Temperature along an elevation gradient determines Galapagos tortoise sex ratios

Area of Science:

• Herpetology
• Climate Change Biology
• Conservation Science

Background:

• Endemic island reptiles, like Galapagos tortoises, are vulnerable to climate change due to small populations and temperature-dependent sex determination (TSD).
• Captive studies show Galapagos tortoises exhibit Type A TSD, where warmer temperatures yield females.
• Limited data exist on wild incubation temperatures and sex ratios, hindering climate change impact assessments.

Purpose of the Study:

• To quantify nest temperatures and juvenile sex ratios in free-living Galapagos tortoises across an elevation gradient.
• To assess the relationship between elevation, nest temperature, and hatchling sex ratios.
• To predict potential impacts of climate change on Galapagos tortoise populations.

Main Methods:

• Nest temperatures were monitored every 4 hours using iButton thermochrons across three elevation zones (14m, 57m, 107m) on Santa Cruz Island.
• Juvenile tortoise sex was determined using coelioscopy on 40 individuals from the nesting zones.
• Data analysis focused on the middle trimester of incubation, the critical period for sex determination.

Main Results:

• Mean nest temperatures during sex determination decreased with elevation (27.09°C lower, 25.75°C upper zone).
• Male proportion increased significantly with elevation (9.5-11.1% lower/middle zones, 80% upper zone).
• A 50m elevation increase correlated with a >1.25°C decrease in nest temperature and a ~70% increase in male proportion.

Conclusions:

• Elevation-driven temperature differences significantly influence Galapagos tortoise sex ratios in the wild.
• Predicted climate change-induced temperature increases may lead to a higher frequency of female tortoises.
• Conservation strategies must consider TSD and climate change impacts on sex ratios for population viability.