Humans are the only mammals who feed their young special complementary foods before weaning their young off breastmilk (in the context of the termination of suckling). ¹ Early weaning is likely due to our slow post-natal growth and likely due to our brain size. ¹ Did you know that the human brain is more than three times larger than our closest relative, the chimpanzee? In human adults, the brain accounts for 2% of body mass but consumes 25% of resting energy expenditure.