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Not all embryos develop inside eggs. In many animals, embryos develop fully inside the parent’s body. This habit has evolved numerous times and in numerous styles. This episode, we discuss the complex and fascinating evolution of Live Birth.
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Look Ma, No Eggs!
Viviparity is a style of reproduction commonly known as live-bearing or live birth. It is the alternative to oviparity, the laying of eggs. In viviparous animals, embryos develop inside the body of the parent instead of developing in eggs outside the parent’s body.
Live birth is commonly associated with mammals (all modern mammals except monotremes give live birth), but the full list of viviparous animals is incredibly diverse, including many species of lizards and snakes; most cartilaginous fish; certain amphibians and bony fish; a wide variety of insects, worms, and other invertebrates; and a handful of extinct reptilian lineages (see below).
Viviparity is thought to have evolved independently well over 100 times, making it one of the most extraordinary examples of convergent evolution. Unsurprisingly, there are many styles of viviparity, varying in how embryos are nourished (through yolk, parental tissues, or other options), where embryos are incubated (ovary, uterus, or oviduct, for example), and how long the embryo spends gestating (ranging from weeks to years). There also isn’t a clear line between oviparity and viviparity, with many shared features, and some taxonomic families – and, in lizards, even some species – exhibiting both reproductive styles.
Live Birth Long Ago
Identifying viviparous animals in the fossil record can be tricky, since most of the specialized structures involved are soft tissues, unlike the durable shells of some egg-laying animals. But now and then paleontologists will uncover incredible adult specimens with the fossil remains of late-stage embryos still inside their bodies. Thanks to such fossils, scientists have identified pregnant examples of many modern groups of animals, and have discovered evidence of viviparity in mosasaurs, ichthyosaurs, plesiosaurs, placoderms, and other extinct groups.
Plenty of research has explored trends in the evolution of viviparity, though these can be difficult to nail down given just how diverse this lifestyle is. Many researchers have identified a link between viviparity and aquatic lifestyles, some have explored apparently correlation between live birth and colder climates, and some have even found evidence of reversions from viviparity back to oviparity.
Messel boa: Live birth in a 47-million-year-old snake
The first known case of eggs plus live birth from one pregnancy in a tiny lizard
Why Did Our Mammal Ancestors Stop Laying Eggs?
What Fossils Can Tell Us About the Evolution of Viviparity and Placentation (technical, sample)
Viviparity and oviparity: evolution and reproductive strategies (technical)
Convergent Evolution of Viviparity, Matrotrophy, and Specializations for Fetal Nutrition in Reptiles and Other Vertebrates (technical)
Matrotrophy and placentation in invertebrates: a new paradigm (technical)
Examples of live birth in extinct groups: ichthyosaurs, mosasaurs, plesiosaurs, placoderms, mesosaurs, choristoderes, early coelacanths, early chondrichthyans.
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