(Related: " Oddest Eggs of the Animal Kingdom.") Salamanders, like the odd-looking axolotyl, are mostly egg layers, as are the 6,000-plus known frog species. The developing babies scrape off the interior lining of the ovary and ingest it, a “kind of an amphibian equivalent of lactation,” Reznick says. Live bearing is common in some amphibians, but more unusual in others.įor instance, about 75 percent of the 180 species of caecilian, a legless amphibian, birth live young, Gibbons says. The female then feeds the baby with milk for another five to six weeks before she leaves the burrow to forage. (Related: " Spiky Baby Killers: Echidna Secrets Revealed.") Echidna researcher and National Geographic grantee Stewart Nicol describes the video, which shows three males actively pursuing a female on a farm in Victoria, Australia.Īfter a three-week pregnancy, the short-beaked echidna of Australia makes a nursery burrow, where she lays her egg directly into her pouch, incubating it for ten days until it hatches into a baby. 8, 2014-Never-before-published footage shows the mating rituals of echidnas-spiny mammals native to Australia that lay eggs. (See " Pictures: Rare "Cyclops" Shark Found.")įeb. In some sharks that bear live pups, “the mother produces egglets that are not fertilized, and the babies are feeding on them while in the ovary,” says David Reznick, an evolutionary biologist at the University of California, Riverside. Live birth is also rare in fish, accounting for about two percent of known species, including guppies and sharks. Only birds, turtles, and crocodilians bear no live young at all. (See " Evolution in Action: Lizard Moving from Eggs to Live Birth.") In general, though there are fewer reptiles in higher altitudes and in colder temperatures, proportionately more of them are live-bearing, he adds. In Australia, yellow-bellied three-toed skinks that live in higher, colder regions bear live young, while those in warmer lowlands lay eggs, which are harder to keep warm under cold conditions. Some reptiles tailor their reproductive strategy depending on where they live. (Related: " Amazing Video: Inside the World's Largest Gathering of Snakes.") Common garter snakes, for example, birth live young, while pythons lay eggs and guard them. ReptilesĪbout 15 to 20 percent of the 9,000 known species of snakes and lizards are live-bearers, Gibbons says. Here's a look at some animals who came to diversify how they multiply. Live bearing means the mother can better protect her developing embryo until it's ready to be born. (See " What Comes Before Snake Sex? A Kama Sutra of Courtship Moves.")īoth methods have their benefits, he says: Egg laying allows greater mobility for the mother who is "not encumbered by large eggs or embryos for a prolonged period," Gibbons says, and a higher likelihood of genetic diversity if there are multiple matings and clutches. Many animal groups have some species that lay eggs and others that birth live young, including fish, amphibians, and yes, even mammals, says Whit Gibbons, evolutionary biologist at the University of Georgia’s Savannah River Ecology Laboratory. While snakes are pretty unique in the animal kingdom, on this one they have to share the spotlight. “Are snakes the only family that evolved multiple birthing options-egg or live birth?” Louis Meluso asked Saturday's Weird Animal Question of the Week.
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