Built-In Shelters
Kangaroos, opossums, and other marsupials give birth to extremely underdeveloped young. A newborn kangaroo joey, no larger than a kidney bean at birth, must climb from the vagina to the teats, which are hidden inside an expandable pouch. As the joey grows, it rides along in this pouch, eventually
becoming large enough to peek out. Even after it leaves its mobile nest, it may clamor to get in again when it wants to nurse or be protected, no matter how large it has grown. The mother eventually has to forcibly restrict access to this comforting haven.
Other kinds of built-in papooses are not as well known. The Surinam toad of South America, for instance, carries its developing eggs in pits on its back, providing both moisture and protection from predators. Some seabirds, such as penguins, balance a single egg on top of their feet so it can’t roll off the rocky cliffs of their rookery. For species that don’t have these built-in nurseries, the next best thing is to create a shelter using natural materials.
Building a Nursery
Nest building is such an important chapter in some species’ behavior that captivity will not squelch it. Zoo beavers, if they are provided with aspen branches, will work furiously to build a lodge, and foxes will dig a den in the dirt floor of their enclosure. Video cameras in these dens offer zoogoers glimpses of domestic life that are impossible to see in the wild. In the same way, a raised walkway may allow you to peer into the construction of a bird nest in the aviary.
Nest building can completely consume the animals for a few days or weeks (usually in the spring), making this one of the most exciting times to visit your zoo.
Though we are not always aware of it, animals are raising their young all around us: on the ground, in the ground, in water, on branches, in tree trunks, and even on cliffs. Some occupy the abandoned nests of other animals or squeeze into natural tree cavities and rock crevices, while others build their homes from scratch. The building plans come in many shapes and sizes, from the hummingbird’s thimble to the huge condominiums of the weaverbirds. Some dwellings, such as the underground burrows of prairie dogs, are real feats of architectural engineering. Others are no more than thrown-together piles of grasses, like those built by many species of waterfowl.
Nests, like paw prints, are so characteristic that you can often identify the species of nest builders even if they aren’t in sight. Although the basics are the same within a species, individuals have some latitude in customizing their nests, and anything in the environment is fair game. An American robin on my college
campus, for instance, had a penchant for the tear-off strips from the sides of computer paper. You may be able to see similar kinds of borrowed building materials in the nests of birds in your zoo aviary.
In addition to found items, animals may personally contribute their glandular secretions, feces, or saliva to help hold the nest together. (Gourmet food lovers may be surprised to learn that the main ingredient in the coveted bird’s nest soup is the hardened spittle of the edible-nest swiftlet!) Gray tree frogs avoid the danger of ground predators by gathering in trees en masse and stirring the females’ secretions into a foamy froth. Tadpoles hatch from the eggs laid in this froth and drop to the ground once they become frogs.
All nests—large, small, sloppy, or sturdy—are designed to protect the young until they are old enough to survive on their own. A nest can cryptically camouflage its tenants, place them out of reach of predators, and shield them from the elements. As you peruse the nests at your zoo, consider what it is about the design that gives offspring a survival edge. By building a nest that works, the parents buy themselves a bit of genetic immortality—a fair trade—no matter how laborious construction may be.
NEST BUILDING. Many aviaries provide visitors with a bird’s-eye view of nature’s master architects.
BIRTH Elephant and giraffe calves are in for a big surprise when they’re born.
Giraffes may drop more than 5 feet to the ground, a fall that gets them breathing, but doesn’t seem to harm them. The young of aquatic mammals, such as whales and dolphins, have a much smoother exit; if they need help taking their first breath, their mother gently buoys them up to the surface. On land, mammal mothers usually lick the placental matter from their young, both to remove the odor of birth and to help dry the fur so that chilling won’t set in. They may also lick the muzzle to encourage breathing or the anus to get the young to defecate.
Where predation is a concern, the mother may eat the young’s feces to keep telltale smells to a minimum.
Birds and most reptiles and amphibians develop outside the mother’s body in protective eggs. Birds usually incubate these eggs with the heat of their own body, sometimes developing a brood patch—a thinly feathered patch on their chest or belly that gives the egg direct access to warm skin. Most baby birds are born with a sharp “egg tooth” on their beak that helps them cut their way out of the shell. Clearing this first hurdle in the outside world is exhausting work, and, depending on the species, the parents may or may not help.
Alligators bury their eggs in a mound of vegetation, which works like a compost heap to warm the eggs. The female checks the nest regularly, wetting, shading, or removing vegetation to adjust for rises and falls in temperature. This thermostat management turns out to be vital for balancing the sex ratio in the population. Eggs that are incubated below 86 degrees Fahrenheit will be all females, while those kept above 93 degrees Fahrenheit will be all males.
Somehow, the mother keeps temperatures in the nest just right, giving birth to a balanced mix of genders. When it comes time for the young reptiles to hatch, they begin to cry out, prompting the mother to come running. She digs them out of their compost incubator, helps break them from their shell, and then carries them down to the water in her mouth, all without a scratch!
In contrast, most frogs, salamanders, and toads practice a lay-them-and-leave-them strategy. They deposit their eggs, coated with protective jellies or shells, in sheltered, moist areas. Since both eggs and hatchlings make great predator fare, they are usually produced in staggering numbers, in the hopes that at least a few will survive to adulthood. Snakes, depending on the species, either give birth to live young (tiny replicas of themselves) or else bury eggs to hatch on their own.
Either way, snake parents are well on their way to other pursuits shortly after passing on their genes.
GETTING TO KNOW YOU Whereas some animals play the odds by producing vast numbers of offspring and leaving them to fend for themselves, others pour energy into raising just a few young. In birds, the relationship between mother and offspring often begins while the chicks are still in the egg.
The young birds peep and the mother answers, as if urging them to come out.
The two keep up this soft and incessant contact for days or even weeks.
Mammals are particularly interested in getting to know their newborns, and
the mothers often use grooming as the vehicle. In cleaning and cuddling sessions, the female learns the sound of her young’s voice, its smell, and perhaps even the taste of its skin. This initial getting-to-know-you period is essential, especially later in life when the mother and young must find each other in the confusion of a herd of animals that look very much the same.
In the first hours or days of life, the young of many species fixate on the first moving object they see, forever associating that figure with “mother.” This process, called imprinting, ensures that the young animal will know exactly where to run when it’s hungry or in trouble. Unfortunately, depending on what they see after birth, animals can easily imprint on a human, a dog, or even a truck or tricycle.
Imprinting comes into play again when adult animals are looking for a suitable mate. Studies have shown that swans seek out mates that look somewhat like their parents (this ensures a species match-up), but that also look different enough. Staying away from animals with the exact markings of their parents keeps them from inbreeding with close relatives. It makes you wonder about our human incest taboo; might our aversion to mating with relatives be biological as well as cultural?