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).
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.
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.
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.
Wolf spiders practice a form of infant care that is unique among spiders. As soon as the spiderlings emerge from their egg sac, they immediately clamber onto their mother's back, where they remain for up to two weeks, researchers reported in a study of several wolf spider species, published in 1964 in the journal Arkansas Academy of Science Proceedings.
The scientists observed that the first spiderling usually hesitated as it poked its head out of a hole in the egg sac. But it soon scrambled out, crawling over its mother's body until it settled on her back, and all of its siblings followed shortly thereafter and crowded aboard. As many as 1,035 spiderlings piled on in the wolf spider species Lycosa rabida, the scientists discovered.
Once the spiderlings were settled on their mother's back, the scene could be quite chaotic, according to the researchers.
"The egg sacs usually emptied within 3 hours, and the spiderlings have stacked themselves on top of each other over the "mother's" abdomen, and may be spilling over onto the sides and onto her phalothorax — which keeps her busy, occasionally, brushing them out of her eyes with her palpi," the study authors wrote.
The grey, tongueless, triangle-headed and curiously flat Surinam toad (Pipa pipa) is almost entirely aquatic, living in lowland rainforests in Bolivia, Brazil, Colombia, Ecuador, the Guianas, Peru and Trinidad.
During mating season, the male helps the female to position up to 100 fertilized eggs on her back, where they are overgrown by skin, according to the Encyclopedia of Life. While encased in her back, the embryos develop within the eggs as tadpoles for around three to four months, finally bursting out of the mother's back as tiny froglets that measure about 0.8 inches (2 centimeters) in length. After the leggy little ones emerge, the mother sheds her skin in preparation for the next mating season, the San Diego Zoo explained in a species description.
Opossums are North America's only native marsupials. There are about 75 species in this family living in both North and South America, and one of the most widely distributed species is the Virginia opossum (Didelphis virginiana).
Females give birth to litters of approximately 4 to 25 young that are "honey-bee-sized," following an extremely short gestation period of 12 to 13 days, according to a description published by Animal Diversity Web (ADW). The newborns drag themselves into the mother's pouch with their muscular front legs — only about eight of them will survive the journey. Those that do, develop for about two to three months and then transfer to the mother's back for another one to two months, as they gradually wean and become more independent.
Keeping track of up to 100 babies is a daunting task for any mother, and female scorpions do so by carrying their scores of young — called scorplings — on their backs until the scorplings' first molt, according to a study published in 2011 in the European Journal of Entomology.
The scorplings are born alive, and their bodies, which look like tiny versions of adult scorpions' forms, are soft and pale. They leave their mother's back after about 10 to 20 days, when their exoskeletons harden and darken.
Scorpion mothers sometimes enjoy an additional benefit from bearing their babies on their backs — easy access to a quick snack. However, this type of cannibalism typically only happens when the mother can't find any prey, the study authors wrote.
For the first year of their lives, giant anteater young — known as "pups" — frequently ride on their mothers' backs, according to a species description published online by the San Diego Zoo.
Giant anteaters (Myrmecophaga tridactyla),usually bear one pup at a time. Newborns weigh about three pounds (1.4 kilograms) at birth and emerge covered in a full coat of hair. They stick close by their mothers for four weeks, nestling under her to nurse and clambering up onto her back for a lift whenever she moves around. Pups grow more independent after about one month, but are still frequent passengers on their mothers' backs, the San Diego Zoo explains, adding that the pups will usually wean by the time they are nine months old, and leave their mothers at about two years old, when they are sexually mature.
Also known as tailless whip scorpions, whip spiders are not true spiders, but rather belong to an arachnid group known as amblypygids, which contains over 155 species. Though they have eight limbs, only six are used for walking, while two whip-like appendages — which can be several times as long as their bodies — act as sensory organs.
Females lay between 6 and 60 eggs, which they carry around in a leathery sac for around three months until the eggs hatch. When the babies first emerge, they are white and very soft, and cling to their mother until after their next molt, according to a species description published online by the Cincinnati Zoo.
Banded horned tree frog
The banded horned tree frog (Hemiphractus fasciatus) has a distinctive triangular "helmet" adorning its head, and is found in parts of Ecuador, Panama and Colombia. It does not have a tadpole stage in its life cycle. Instead, fully-formed froglets — miniature versions of adults — emerge after developing from eggs attached to the skin on their mother's back, according to a study published in 1974 in the journal Occasional Papers Museum of Natural History at the University of Kansas.
Females can grow to be nearly 3 inches (69 millimeters) in length, and their eggs measure about 0.2 inches (between 5 and 6 mm) in diameter. After the froglets have emerged from the eggs, depressions remain visible on the mother's back, the study authors wrote.
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Mindy Weisberger is a Live Science editor for the channels Animals and Planet Earth. She also reports on general science, covering climate change, paleontology, biology, and space. Mindy studied film at Columbia University; prior to Live Science she produced, wrote and directed media for the American Museum of Natural History in New York City. Her videos about dinosaurs, astrophysics, biodiversity and evolution appear in museums and science centers worldwide, earning awards such as the CINE Golden Eagle and the Communicator Award of Excellence. Her writing has also appeared in Scientific American, The Washington Post and How It Works Magazine.