Northwestern University researchers have found out why human children grow slowly when compared to closest animal relatives.

The researchers said that a five-year-old's brain is an energy monster, meaning, toddlers use twice as much glucose as that of an adult's brain. The brain uses major part of the energy in the early years. As a result, the brain develops at a faster rate than the body.

"Our findings suggest that our bodies can't afford to grow faster during the toddler and childhood years because a huge quantity of resources is required to fuel the developing human brain," said Christopher Kuzawa, first author of the study and a professor of anthropology at Northwestern's Weinberg College of Arts and Sciences, in a press release. "As humans we have so much to learn, and that learning requires a complex and energy-hungry brain."

For the study, researchers used PET and MRI brain scan data to measure glucose uptake and brain volume, respectively. They wanted to determine at what ages the brain consumes most amount of resources and when the body growth is slowest.

The researchers found that at 4 years of age, the "brain drain" is at its peak and body growth at its minimum. The brain consumes resources at a rate equivalent to 66 percent of what the entire body uses at rest.

The finding, thus, provides evidence to a long-standing hypothesis in anthropology that children grow slowly and are dependent for long.

"After a certain age it becomes difficult to guess a toddler or young child's age by their size," Kuzawa said. "Instead you have to listen to their speech and watch their behavior. Our study suggests that this is no accident. Body growth grinds nearly to a halt at the ages when brain development is happening at a lightning pace, because the brain is sapping up the available resources."

The finding is published in the journal Proceedings of the National Academy of Sciences.