Animals are born anywhere from helpless to nearly self-sufficient, and scientists place them on a spectrum. Giraffes are at the self-sufficient end, and they are precocial, which means they are born ready to move, and a newborn giraffe's nervous system is ready at birth, like that of a one-year-old human child ready to walk. Scientists call chicks that hatch helpless and stay in the nest altricial, and they call chicks that hatch well developed and ready to move precocial. Pigeons, songbirds, and owls and eagles are altricial, while most chickens, ducks and turkeys are precocial, and pigeons are the exception among farm birds. Birds born in open country hatch ready to run, and being able to move right away helps them avoid predators. Songbird chicks, for example, hatch helpless, with their eyes closed and almost no feathers, and they stay in the nest to wait for their parents to bring them food.
Marsupials and monotremes show other solutions: a marsupial mother's pouch is a safe place where her young finish developing, and a wombat's tiny, underdeveloped baby grows in the pouch for 6 to 10 months. A joey spends its first six months relying on its mother's pouch for protection and milk, and then it comes out to learn how to forage and avoid predators. Two kinds of mammals lay eggs, the platypus and the echidna, which are called monotremes. A baby echidna comes out of its egg after about 10 days, blind and hairless, and it then drinks milk in its mother's pouch for two or three months. When it grows prickly spines, its mother takes it out of her pouch and builds a burrow for it. A mother platypus keeps her eggs warm by curling around them and holding them against her belly with her tail.
Milk is expensive to make: in a drone study of humpback whales, calves grew in body volume by nearly 395 percent and in length by almost 60 percent over six months, while their nursing mothers shrank in body volume by about 17 percent. In Hawaii, the energy a mother spent nursing was more than the whole energy cost of carrying her calf for a year. Orangutan mothers invest in a different way, because they teach their babies which foods to eat, where to find them, in which trees and in which seasons, and the babies are weaned at about age six or seven. A Sumatran orangutan mother may wait at least eight years before she has her next baby. Humpback mothers lose about a fifth of their body volume over 60 days of nursing in Hawaii, and a nursing whale mother can make 50 gallons or more of milk in a day.
Emperor penguins schedule their nesting around the food supply, starting in the dead of winter because incubating an egg takes around four months, so that the chick hatches when there is the most food in the ocean nearby. The dad keeps the egg warm on top of his feet, and the skin of his brood pouch is full of nerves that sense temperature, so he can feel the temperature of the egg. While the moms go to sea to feed on fish, the chick stays on the parent's feet under a warm flap of belly skin for the first weeks. In a 48-hour blizzard the whole huddle can drift as much as 200 meters downwind, and the dads take turns moving from the cold edge of the huddle to the warm middle, and a chick needs about 42 kilograms (93 pounds) of food from each parent.
In the early 1800s, John James Audubon tied thin silver threads to the legs of young Eastern Phoebes at a cave nest in Pennsylvania, and he is probably the first person to band birds in North America. He wrote that two of his banded phoebe chicks came back to nest near their birthplace, but modern studies doubt it: in one large Indiana study, only 218 of 11,847 banded phoebe nestlings, or 1.8 percent, returned to the study area. A bird historian, Matthew Halley, argued that part of the famous banding story might never have happened, which shows that scientists must check even famous old reports against the evidence. Today a person needs a federal permit to band or mark wild birds in the United States, and banding data help scientists with bird science, management and conservation. In the United States, the U.S. Geological Survey's Bird Banding Laboratory runs the permit program with Canada's wildlife service.
The Wytham Woods great tit study is the longest continuous study of an individually marked animal population in the world. It began in 1947, when John Gibb and David Lack put up 100 nest boxes in one part of the woods, and around 1960 it grew to over 1,000 boxes across the whole 385-hectare wood. Great tits were chosen because they readily use nest boxes, nest close together, do not travel far from where they hatched, and cope well with being watched. In 75 years, researchers have monitored every nesting attempt and identified every parent and chick, across more than 55 generations. Data from Wytham Woods showed there is a best number of eggs for a great tit to lay, which depends on how much food, such as caterpillars, is around when the chicks need feeding. Long records show that spring has moved earlier: in 2022 the first great tit egg at Wytham Woods was counted on 28 March, almost exactly a month earlier than 75 years before.
