Quintana, I., Norris, D., Valerio, A., Becker, F.G., Gibbs, J.P. and Michalski, F. (2019), Nest removal by humans creates an evolutionary trap for Amazonian freshwater turtles. Journal of Zoology, vol. 309, pp. 94-105. https://doi.org/10.1111/jzo.12689
Avoiding predators is important for the survival of prey species, and turtles have evolved a number of characteristics that have protected them over millennia. For example, the hard shell cases of adult turtles help protect them from many different types of predators, and females of many turtle species also bury the eggs they lay in underground nests to hide them from predators. However, hunting of adults and eggs by humans is threatening many freshwater turtle populations.
The Yellow-spotted River Turtle (Podocnemis unifilis) is widespread across the Amazon Basin, but like many other turtles their populations have been decimated by humans. Both eggs and adults of P. unifilis have been exploited since the pre-colonial period (pre 18th century) and are still consumed by indigenous and riverine communities across the Amazonia. Although the Yellow-spotted River Turtle has been widely studied over decades, we still have no idea if they have any behavior or response that can help the members of surviving populations avoid predation by a “super-predator” (humans) that hunts both adults and eggs. Despite many studies that evaluated environmental and human impacts on nesting in freshwater turtles, studies conducted over large spatial scales and with a large number of nesting areas are still lacking. To understand if female Yellow-spotted River Turtles were able to avoid human hunting, we monitored 73 nesting areas along 118 km of Amazon rivers.
In our study area, removal of eggs by humans is synchronized with the peak turtle nesting times (month of November). Yellow-spotted River Turtles synchronize their nesting (i.e. peak timing) so that the hatchlings will emerge before the river levels rise and flood the nests. Yet, it is exactly these “peak” nests that humans will remove. The nests that are laid after the peak may not be removed, but are most likely to become flooded. As such, the removal of eggs by humans is most likely to be an additive or even synergistic effect beyond the natural losses.
Female turtles can receive and respond to environmental cues prior to nesting via touch, sight and/or smell. They can also show behavioral changes to avoid predators, for example, in Australia female turtles reduced exposure to a recently introduced invasive predator (foxes were introduced 130 years previously) by nesting closer to the water. We would therefore expect to detect some changes in nesting area and/or site selection by females over the different human consumption and environmental conditions encountered in our study area. These changes could include nesting further away from houses and/or reduced nesting in areas with greater hunting pressure.
Although freshwater turtles can receive a variety of environmental cues, these cues seem unlikely to help Yellow-spotted River Turtles avoid or escape nest removal by humans. Nesting usually occurs during the night, and in our study area during the peak nesting season humans may wait concealed nearby the nesting areas until the nests have been laid. Once this had occurred (e.g. in the early morning), humans would then follow the tracks left at the nesting area to locate and harvest the newly laid nests. We found that to avoid human removal of nests, female turtles should nest on bank-side nesting areas away from houses. However, the density of nests tended to increase both on islands and within 1 km of houses. These are exactly the areas with higher levels of nest predation by humans. The higher nest density found on islands could lead to an increase in egg harvest, because clumped nests experience a higher predation rate than scattered nests. Despite the wide availability of alternative nesting habitats, turtles do not appear to be capable of avoiding hazardous nesting sites associated with increased egg harvesting by humans. Because signals used by female turtles to select nesting areas are apparently not sufficient for avoiding nesting along rivers accessible to humans, direct conservation action will be vital for the persistence of these and likely other Amazonian freshwater turtles.
Our study represents a significant advance for the understanding of freshwater turtle nesting patterns. This advance comes from the scope (the number of nesting areas and the length of river surveyed) and the novelty of the clearly contrasting size of nesting areas to those presented in previous studies, which have generally focused on a few (<10) large nesting beaches. Our study area (and perhaps many of the rivers across the species’ range) does not contain such large nesting areas. We demonstrated that nest removal by humans is a major threat to P. unifilis nesting success around sustainable use protected areas.