High temperatures reduce hatchling output and skew sex ratios

  • Scientists consider that for conservation efforts to succeed, it is necessary counteract the impact of fisheries; and that egg clutches must be relocated only under extreme conditions that threaten their survival.
  • Study was made in Playa Grande, Las Baulas National Park in Costa Rica, utilizing nine years of data collection with input from researchers, assistants, and volunteers.

A recent study concluded that in the nesting beaches of the Costa Rican northern Pacific, high temperatures increase the ratio of female-biased nests during the incubation, but reduce the output of female hatchlings in the leatherback turtle (Dermochelys coriacea).


Sex of offspring in most turtles is determined by temperature-dependent sex determination (TSD). In sea turtles, higher incubation temperatures produce female hatchlings and primary sex ratios are often highly female-biased. Because of the current rate of climate warming, highly female-biased sex ratios have raised concern among scientists and managers because populations might become too female-biased for genetic viability.

Scientists tested the effects of higher incubation temperatures on embryo and hatchling mortality and on sex ratios in a population of leatherback turtles in the East Pacific. The long-term study provided a large sample size in a location influenced by El Niño Southern Oscillation that resulted in highly variable climatic conditions between seasons. High temperatures reduced emergence success.

Output of female hatchlings increased with incubation temperature as it reached the upper end of the transitional range (range of temperatures that produce both sexes) (30°C) and decreased afterwards because high temperatures increased mortality of ‘female clutches’.

Effect of temperature on female hatchling output lessened female-biased sex ratios from 85% female primary sex ratios to 79% secondary sex ratios (sex ratios of total number of hatchlings emerged). If male turtles reproduce more often than females, operational sex ratios will be closer to 1:1.


Output of female hatchlings increases with incubation temperature as it reached the upper end of the transitional range (range of temperatures that produce both sexes) (30°C) and then decreases because high temperatures increase mortality of ‘female clutches’.

Biological Conservation, volume 176

Female-biased primary sex ratios should not raise concerns by default, but climate change may still threaten populations by reducing hatchling output and increasing frequency of seasons with 100% female production.


Clutch relocation to cooler conditions may alter sex ratios and should be used cautiously unless temperatures are so high that no hatchlings survive. In addition, it is unknown what differential survival of male versus female hatchlings may have on the eventual adult sex ratio after they enter the ocean and disperse.


The study was made taking data from 9 years of field work; all the data collected was analyzed by scientists from Institut Mediterrani d’Estudis Avançats, The Leatherback Trust, Indiana-Purdue University, Ministry of Environment and Energy of Costa Rica, Michigan-Dearborn University and Drexel University.

The full text can be found in Biological Conservation 176, 71-79. It also can be accessed online in www.sciencedirect.com.