No one remembers the experience of being born. When they are thrust out of their cosy incubator, newborn mammals have to cope with oxygen flooding into their bodies from the environment, all while grappling with feeding and growing. It's a time of turmoil. But just how does the metabolism of tiny newborns adjust as they grow and develop, in some cases from helpless, blind, naked pups to independent furry adults? To find out, Jessica Li, Katie Marshall and Bill Milsom from University of British Columbia, Canada, measured the oxygen consumption of tiny newborn thirteen-lined ground squirrels (Ictidomys tridecemlineatus) every day over the first 30 days of life, and then at 60, 90, 120 and 465 days, to find out how they grow and how their metabolism changes during this critical time of life.
Incredibly, the tiny (4.3 g) newborns’ metabolic rate doubled during the first 24 h of life from 0.07 ml O2 min−1 to 0.14 ml O2 min−1, before they gained 50% more mass in the first 3 days. From then on, the pups continued growing at ∼1.2 g day−1 until the age of 30 days, while their metabolism increased gradually, reaching ∼1 ml O2 min−1 at 22 days. But then their metabolic rate doubled over the next 8 days, before continuing to ramp up over the next month to 5.5 ml O2 min−1 at 60 days before the pups reached full size at ∼80 days. However, after 120 days, the youngsters’ metabolic rate began to decline, almost halving by the time they were just over a year and 3 months old. So, the youngsters’ metabolic rate varies enormously during the first year of life, and when the team adjusted the pups’ metabolic rate for their size, it turned out that it rocketed to an incredible ∼50 ml O2 min−1 kg−1 at 30 days.
But how do all of these extraordinary metabolic changes relate to the youngsters’ growth and development? During the first 2.5 weeks of life, the blind and naked youngsters are mainly growing, but at around 18 days – a couple of days before they begin to open their eyes and wean off milk – their mass-adjusted metabolic rate increases dramatically, probably in preparation for the next 12 days as they prioritise developing independence over growth. Once the pups are fully weaned and their mass-adjusted metabolic rate has peaked, their growth spurt kicks in at ∼30 days, as they grow from ∼40 g to adult size (∼300 g) at ∼90 days.
The metabolism of thirteen-lined ground squirrels varies enormously during the first year of a pup's life, prioritising costly physiological development when weaning, followed by a growth spurt until fully grown at ∼90 days. And the team suspects the metabolism of other newborns is likely to be equally versatile, but each species will probably fine-tune their respective growth and development trajectories to their specific circumstances.