Other long projects follow other young animals: the Amboseli Baboon Research Project in Kenya has followed individually known baboons from birth through their whole lives since 1971, and since 2000 researchers have studied baboon stress and growth without disturbing the animals by testing hormones in their droppings. To follow young salmon, scientists tag wild Chinook parr in up to 15 Idaho streams with tiny PIT tags, then detect them at monitoring sites and dams as they swim downstream. These studies estimate how many young Chinook survive from the parr stage to the smolt stage, and how fast they grow and move. Young Chinook in freshwater have dark marks on their sides called parr marks, which help camouflage them, and before they go to sea they lose the marks and grow a dark back and light belly like other open-water fish.
Some of the most famous experiments concerned what a hatchling follows: in the 1930s, Konrad Lorenz showed that birds hatched in an incubator, with no parents around, follow whatever they first see. He got a one-day-old duckling and found that it followed him as if he were its parent, and he called this early following response imprinting. Lorenz shared the 1973 Nobel Prize in Physiology or Medicine with Karl von Frisch and Nikolaas Tinbergen for studying how animals behave. A chick that makes the first hole in its shell stops and rests for three to eight hours before it breaks out, and a few hours after hatching it is dry and fluffy and starts to walk around. Some baby birds, like chicks, ducklings and quail, hatch with feathers or fluff and can stand, walk or run soon after hatching, and before it hatches, a chick grows a small, sharp egg tooth at the tip of its beak, which it uses to peck through the shell.
Survival numbers can be stark: in places with many lions and other predators, half or more of newborn giraffes may die in their first days or weeks. Only about one in 1,000 to one in 10,000 sea turtle hatchlings is estimated to survive to adulthood. Sea turtles come out of the nest together: when the first hatchling breaks free and digs, the movement sends vibrations through the nest, and the others begin to dig out too. Many years later, a grown female usually returns to lay her own eggs on a beach near where she hatched. Hatchlings find the sea by the slope of the beach and by the moon or starlight shining on the water, and after they reach the water, young sea turtles swim out to the open ocean, where they live for several years.
Frogs show how flexible development can be: a single frog can lay up to 20,000 eggs at once, but only a small percentage of the eggs survive to adulthood. Bullfrog tadpoles may take several years to turn into frogs, and tadpoles in ponds that are drying up can speed up their change into frogs, or they can delay it. Some frog species skip the swimming tadpole stage and develop into small frogs inside the egg. As a tadpole changes into a frog, it grows lungs and loses its gills, so it can breathe air. A tadpole's tail is not shed, but the body slowly absorbs it and uses it as a source of energy. Frogs are indicators of ecosystem health: changes in the timing or success of any life stage can signal wider environmental pressures.
Insects change too: a monarch caterpillar is less than 1 centimeter long when it hatches, and it grows to about 5 centimeters, and it grows to almost 2,000 times the mass it had when it hatched. Going through all four life stages, egg, larva, pupa and adult, is called complete metamorphosis. Inside the chrysalis, the caterpillar's chewing mouth changes into the straw-like tongue of a butterfly, used for sipping nectar. The other way insects develop, incomplete metamorphosis, skips the pupa stage and has only three stages: egg, nymph and adult. A new butterfly comes out of the chrysalis with crumpled wings, and it must pump fluid into them and let them dry before it can fly. A monarch egg is about the size of a pinhead or the tip of a pencil.
Some questions remain open: giant pandas are born smaller, compared with their mothers, than nearly any other mammal except marsupials, and as of November 2026, scientists still do not know exactly why. Zookeepers think a panda mother's bamboo diet may be part of why cubs are born so small. Bamboo has few nutrients, so she puts her energy into making rich milk instead of growing a big baby. A newborn is pink and blind, and its pink skin is covered with white fur within about 48 hours, while the fur is all filled in by about three weeks. A newborn panda is about the size of a stick of butter and weighs only about 1/900th as much as its mother, and she feeds it up to 14 times a day in its first weeks while the cub gains weight fast.
Words to know
- altricial: helpless at birth
- precocial: ready at birth
- metamorphosis: body change
- monotremes: egg-laying mammals
- imprinting: early following
- marsupial: pouch mammal
