Comparative Ecology of Sympatric Gonatodes (Squamata: Gekkonidae) in the Western Amazon of Brazil
Two sphaerodactyline geckos, Gonatodes hasemani and G. humeralis were studied in eastern Rondônia, Brazil, to determine ecological factors allowing coexistence. Gonatodes hasemani lives primarily on fallen logs in undisturbed forest, whereas G. humeralis lives primarily on tree trunks. Although both species occur in most forest patches, G. hasemani is more common within terra firme (never flooded) forest, and G. humeralis is more common in forest adjacent to rivers. Gonatodes hasemani is slightly larger in body size and is more robust than the more arboreal G. humeralis. Both species are active at the same time during the day, maintain the same body temperatures, and are most frequently found in shade. Dietary overlaps based on numbers (0.831) and volumes (0.877) of prey types were high indicating relatively similar diets. Differences in prey types appear to reflect differences in microhabitat use, and both species eat prey of the same size. Reproductive characteristics are similar but the larger bodied G. hasemani produces slightly larger eggs. The niche axis on which these two lizards most clearly separate is microhabitat: G. hasemani uses lower perches with larger diameters in the forest primarily because individuals typically are found on fallen logs, whereas G. humeralis uses higher and thinner perches because individuals are typically found on tree trunks and vines. Limited data on snakes that prey on small lizards in Amazon forest suggests the possibility that niche differences between these Gonatodes species may be mediated by predators. Dois lagartos geconídeos Sphaerodactylinae, Gonatodes hasemani e G. humeralis, foram estudados no leste de Rondônia, Brasil, com o intuito de se determinar os fatores ecológicos que permitem sua coexistência. Gonatodes hasemani vive primariamente sobre troncos de árvores caídos, em mata não perturbada, enquanto G. humeralis vive primariamente sobre troncos em pé. Embora ambas as espécies ocorram na maioria das áreas de mata, G. hasemani é mais comum em mata de terra firme, enquanto G. humeralis é mais comum em matas adjacentes a rios. Gonatodes hasemani é ligeiramente maior quanto ao tamanho do corpo e mais robusto que seu congênere mais arborícola, G. humeralis. Ambas as espécies estão ativas nas mesmas horas do dia, mantêm a mesma temperatura corporal, e são mais frequentemente encontradas na sombra. Uma grande sobreposição na dieta, tanto baseada em número (0.831) quanto em volume (0.877) de tipos de presas indica dietas relativamente similares. As diferenças observadas nos tipos de presas parecem refletir diferenças no uso de microhabitats, sendo o tamanho das presas semelhantes para as duas espécies. As características reprodutivas são semelhantes, mas G. hasemani, que possui um corpo maior, produz ovos ligeiramente maiores. O eixo no qual esses dois lagartos mais claramente se separam, no que diz respeito ao nicho, é o microhabitat: G. hasemani utiliza substratos mais baixos e de maior diâmetro na mata, primariamente porque indivíduos são tipicamente encontrados em troncos caídos, ao passo que G. humeralis utiliza substratos mais altos e mais finos, já que indivíduos são encontrados tipicamente sobre troncos de árvores vivas e cipós. Os dados, ainda que limitados, sobre cobras que se alimentam de pequenos lagartos na floresta amazônica sugerem a possibilidade de que as diferenças de nicho entre essas duas espécies de Gonatodes possam ser mediadas por predadores.Abstract

Distribution of body sizes (snout–vent length) for Gonatodes hasemani and G. humeralis from Rondônia, Brazil. Arrows indicate size at sexual maturity for each species and sex

Plot of unrotated factor scores for PCI and PCII on size-adjusted morphological variables for Gonatodes hasemani and G. humeralis from Rondônia, Brazil. Factor I represents a gradient based on a combination of four variables, body width, body mass, head width, and body height. Factor 2 represents a gradient based on a combination of hind-leg length, head length, and foreleg length

Photographs of (A) adult male Gonatodes humeralis from the Rio Juruá, Acre, Brazil; (B) adult female G. humeralis from the Rio Curuá-Una, Pará, Brazil; and (C) adult male and (D) female G. hasemani from the Rio Juruá, Acre, Brazil

Use of habitat patches by Gonatodes hasemani and G. humeralis

Use of microhabitats by Gonatodes hasemani and G. humeralis

Comparison of perch heights (left) and perch diameters (right) used by Gonatodes hasemani and G. humeralis. Means are indicated by arrows. Differences in perch height and diameter are significant at P = 0.0002 (see text)

Daily activity patterns for Gonatodes hasemani and G. humeralis based on observations of lizards in the field

Relationship between stomach volume and lizard snout–vent length for Gonatodes hasemani and G. humeralis. The line estimates the maximum possible fullness of stomachs. Its slope is based on the regression line for all points shown. Its elevation estimates the fullest stomachs

Distribution of prey volumes for Gonatodes hasemani and G. humeralis

Relationship between mean prey volume per lizard and lizard mass for two species of Gonatodes. The common regression and is shown with 95% confidence limits of the dependent variable. The formula for the regression line is log-mean prey volume = 0.447 + 1.58 (log-mass)