As a rule, reptiles lay eggs, while mammals deliver young through live birth. According to new research, however, this distinction is a bit more fluid than most assume—for reptiles, at least. Ancient snakes and lizards seem to have flip-flopped between these two birthing strategies around 175 million years ago before most finally settled on egg-laying.
In the fossil record of scaled reptiles, several embryos have been found still situated within the fossilized skeletons of plesiosaurs and mosasaurs. This evidence inspired other researchers to examine the history of live birth in reptiles, which turns out to have evolved in those species at least 115 times through evolutionary history.
The authors of this new study used genetic sequencing to explore the birth history of around 2,000 species of lizards and snakes, Nature World News explains. They found that snakes and lizards first evolved live birth around 175 million years ago. Today, around 20 percent of scaled reptiles reproduce using live birth. Until now most researchers assumed that reptiles did not go back to egg-laying after making the shift to live birth. But this study found evidence of a "complex pattern of subsequent transitions" between egg laying and live birth. Birthing method, they say, appears to be "a trait that shifts frequently in response to ecological conditions." Colder temperatures seem key to triggering the switch to live births, because those conditions encourage a female to retain her eggs for longer and longer periods of time.
Even species that favor live birth still seem to retain their ability to produce eggs, as one recent example shows. A species of skink found in Australia lays eggs—except when it lives higher up in the mountains, in colder environments, National Geographic reports. In that case, those more northerly animals are almost all give birth to live young.
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