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A human mother carries a growing fetus in her womb for approximately nine months, but even after the baby is born, the helpless newborn still needs to be carried. In fact, many animal mothers transport their young, sometimes many dozens of them at a time, and sometimes lugging them around for years.
Animals tote their babies in a variety of ways — marsupials like kangaroos, koalas and wallabies have specialized pouches that cradle their still-developing infants, while fish, crocodilians and certain mammals often transport their young using their mouths.
But a surprising variety of animals carry their young on their backs, and for Mother's Day, Live Science took a closer look at some of these "piggybacking" mothers (but despite this behavior's nickname, it is not practiced by hogs or pigs).
ChimpanzeeSlide 2 of 21
Great apes — gorillas, chimpanzees, bonobos and orangutans — are our closest primate relatives, and all are known to carry their young on their backs. In most primate species, newborns are unable to walk or care for themselves, and are not protected by nests. Their slow development requires that their mothers keep them close, for frequent nursing and for transportation and protection. Infants are usually transferred from the front of the mother's body to her back when they are strong enough to grip her securely — typically when they are few months old, according to a study published April 2008 in the journal Naturwissenschaften.
Chimpanzees are the most social of the great apes, and they also demonstrate a long period of dependency between mothers and offspring. Infants nurse for up to five years, and frequently stay close to their mothers for several more years after they are fully weaned, according to the nonprofit conservation organization Center for Great Apes.Slide 3 of 21
Horned marsupial frogSlide 4 of 21
Horned marsupial frog
The term "marsupial" typically conjures images of mammals that tote their young in furry pouches, such as kangaroos, koalas, and other denizens of the Australian continent. But the rare and endangered horned marsupial frog (Gastrotheca cornuta), which lives in the forests of Panama, Columbia and Ecuador, also bears a stretchy baby-bearing pouch — on her back.
Inside her pouch, the mother frog incubates a small clutch of the largest known amphibian eggs, which measure about 0.4 inches (10 millimeters) in diameter. To put that into perspective, the mother's entire body measures about 3 inches (77 mm), herpetologist Jay M. Savage, an adjunct professor of biology at San Diego State University, wrote in "The Amphibians and Reptiles of Costa Rica" (The University of Chicago Press, 2002).
After a male fertilizes the females' eggs, he guides them into her pouch, where the embryos develop into froglets. The pouch is a permanent structure, but it changes greatly during reproduction, with separate chambers forming to encase each tiny embryo. It is thought that air circulates to the developing froglets' gills through a network of veins in the pouch, Savage wrote.Slide 5 of 21
SwanSlide 6 of 21
Swans, the world's largest waterfowl, are widely recognized for their loyalty to their mates and are known to pair up for life. But swan mothers have also been observed providing especially devoted attention to their young — known as cygnets — by serving as a temporary flotation device to help the little ones as they learn to swim.
Of the six knowns swan species, orange-billed mute swans (Cygnus olor) are the most common sight, visible in ponds and lakes in Europe, northern-central Asia and in North America, where they were introduced in the late 19th century. They were brought to the U.S. as "decorative" birds in zoos, parks and private estates, but feral populations spread to the Northeast, Mid-Atlantic, Great Lakes, and Pacific Northwest regions, according to the Cornell Lab of Ornithology.
Female swans typically lay five to seven eggs, which incubate for 36 to 38 days, according to the University of Michigan Museum of Zoology. Cygnets are covered in white or grayish down, and can swim and dive about 24 hours after hatching. Their mothers and fathers share parental care, frequently carrying the cygnets on their backs, with their wings curled protectively over their babies.Slide 7 of 21
Wolf spiderSlide 8 of 21