From: Megan Talkington, ScienceNOW
Published December 18, 2009 06:48 AM

Brain Size Predicts the Time it Takes Mammals to Walk

Many animals test their legs and totter forth only hours after they are born, but humans need a year before they take their first, hesitant steps. Is something fundamentally different going on in human babies? Maybe not. A new study shows that the time it takes for humans and all other mammals to start walking fits closely with the size of their brains.


In past studies to develop a new animal model for the brain events that support motor development, neurophysiologist Martin Garwicz of Lund University in Sweden and his colleagues discovered that the schedules by which ferrets and rats acquire various motor skills, such as crawling and walking, are strikingly similar to each other; the progress simply happens faster for rats. That made them wonder how similar the timing of motor development might be among mammals in general.

They compared the time between conception and walking in 24 species and looked at how well this duration correlated with a range of variables, including gestation time, adult body mass, and adult brain mass. As they report in this week's issue of PNAS, brain mass accounts for the vast majority (94%) of the variance in walking time between species.

Species with larger brains, such as humans, tend to take longer to learn to walk. Strikingly, a model based on adult brain mass and walking time in the other 23 species almost perfectly predicts when humans begin to walk. "We've always considered humans the exception," Garwicz says, "But in fact, we start walking at exactly the time that would be expected from all other walking mammals."

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