Population size, habitat choice and sexual dimorphism of the Amazonian tortoise (Geochelone denticulata) in Tinigua National Park, Colombia

The Amazonian tortoise, Geochelone denticulata, has been classified as vulnerable; nevertheless, necessary ecological and demographic information to formulate conservation plans is limited. In this study we present data on population density, habitat preference, diet and morphological measurements of a population inhabiting a lowland rainforest in Colombia. We used line transect methods to estimate population density over three different years, and we measured and marked 73 individuals in two additional years. The estimated population density based on 158 encounters was similar over the three years (41 ± 20, 38 ± 6, 34 ± 8 ind/km2 ±SE, respectively). However, we found marked differences between seasons within years, because it was easy to detect individuals (particularly males) during mating periods. There was no evidence of large scale seasonal migrations; thus, it is possible that individuals were missed in the non-reproductive season and therefore our figures may underestimate population size. For the same reason, sex ratios during the mating season may be more biased towards males than for whole year estimates (1.7:1 vs. 1.2:1). Monthly density estimates were significantly correlated with mating frequency and with estimates of fruit abundance, but were not associated with rainfall, temperature or sun radiance. These results suggest that males might use available energy from fruit to travel extensively during the mating season in search of females. However, the most productive forest type in terms of fruit was not preferred by tortoises, to the contrary, they were more commonly encountered in forests with a high proportion of gaps. Males tended to have larger and more elongated bodies than females. This result supports the hypothesis that since males must travel more in search of mates, it is an advantage for them to have an elongated carapace in order to move through dense understory. More ecological information is required from other geographical locations to establish which ecological variables are most important in determining the carrying capacity for terrestrial tortoises. So far the demographic structure of the populations suggest that predation of young individuals plays a major role.