The need for milk is an essential part of the development of any young mammal, and being aquatic makes breastfeeding considerably harder.
Species from three orders – Carnivora (including seals and sealions), Cetacea (dolphins and whales) and Sirenia (manatees and dugongs) – live and feed at sea.
Seals and sealions have retractable nipples that tuck inside the body when the baby is not feeding, but animals that are fully restricted to the sea, such as whales and dolphins, have evolved ‘mammary slits’ – special folds of skin that enclose the feeding glands.
We are still unsure, but it is thought that either the calves can curl their tongues to channel released milk, or that specialised muscles actually contract the mammary glands, squeezing milk into the baby’s mouth.
The blue whale has the largest mammary glands on Earth – each is about 1.5m long and weighs as much as a baby elephant.
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