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Do mammals lay eggs? - What is a mammal?

Unless you were read animal encyclopedias as a child (raises hand), you may not know some of the fascinating features that wildlife across the globe can exhibit.

A short-beaked echidna searches for food.
Monotremes like this short-beaked echidna have a unique reproduction technique.

Mammals are a group of animals that most wildlife lovers can point at and say, "That is a mammal." But, for those who are less informed, what makes a mammal a mammal? Do all mammals give live birth? Join the flock to get a simple answer to these tough questions!

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What is a mammal?

Mammals can be categorized as a group of animals that have backbones, bodies insulated by hair, that nurse their infants with milk, and all share a unique jaw articulation. This is quite the scientific simplification of mammals, but it at least allows us to begin forming a picture of what is and is not a mammal according to this scientific definition.

What is the first mammal you picture in your mind? Is it a bison? A bear? A human? Comment below!

How many types of mammals exist on Earth?

It is believed there are between 5,000 - 6,000 types of mammals on this planet. However, depending on which mammalogists you ask, that number may differ wildly! The diversity in the form and function of these 5,000+ species is mind-blowing. The smallest mammal is a bat, the bumblebee bat, which weighs only 1.5g! The largest mammal is the blue whale, which comes in over 100 million times (up to 200,000 kg) the weight of that tiny bat!


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How do most mammals reproduce?

The majority of mammals are live-bearing, placental mammals that produce young mammals by utilizing a special organ inside the uterus called the placenta. When young mammals are born, they are alive, and in the majority of mammals, they are connected to their mothers through an umbilical cord. Marsupials differ slightly, as their placentas differ in form and function. However, mammalogists still consider them placental mammals. The egg that forms inside marsupials is much more like a reptile egg than that of other mammals, but the young are born alive, still. However, unlike other mammals, very little development occurs inside the egg. Instead, the young are born at an early age and develop primarily in the pouch of the marsupial.

BUT... there is an order of mammals that do not give 'live' birth to their young. The monotremes, which include the platypus and echidnas, utilize a different strategy to reproduce.

Do any mammals lay eggs?

Most mammals do not lay eggs; however, there are five known extant species of mammal that do lay eggs. The five mammals which lay eggs are the platypus, the eastern long-beaked echidna, Sir David's long-beaked echidna, the short-beaked echidna, and the western long-beaked echidna.

A platypus searches for food in Tazmania.
The adorable platypus lays eggs. It is one of five mammal species to do so!

Where can I find echidnas and the platypus?

Some of the most stable and dense populations of platypuses occur in Tazmania. There is a stable population that was introduced to Kangaroo Island in Australia. There are some populations found throughout western and southern Australia, though they becoming more scarce every year. Echidnas can be found throughout Papua New Guinea and Australia. The Short-beaked Echidna is a stable species in Australia, while the other three echidnas are only in Papua New Guinea and Papua Province (which is Indonesia). Sir David's Long-beaked Echidna is found only in the Cyclops Mountains in extreme northern Papua Province.


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