Human biology and luck helped a baby survive without food or water for six days after being dumped, paediatricians say.
Kim Oates, an emeritus professor of paediatrics at the University of Sydney said healthy babies were born with enough fluid to help them survive for some time without feeding.
"When full-term babies are born, they are usually born with fluid on board. They have a good amount of fluid storage and extra glucose stored in their liver as well," he said.
"I think the evolutionary basis of that is that if the mother had a difficult birth and passed out and couldn't recover and feed the baby for a little while, the baby would be OK.
"They have enough to survive for the 24 to 48 hours without much in the way of feeding. I am surprised: six days seems a long time to me."
After six days the baby was likely to be suffering from low blood sugar and at risk of getting cold.
"It's possible, it's surprising. If it was a premature baby it would have had much less chance of surviving because they don't have as much glucose and fluid on board," Professor Oates said.
Andrew McDonald, a Sydney paediatrician and Labor MP, said babies, born without complications such as respiratory problems, were very resilient and, if kept warm, could survive without fluid for four days.
"It was a very lucky baby. But evolution was kind to us," he said.
Dr McDonald said newborns were designed in a way that they did not need much fluid until their fourth day of life.
"That's when the mother's milk comes in," Dr McDonald said.
"They aren't given much fluid for the first four days ordinarily. They are born full of fluid.
"Some babies can lose up to 10 per cent of their birth weight in their first four days of life."
The combination of hot weather and the coolness inside the drain may also have helped the baby stay warm enough.
Dr McDonald said the child appeared to have reached full term before he was born, which would have helped improve his chances of survival.
"You see this in earthquakes, babies living for some days," Dr McDonald said.
"What was surprising was that the baby had a vigorous cry after five days.
"He wouldn't have lasted much longer," he said.
Dr Howard Chilton, a consultant neonatologist at Prince of Wales Private Hospital and the Royal Hospital for Women in Randwick, said newborn babies were "water logged" and had 15 per cent fat in their bodies, allowing them to survive without food for days.
"We know the physiology of babies is designed for toughness and resilience in their early days," Dr Chilton said.
"The great problem for babies is cold. But the weather has been reasonably good."
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