What Are Some Common Traits Between Reptiles and Mammals?
As children, we mostly learn about the differences between reptiles and mammals. Reptiles are “cold-blooded” while mammals are “warm-blooded”. Reptiles have scales while mammals have fur. Reptiles lay eggs, while mammals give birth to live young.
But there’s more to these two groups than meets the eye. While reptiles may superficially resemble the slimy, water-loving amphibians, they are actually more closely related to mammals and share several unexpected traits.
For example, reptiles and mammals are both “amniotes”. Amniotes are animals that are able to reproduce away from water due to internal fertilization (sperm fertilizes the eggs inside the female instead of outside in the environment) and an egg with a thick, relatively hard shell that doesn’t need to stay moist to avoid drying out.
Now you may be thinking, “Wait a second, mammals don’t lay eggs!” However that’s not entirely true. Primitive mammals like platypuses and echidnas (also called monotremes) do lay eggs. Way back in evolutionary history, all early mammals probably laid eggs. Eventually, most mammal lineages evolved internal gestation and live birth, but we still retain a reminder of our egg-laying past in the form of a yolk sac that forms part of the mammalian placenta.
Reptiles only have one inner ear bone, while mammals have three. However, those two extra ear bones are still found in reptiles, they’re just past of the jawbone instead of the ear. At some point in evolutionary history, those bones migrated from the jaw to the ear in early mammal-like-reptiles. Over time this left mammals with a single jawbone and three ear bones.
Just like with egg-laying, the line between warm-blooded (or endothermic) and cold-blooded (or ectothermic) is not as clear cut as many people think. A large species of lizard called the Argentine black and white tegu is able to generate its own body heat at certain times of year. There are also mammals, like naked mole rats, that have lost their ability to thermoregulate (or cool and heat their own bodies) and become essentially “cold-blooded”.
On a more basic level, reptiles and mammals are both vertebrates and both tetrapods. This means that they both have a backbone and they both have a four-legged body plan (or come from ancestors with a four-legged body plan, in the case of snakes and legless lizards).
Mai, Ulrike (photographer). (2017). Iguana Reptile Lizard [photography]. Retrieved from