• Question: Why do only some animals lay eggs?

    Asked by imogenxmas18 to Paul, James, Hannah, Freya, Ceri-Wyn, Anthony on 11 Jan 2019.
    • Photo: Anthony Redmond

      Anthony Redmond answered on 11 Jan 2019: last edited 11 Jan 2019 12:29 pm


      Actually most animals lay eggs, and this is the ancestral condition! It is only a very few species that give live birth in comparison, but live-birthing has evolved multiple times (in sharks, mammals, amphibians, and many many times in reptiles), and that seems unlikely unless there are selective pressures driving it!
      It’s probably the case that live-birthing evolved to help defend unborn young from changes in the environment or harsh environments in general, such as predator behaviour and temperature changes.
      Live-birthing keeps offspring safe and sound in the mothers body until birth, e.g. with constant temperatures and nutrient supply. However, there are some trade-offs: live-birthing takes up more physical space inside the mother, meaning that smaller litters are produced, and also increases the time until the mother can mate again, further reducing the number of potential offspring. There is also a huge risk to the offspring if the mother is injured or killed, or faces other adverse conditions. On top of this, egg laying species can develop strategies to protect their eggs, such as building nests, camouflaging their eggs (some shark egg cases, like mermaids purses, are nearly indistinguishable from seaweeds), or even carrying their eggs around with them!

      So on top of simply being the default state for animals due to evolving before live-birthing, egg laying has its advantages! In fact, some studies suggest that some reptiles from live-birthing lineages have reverted to egg laying!

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