Whale calves arrive tail first so they don't drown during birth
Whales, dolphins and porpoises usually give birth tail first, which lowers the risk of the calf drowning before it is fully out; the mother then nudges it up to breathe. Across the animal kingdom, birth is tailored to how helpless a newborn will be, and in every mammal it demands a dramatic rewiring of the baby's heart.
In mammals, hormones trigger the muscular uterus to contract and expel a fetus once it is developed enough to breathe and feed. Some newborns, called precocial, can move almost straight away; others, called altricial, depend completely on their parents. Marsupials push this to an extreme, bearing tiny, immature young after a short pregnancy that finish developing in the pouch. Live birth is not exclusive to mammals: some reptiles, amphibians, fish and invertebrates hatch eggs internally, while others nurture embryos inside the body.
The first breath forces a plumbing overhaul. In the womb, the placenta supplies oxygen, and fetal blood largely skips the lungs by passing through the foramen ovale, a hole between the two upper chambers of the heart. Once the cord is cut and air fills the lungs, blood from the right ventricle flows there instead. Pressure on the left side of the heart rises above the right, pushing the foramen ovale shut, and vessels such as the umbilical arteries and the ductus arteriosus gradually shrink into ligaments.
Large mammals such as elephants, giraffes, horses and whales typically carry one young at a time, and most of their babies are precocial, standing, walking or swimming soon after birth. Twins are rare but not unheard of: in April 2018, elephant twins about eight months old were spotted with their mother's herd in Tanzania's Tarangire National Park.
Farm and household animals show the stages clearly. A cow in labour slips away from the herd, restless, until the cervix is fully open; then the calf's hooves and head appear, she pushes it out, licks it vigorously, and within minutes it struggles upright. She usually eats the placenta, remarkable for a herbivore. Dogs follow a similar three-stage pattern, called whelping, with puppies arriving one at a time in grey membrane sacs that the mother breaks open before licking each pup to start it breathing.
Source: Birth
