The Remarkable Evolution of Lactation

Lactation is a uniquely mammalian trait that has an ancient origin. Mammary glands apparently evolved in early synapsids from apocrine-like glands that supplied moisture to eggs. Incorporation of antimicrobial constituents and nutrients into mammary secretions allowed milk to supplant egg yolk in the rearing of young. All extant mammals, including monotremes, marsupials and eutherians (placental mammals) produce milk by similar secretory mechanisms, but there is large variation in the amount and composition of the milk produced. For example, milk triacylglycerol ranges from trace amounts in rhinos to more than 50% in some seals. Three different scenarios that have led to the evolution of high fat milks will be discussed. In bears females fast during lactation and the composition of milk is central to this achievement since the low sugar and high fat minimizes the need for maternal glucogenesis. In sea lions and fur seals, high fat milks allow females to undertake foraging trips to sea of 1 day to 3 weeks duration and still deliver sufficient energy to pups during brief suckling periods on land. The highest milk fat is found in hooded seals, which lactate for less than four days. Rapid transfer of energy-dense milk permits pups to gain 7 kg per day before being weaned. Mammals have evolved remarkably diverse lactation strategies in which both the types and quantities of milk constituents very greatly.