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Temperature-dependent sex determination occurs in many species of turtles. Because substrates differ in their heat retention capacities, a relatively small change in ambient temperature can exert great influence on sex ratios of hatchlings from clutches deposited in different substrates even within the same geographical area. Since Podocnemis unifilis is one of the Amazonian turtle species (Podocnemididae) with high behavioural plasticity in the use of nesting sites, we tested the influence of soil types on hatchling sex ratios in the Piagaçu Purus Sustainable Development Reserve along the lower Purus River, Brazil. During the 2013 and 2014 nesting seasons, we evaluated 26 nests on a sand bank (n = 14) and clay banks (n = 12). Approximately half of the hatchlings that emerged from each nest were sexed via gonad histological analysis (n = 341). We found a higher incidence of females in nests from the sandy substrate and more males in nests from the clayey substrate (= 40.466, df = 1, P < 0.0001). Ratios of 1 male to 2.28 females were recorded from sand bank nests, and 1.9 males to 1 female from clay banks. Overall, however, the hatchling sex ratio approximated to 1:1. Due to the gradual increase in global temperatures, it has been suggested for species with Temperature Sex Determination (TSD) that some turtle populations become increasingly female-dominated. However, it is likely that the use of different nesting substrates may help to mitigate the effects of global warming on hatchling sex ratio, especially in species with greater behavioural plasticity, such as P. unifilis. [ABSTRACT FROM AUTHOR]

A change in seasonal flooding cycles in the Amazon may negatively impact nesting success of the Giant South American Turtle (Podocnemis expansa). Our aim was to devise a technique that could be replicated in the entire Amazon basin, for monitoring alterations in fluvial cycles and their effects on turtle nest mortality. We mapped the spatial distribution and height of P. expansa nests and tested the effects of different inundation scenarios within the Trombetas River Biological Reserve, Para state, Brazil. We also used historical data on water level and hatchling production to test whether the sharp decline in the Trombetas River P. expansa population over the past thirty years was related to detected changes in the flood pulse. Our models indicate that an increase of 1.5m in the water level is sufficient to decrease the time of exposure to less than the minimum required for incubation and hatching (55 days above the water) in 50% of the nesting area. This model explains the low hatchling production in dry seasons when the total nesting site exposure was less than 200 days. Since 1971, there was an average decline of 15 days per decade in sandbank exposure during the nesting season (a total of 62 days from 1971 to 2015). However, the decrease in sandbank exposure was not significantly correlated with the sharp decline in hatchling production. Changes to the water cycle in combination with the main sources of decline (overharvest, construction of dams, and dredging of riverbeds) might have an accumulative effect on P. expansa populations. [ABSTRACT FROM AUTHOR]