When North American painted turtles break out of their shells, they face an extreme endurance test; they spend their first six months of life entombed in the nest with neither food nor drink. But they aren't completely empty during their incarceration; the newly hatched turtles seem to dine on egg-shell and soil. Along with these deprivations, the youngsters also have to survive perishingly cold winter conditions by supercooling; remaining unfrozen at temperatures lower then the freezing point of their tissue. But if the frosts come too soon, the youngsters are in danger, as it takes a while for them to harden to the cold. Jon Costanzo and Richard Lee wondered why it took the turtles several weeks to develop their supercooling ability. Were the tiny reptiles ridding themselves of life threatening ice-nucleating agents by emptying their guts? The team tested how well hatchlings that had not come into contact with soil survived cold conditions and discovered that a gut full of soil costs the youngsters dearly ()!
But Costanzo had to find turtle eggs that had never touched soil before he could test the effects of soil ingestion on the hatchlings. The team travelled to the Nebraska Sandhills, where mother turtles go to lay their eggs. Costanzo explains that the only way to prevent contamination by soil was to find a female preparing to lay her clutch and transfer her to a clean plastic box to deposit her eggs. Back in the lab, Costanzo and his colleagues helped some of the young turtles hatch from their eggs to make sure that they didn't inadvertently swallow eggshell, and then he transferred them to live on moist paper. Others were allowed to hatch naturally and transferred to live on soil or vermiculite. The team slowly lowered the temperature over several months to simulate the onset of winter, and then tested how well the hatchlings survived as the temperature plunged.
Comparing the summer and autumn youngsters' tolerance to cooling, Costanzo found that the autumn turtles supercooled to much lower temperatures than turtles acclimatised to summer conditions; empty stomachs helped supercooling. But animals that lived on vermiculite and paper always supercooled better than the soil-reared youngsters.
Was the soil leaving something behind that compromised the youngsters'chances? Costanzo washed bits of eggshell, soil and vermiculite from the turtles' intestines and tested the solutions' ice nucleating capacity. Only the soil wash nucleated ice. The youngsters' soil meal was leaving behind soluble ice nucleating agents, even when they were empty.
But why fill up on soil when it risks the youngsters' survival? Costanzo thinks that the turtle's soil meal could have something to do with their mother's final parental act; she urinates over the nest as she lays her eggs. He suggests that the youngsters eat soil to inoculate their intestines with the gut flora their mother left behind. Unfortunately, the hatchlings pick up a dose of potent ice nuclei at the same time, but the benefits of setting their guts up for life must outweigh the icy risk.
