Why are some babies able to stand after birth and others aren’t?

Brain size decides whether babies are independent or incapable

Human babies are unable to hold their heads up and take a long time to learn to walk because their brains aren’t fully developed when they are born. Horses, in contrast, stand up within hours and are ready to run from danger soon after. According to a 2009 study from Lund University in Sweden, it all has to do with the size of our brains.

The scientists made a model that predicts the time of a baby animal’s first steps based on the weight of its adult brain. To do this they looked at 24 different mammal species, representing most of the orders of walking mammals alive today. They found that when brain development hits a certain point relative to the final size of the brain, animals are ready to walk.

If you trace development forward from the moment when the sperm met the egg, horses and humans take their first steps at the same point in developmental time – horses are just born further along into their brain development than we are. This difference in development at birth stems from the increase in size of the human head and the evolution of the pelvis as humans began to stand upright; if babies stayed in the womb any longer their heads would be too large for a safe birth, so they’re born while they still fit through their mothers’ pelvises.


For more answers to your animal questions, make sure you pick up a copy of World of Animals issue 66!