Every parent knows how much they invest in their kids. But few devote as much to their young as Children's python mums, investing up to 30% of their body mass in a single clutch of eggs. This is a major incentive for each mum to ensure their offspring's survival by brooding the eggs until they hatch. According to Zachary Stahlschmidt and Dale DeNardo one of the main risks faced by the developing eggs is desiccation. They explain that without the mother's protection, the eggs lose water through their paper-thin cases 25–30 times faster than when mum is safely coiled around them. But at what cost? Could the mother's coils restrict her youngsters' access to air? Stahlschmidt and DeNardo decided to monitor the behaviour of brooding Children's python mothers while measuring oxygen levels in the enclosed clutches, to see if the mothers' coils restricted their youngsters' oxygen supply (p. 1535).
Monitoring the snake mothers' activity levels, Stahlschmidt filmed the reptiles for 12 hour periods during the early, middle and late stages of their eggs' development. According to Stahlschmidt, the snakes are inactive and tightly coiled around their eggs for 90% of the time; `I watched the films on fastforward' he admits. The remaining 10% of the time, the snakes loosened their coils and adjusted their posture. And when Stahlschmidt correlated the oxygen levels in the clutch with the snake's activity, he realised the oxygen level dropped steadily while the mother was tightly coiled around, but increased dramatically within three minutes of the snake shifting position. There was a cost to protecting the eggs from desiccation; the eggs' air supply was restricted. But was the restriction sufficient to compromise the eggs' metabolism?
Measuring the metabolic rates of clutches of artificially incubated eggs at oxygen levels ranging from 10 to 20 kPa (21% – normal – oxygen), Stahlschmidt realised that the oxygen level inside the brooding mother's coils was sufficient to meet the youngest fetuses metabolic demands. However, as the snake foetuses grew their metabolic demands increased, requiring 15.1 kPa oxygen to maintain their metabolism during the middle stages of development and 19.4 kPa oxygen towards the end of incubation. But the oxygen levels deep in the snakes coils fell below 15.1 kPa almost 20% of the time during the middle stages of development, and never rose above 19.4 kPa during the final days of incubation. While the eggs were occasionally oxygen starved during the middle stages of development, they were constantly starved of oxygen during the final stages of development. The pythons were prepared to risk suffocating their eggs to protect them from desiccation.
Surprised that the environment in the mother's coils was hypoxic during the final stages of the fetuses' development, Stahlschmidt wondered if the pythons might adjust their behaviour to increase their young's oxygen supply. But returning to the incubation footage, Stahlschmidt saw that the mother's behaviour was unaltered by the risk of oxygen deprivation. The mothers either seemed unaware of their offspring's low oxygen levels, or were unable to respond to the metabolic restriction in case their eggs dried out.
- © The Company of Biologists Limited 2008