Fatness at Birth

Humans are among the fattest mammals at birth  reflecting rapid fat deposition during the third trimester of pregnancy. For most mammals, roughly 2-3% of birth weight is fat. Chimpanzee newborns average 3% of birth weight as body fat.  The human level of newborn fattness exceeds that of all documented mammals other than a single seal species, which is born with a comparable level. Most human baby fat is subcutaneous white adipose tissue, rather than heat-producing brown fat. The evolutionary origins of the distinctive human pattern of fat deposition is not known, but several explanations have been suggested. Subcutaneous white fat is an insulator in some mammals; it was traditionally assumed that humans evolved extra baby fat to compensate for the loss of fur. The evolutionary importance of fat as a source of insulation in humans is debated, and there is minimal evidence that human baby fat evolved to serve this role. More recently it has been proposed that human baby fat evolved to protect brain energetics during starvation or nutritionally-stressful childhood infections. In humans, the unusually large brain consumes the majority of the body's energy at birth, and is rapidly damaged by a shortfall of energy or oxgen. Human metabolism shows evidence for an evolved capacity to feed the brain on body fat during starvation, as evidenced for instance by the human brain's unusual ability to utilize fat-derived ketone bodies in lieu of glucose. Testing whether human baby fat evolved as an energetic buffer for the brain awaits data on brain size and body composition from a wider array of mammal and especially primate species.