Life’s beginning varies a lot among mammals. We can group mammals into three main categories based on how their babies come into the world. The main types of mammals are called monotremes, marsupials, and placentals.
Most mammals give birth to live babies instead of laying eggs. However, there are a couple of exceptions: platypuses and echidnas. They belong to a special group called monotremes. Even though they are unusual, monotremes, like the platypus, still provide milk for their young after they hatch. Monotremes are found in Australia and surrounding islands.
Marsupials are another interesting type of mammal. They have a very short gestation period – that is the amount of time they are inside their mom’s belly. When they are born, they are very tiny and need a lot of care. They crawl into a pouch or open area on their mother’s belly, where they stay warm, safe, and can drink milk while they grow. A famous example of this is a kangaroo with a joey in its pouch. The kangaroo, like most marsupials, lives in Australia. South America is also home to many marsupials, and there is even one marsupial in North America: the opossum.
The largest group of mammals is called placentals. They give birth to live babies after a longer gestation period. A special organ called the placenta helps give these mammals the nutrients they need while still in the womb. Placental mammals are born more developed than marsupial babies. Some, like baby deer (called fawns), can even stand up and start walking the same day they are born. Placentals are found in many habitats all around the world.
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References and Further Reading
- Ferner, K., Schultz, J. A., & Zeller, U. (2017). Comparative anatomy of neonates of the three major mammalian groups (monotremes, marsupials, placentals) and implications for the ancestral mammalian neonate morphotype. Journal of anatomy, 231(6), 798-822. Available via Wiley Online Library: https://onlinelibrary.wiley.com/doi/10.1111/joa.12689
- Thomas, L. (2017). Monotremes, Marsupials, and Placentals. UC Santa Cruz. Available: https://norriscenter.ucsc.edu/collections/mammals/monotremes-marsupials-placentals.html
- University of Hawai’i at Manoa (n.d.). What is a Mammal. Exploring our Fluid Earth. Available: https://manoa.hawaii.edu/exploringourfluidearth/biological/mammals/what-mammal
- Wible, R. (2022). DO ANY MAMMALS LAY EGGS? Carnegie Museum of Natural History. Available: https://carnegiemnh.org/do-any-mammals-lay-eggs/
- Used for editing word-choice and tone: OpenAI. (2023). ChatGPT (September 25 Version) [Large language model]. https://chat.openai.com