A study of Australian river turtles may help to explain why turtle eggs tend to hatch at the same time. Researchers believe that the buildup of carbon-dioxide in the nest could be a signal to embryonic turtles to increase metabolic rates and coordinate hatching times. To test this theory, scientists separated a batch of eggs, keeping half at a low temperature and half at a high temperature for two-thirds of the incubation period. The eggs were then brought together for the final days. The result: the cooler eggs, which would typically take longer to hatch, caught up to the warmer eggs, and all hatched together.
"An egg is actually breathing. It’s sucking in oxygen and expiring carbon dioxide,” explained Ricky-John Spencer at the University of Western Sydney. “If you’ve got a lot of well-developed eggs in the nest, there would be more CO2."
Females are hot, males are cool: