Are There Mammals That Lay Eggs?
Mammals give birth to live young, right? That’s a huge component of what it means to be a mammal. Mammals are hairy, produce milk, are warm-blooded, and give birth to live young…except for two animals.
That’s right. There is a group of mammals, called monotremes, that lays eggs instead of giving birth to live young.
Monotremes are a pretty exclusive bunch. There is just one species of platypus that’s alive today. There are three or four types of echidna, with several subspecies. All of the world’s monotremes live in Australia or New Guinea. Although they’re warm-blooded and they have hair, monotremes aren’t quite like the rest of the mammal world.
The Extant (Currently Alive) Species of Egg-laying Mammals Are:
- The Duck-Billed Platypus. Ornithorhynchus anatinus is a bizarre-looking animal that’s found in streams with banks suitable for burrowing. They are sleek and brown, with a beaver-like flat tail. Their face has a large, duck-like bill. The males have venomous spurs on their rear ankles. This species eats bottom-dwelling invertebrates such as crabs. They live in eastern Australia, including Tasmania.
- The Short-Beaked Echidna. Tachyglossus aculeatus is a widespread animal that’s found across Australia and into New Guinea. They are spikey and have powerful front limbs made for burrowing. These echidnas eat insects and hibernate through the winter.
- The Eastern Long-Beaked Echidna. Zaglossus bartoni eats almost exclusively earthworms. They are also spined. They live in humid forests, especially above sea level, in New Guinea. There are four subspecies of eastern long-beaked echidna, primarily separated by geographic area.
- Sir David’s Long-Beaked Echidna. Zaglossus attenboroughi lives in the Cyclops mountains of New Guinea. These are the smallest of the Zaglossus genus. It is feared that this species is highly endangered or even extinct, as a specimen has not been collected since 1961. Interviews with locals in 2007 (and signs of digging) suggest that the species is still alive – but no scientists have seen one yet.
- Western Long-Beaked Echidna. Zaglossus bruijni is the largest extant monotreme, tipping the scales at 5 to 10kg. They prefer to live in alpine meadows and montane forests. They are considered critically endangered. They live in New Guinea.
Egg-Laying Mammals are Different From Other Mammals in Several Fascinating Ways:
- They have soft beaks or bills, quite unlike the muzzles and mouths seen in other mammals.
- Their metabolic rates are lower than that of most other mammals.
- Adult monotremes don’t have teeth. Young platypuses do have tribosphenic molars, but they’re lost as adults.
- The adult male platypus is venomous. Echidnas also have the spurs, but lack venom. Fossil evidence suggests that their last common ancestor was venomous. A few other mammals, including the slow loris and several species of rodents, are also venomous.
- The shoulder girdle of monotremes is quite different from that of other mammals. This leads to a reptile-like gait where the animal’s legs are to the side, rather than directly beneath the animal.
- Monotremes are capable of electroreception. This is the biological ability to sense electrical stimuli. Sharks are also capable of this.
- Egg-laying mammals don’t have nipples. Instead, they excrete milk from mammary glands on their skin.
- Like marsupials, very young echidnas spend time inside of a protective pouch.
- Monotremes only have one external orifice, called a cloaca. This orifice is used for reproduction as well as for the passage of urine and feces. Birds and reptiles also have a cloaca rather than separate orifices.
Monotremes were apparently a bit more common long ago. Fossil records indicate that egg-laying mammals diverged from the marsupials and placental mammals (all the other mammals like dogs and cats and people) as early as the Triassic. All of this is pretty unknown, though. The fossil record of mammals simply isn’t very good that far back. The extinct egg-laying mammals were more diverse and numerous than the few species we have today. Ancient egg-laying mammals extended beyond Australia and New Guinea to South America.
How Do Egg-Laying Mammals Raise Their Young?
All of the extant species of monotremes are quite involved parents. The eggs are retained for quite a long time, absorbing nutrients from the mother. Egg-laying mammals generally hatch within just 10 days of being laid – a much shorter time than that of similarly-sized egg-layers.
Echidnas have backward-facing pouches (somewhat like the pouch of a marsupial). The eggs are laid directly into this pouch. In contrast, platypuses lay their eggs inside of a burrow and then curl around the eggs to incubate them.
Like baby marsupials, newly hatched monotremes are extremely helpless. They’re almost larval-like and are extremely undeveloped. As such, their mothers spend a lot of time and energy caring for them.
Monotremes are long-lived and produce small litters, which is fairly typical for species that need to spend a lot of time and energy on rearing their young. The young echidnas and platypuses spend time inside of their mother’s protective pouches.
Since female egg-laying mammals don’t have nipples, they secrete their milk from glands in their skin. Monotreme milk has elevated levels of an antibacterial protein that isn’t found in other animals. Scientists think this may be because there’s a higher risk of infection with this method of nursing.