Difference Between Marsupials, Placentals and Monotremes
What is a Mammal? Three Types of Mammals
The most important characteristic of all mammals is that they feed their babies milk from their mother's body. Hence the name "mammal" derived from the Latin word mamma, which means teat or nipple.
Mammals also breathe air, have backbones, are warm-blooded, have a four-chambered heart, and have fur or hair. The young of most, but not all, mammals are born alive.
Placental Mammals Baby Inside The Body
Placental mammals constitute over 5,000 different species of animals and include those as varied as humans, aardvarks, cats, horses, and whales.
The key characteristic of a placental mammal is that it gives birth to babies that are far more advanced in their development than monotremes and marsupials. They do this by developing the baby inside their body in a uterus. The baby is attached to the uterus by an umbilical cord, which in turn is attached to an organ called a placenta, which is connected to the mother’s blood supply. In this way, the baby gets all the nutrients it needs to grow from its mother’s blood supply. Because the offspring of these animals are heavily dependent on the placenta, they are referred to as placental animals.
The term “placental” is somewhat misleading because marsupial mammals also have a rudimentary placenta. The key difference is that in a placental mammal, the baby remains attached to the placenta inside the mother’s uterus for a relatively longer period of time compared to a marsupial. Placental mammals are sometimes also called eutherian mammals to try to clear up this confusion. Because they have a uterus, placental mammals do not have a pouch. Typically the offspring of a placental mammal weighs about 5% of its mother’s bodyweight.
Do all Animals have Belly Buttons?
No. Not all animals have belly buttons or navels. Only placental mammals such as you and me, cats, cows, whales, etc. have belly buttons. That’s because placental mammal babies have an umbilical cord which originally connected there.
Placental mammals provide nourishment for their young by providing them with milk through teats or nipples. Because the baby is already relatively well developed before it is born, the period of lactation is much shorter than for equivalent marsupial and monotreme mammals.
In general, placental mammals have a body temperature of 38°C. This is about 3°C higher than most marsupials and monotremes.
The basal metabolic rates (BMRs) of placental mammals is about 30% higher than marsupial and monotreme mammals. This means they consume far more energy than an equivalent sized marsupial or monotreme.
In general, placental mammals have fewer teeth than marsupials. They also grow two pairs of teeth, namely milk teeth, and adult teeth.
Adult placental mammals do not have a cloaca. Embryonic placental mammals, however, have a rudimentary cloaca that evolves into a separate anus and genitalia such as the urethra and vagina in a female and urethra and penis in a male. (Note: there are a few rare exceptions).
Marsupial Mammals Baby Outside the Body in a Pouch
Marsupials also give birth to live babies like placental mammals. They, too, have a uterus and placenta. The key difference is that the marsupial placenta is more like a yolk sac, and the marsupial baby is attached to it for an extremely short period compared to a placental mammal. A tiny and underdeveloped offspring is then born. Typically the offspring of a marsupial mammal weighs just 0.003% of its mother’s bodyweight. It's tiny!
These babies are blind at birth, have no ears, and hardly any back legs. They have strong stumpy front legs and a good sense of smell. With these two assets, the young baby crawls from the mother's birth channel into the pouch, where it attaches to one of her teats and remains there for many months, slowly growing into a viable young animal.
Marsupial babies are nourished with milk supplied by their mothers through teats inside their pouches. Because their young are born relatively underdeveloped, these young animals lactate for a very long time compared to equivalent placental animals.
In general, marsupials have a body temperature of 35°C, which is lower than placental mammals that have a temperature of about 38°C. Usually, a marsupial has a body temperature that is about 2.5°C lower than that of a comparably sized placental mammal. A lower body temperature means less energy used to keep warm.
Marsupials have basal metabolic rates (BMRs) that are 30% lower than that of equivalent sized placental mammals. They are far more efficient uses of energy.
Marsupials, in general, have more teeth than placental mammals. They also grow only one set of teeth, some of which are replaced during their lifetime. (They have no milk teeth).
Marsupials also have a cloaca. However, it is a hybrid design. Faeces and urine are excreted through the cloaca, but there is a separate reproductive tract. For example, in the male marsupial, urine is not excreted through the penis. Instead, it comes out of the cloaca/anus.
Monotreme Mammals Baby is An Egg First
The female lays a single leathery-egg directly into a shallow pouch in her belly. There the minuscular baby animal hatches, usually in about ten days.
Monotremes have no teats or nipples. Milk seeps out of pores in the mother's abdomen, and the young animal laps it up.
Monotremes have the lowest body temperature amongst mammals at 30°C. This 8°C lower than a placental mammal and 5°C lower than a marsupial.
Monotremes have basal metabolic rates (BMRs) 25-30% lower than those of most placental mammals
Monotremes are the only mammals with a fully functional "true" cloaca, a single rear opening (orifice) to the outside, similar to a bird or reptile. The animal’s digestive, reproductive, and urinary organs are connected to its cloaca.
Summary of Differences Between Placentals, Marsupials & Monotremes Differences Summarised
|Number of Species||5,000+||334||2|
|Gestation||Uterus||Uterus & Pouch||Pouch|
|Birth||Live Babies weighing 5% of mother's weight||Live Underdeveloped Babies 0.003% of mother's weight||Eggs|
|Teeth||Milk & Permanent||Permanent Only||No Teeth|
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