This article is from the WSSF 2007 AFRMA Rat & Mouse Tales news-magazine.

Did You Know?  Orphan hand-raised (artificial reared) rats have a lower brain weight due to deprivation of maternal and sibling contact. Moms (or foster moms) and litter mates are vitally important to the development of young rats.

This lack of contact when little also reduced their social learning, thermal, tactile, and other stimulations later in life, including the development of maternal behavior.