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What Are Monotremes? 5 Mammals That Lay Eggs

Here's everything you need to know about monotremes, a small group of mammals that lay eggs including the platypus and echidna.

Written by
Samantha Bailon

When you think of eggs, your next thought is likely of chickens, followed by birds in general, and then reptiles. But it turns out that there are mammals that lay eggs, too! The select group is called monotremes.

Monotremes are a unique bunch of animals found primarily in Australia and New Guinea. The group is also a relatively small one, comprised of only five species, including the platypus and several kinds of echidna, or spiny anteater.

The fab five are relatively elusive, and most active at night when hunting the small invertebrates (think ants and worms) that comprise their diet. Monotremes are introverts, posing no danger to humans, no teeth, and a habit of rolling into a frightened ball when in need of protection. 

Once a dominant force in Australia, the monotremes lost ground to the marsupials between 54 and 71 million years ago, thought to survive by taking to the water. These days, humans are to blame for the decline of the monotremes, developing the group’s remaining habitat and poaching the friendly mammals for sport. All that to say, aside from their egg-laying abilities and minimal social prowess, monotremes have something else in common: all five are listed as endangered or near-threatened species. 

Read on to learn more about this small contingent of mammals that lay eggs, the better to appreciate—and strive to protect—them. 

5 Mammals That Lay Eggs

1. Duck-Billed Platypus

The sleek and adaptable platypus makes its home on land and water and is odd enough that the first scientists to learn of the mammal thought they were being pranked. A platypus typically lives for around 17 years, and mates more than once within that timespan. One mother platypus usually produces 1-2 eggs that hatch within 10 days. 

The platypus is an independent animal, able to survive along as early as 3-4 months after hatching thanks to a highly sensitive snout (aka food locator). To protect themselves, platypuses produce venom—another rarity for mammals—from spurs on their hind legs, the poison potent enough to kill small animals and maim others. 

2. Eastern Long-Beaked Echidna

The eastern long-beaked echidna can be found in high-altitude environments like alpine forests and the New Guinea highlands. As with all echidnas, these small creatures are covered in spines—with less fur than their short-beaked counterparts—and use their snouts and tongues to feed on the likes of termites and ants. 

The echidna breeding season occurs within the summer months, during which the mammal will typically lay one egg. 

3. Western Long-Beaked Echidna

As with all echidnas, the western long-beaked echidna makes its home in Australia, New Guinea, and Tasmania. This is the largest of the group and can weigh nearly 40 pounds. Nocturnal and solitary, the echidna is an excellent digger thanks to its collection of claws, which it uses to forage for food in tropical hill forests and upper montane forests.

After 10 days, the echidna’s egg hatches to reveal what is called a puggle, which at birth is the size of a lima bean. 

4. Short-Beaked Echidna

The general echidna story continues, this time in Tasmania and the New Guinea lowlands where the short-beaked iteration makes its home. Short-beaked echidnas have dark fur and a wealth of hollow spines, along with a pouch on their stomach (a physical feature of males and females alike). 

Now back to the puggles! Once hatched, the puggle hangs out in the pouch for a month or so, until its spines begin to develop. At that point, it’s moved into a burrow to mature for a few more months. 

5. Sir David's Long-Beaked Echidna

Named for historian and naturalist Sir David Attenborough, this echidna is the smallest of the family—and also the most elusive. This little spiked mammal is found in the Cyclops Mountains of New Guinea, with non-venomous spurs on its legs.

Not much is known about this echidna’s reproductive patterns, but it can be deduced that they, too, lay eggs that become puggles.