By A.M. -IANtB Science Editor.
Deer are what’s called a “caching” species, which means they leave their babies alone for long periods of time while off foraging, etc. Their milk has more fat and protein to sustain them during this time away from mom. (This is why you should not assume a lone fawn in the woods has been abandoned! If you come across one, leave it alone because mom will be back.) Comparatively, human milk has less protein and fats, which more similarly resembles “carrying” mammals, which carry their infants everywhere they go and breastfeed often. (In fact, human milk has the lowest fat and protein content of all the mammals.) This is consistent with most of our other primate cousins, and explains why many infants seem to cry every time they are put down. They’re just trying to survive!
Mckenna, James J., and Thomas Mcdade. “Why Babies Should Never Sleep Alone: A Review of the Co-sleeping Controversy in Relation to SIDS, Bedsharing and Breast Feeding.” Paediatric Respiratory Reviews 6.2 (2005): 134-52. Print.
McGurk, Harry. Childhood Social Development: Contemporary Perspectives. Psychology Press, 1992. Page 16.