoffroanus is the first freshwater turtles species to lay eggs in this area, creating a seasonal food resource for the lizards.

There are some studies reporting that T. teguxin is sensitive to habitat alteration and the problem of fragmentation by habitat destruction near Santa Cruz, Trinidad, and in Rurrenabaque and the Chapare region of Bolivia, and by highways (Embert 2007). However, the impact of deforestation on this lizard and the consequences of its active foraging on the eggs of P. geoffroanus, affecting its survival in the Guaporé River, needs further study.

Beyond being threatened by the lizard predation, P. geoffroanus is facing the loss of its habitat. Intense habitat fragmentation and destruction caused mainly by human activity has changed the original landscape throughout much of this species' original geographical distribution. Consequently, P. geoffroanus is facing challenges imposed by new human-created ecosystem mismanagement. In addition, due to the cattle culture in this area, nests of P. geoffroanus have been trampled by cattle going to the river to drink water, resulting in egg destruction.

Another threat faced by P. geoffroanus is the local farmers who raise fish in artificial ponds. They kill these turtles when they enter the ponds because they worry that the turtles are there to eat their fish. This is a known practice in the area, but it is not often mentioned.

In this area the IBAMA has developed a program to protect the beaches where P. expansa and P. unifilis nest, but no protection is devoted to P. geoffroanus. This is due to the fact that there is no human consumption of this species. However, the species is still indirectly affected by its habitat loss. Further studies are needed to document the long term survival status for this species, especially since it has not been directly threatened by human predation, but it has been indirectly threatened by deforestation habitat loss and altered predation strategies for the lizard that destroys this species' eggs.