Introduction

Lactation is one of the most important innovations that make mammals different from the other vertebrates. During lactation the female mammal feeds her newborn young with milk – a highly nutritious secretion of the mammary glands unique to mammals. Lactation allows mammals flexibility in where and when they reproduce, as well as the types of resources that can support them. The resources used during lactation can come from body stores before being converted to milk. So lactation allows mammals to harvest scarce resources over a long period, then feed the young at a suitable place and time (Pond 1984). This is especially important in large mammals where the developmental time is much longer than the seasons of plentiful resources, such as the arctic summer. Ice-breeding seals store resources harvested over months or years, then transfer them in milk to their babies in as little as four days. Bears use stored resources to lactate during their winter dormancy when they cannot be active and feeding. Lactation also allows the larger, more experienced mother to feed young that could not survive on an adult diet because they are too small, inexperienced or have an immature gut without symbiotic bacteria. This is especially important in herbivores that need to develop the gut before being able to survive on an adult diet, or others where the young need to learn how to hunt or forage.