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There are almost six and a half thousand types of mammals that inhabit planet earth, although some are recently extinct. While they do share many of the same traits and characteristics, the way that they give birth does vary somewhat. There are three different ways in which mammals give birth, although once the young are born, they are all nourished with their mother’s milk.
What is a Mammal?
A mammal is an animal from the class of Mammalia. This is a class with several thousand different species that all share some common traits. The most common type of mammal is the rodent which includes animals like rats, mice, and squirrels.
Most people understand mammals as being animals that give birth to live young. While this is true in most cases, there are mammals that lay eggs, such as the duck-billed platypus, and I’ll go into more detail on this later on.
However, one thing that all mammals have in common is their ability to produce milk to feed their young. This is where the group gets its name, but it’s not the only feature they share.
In order to be classed as a mammal, an animal must have some kind of body hair or fur as well as being a vertebrate. That raises the question of why dolphins and whales do not have fur but are considered mammals. In this case, hair would not serve a purpose, so it is replaced with a layer of fat under the skin to keep the animal warm in the water.
What’s more, all mammals have the ability to breathe using lungs and are warm-blooded.
Placental mammals are those that develop a placenta during gestation. This is a temporary organ that attaches to the inside of the uterus and connects to the young via an umbilical cord. It is designed to filter nutrients through to the growing embryo as well as removing waste products. Once the mammal gives birth, it will also pass the placenta which is no longer needed. During any subsequent pregnancies, a new placenta will form.
Placental mammals usually have a much longer gestation period than other types of mammal including the blue whale which carries its young for around one year. However, one of the main benefits of this is that the young are born fully developed. For prey animals like zebras, this is advantageous as they are able to start walking and running soon after birth.
Human pregnancy begins from the moment the sperm fertilizes the egg, and this can happen as little as 35-40 minutes after intercourse. However, most women do not realize they are pregnant until they’re somewhere between four and eight weeks into their gestation period.
Humans can also become pregnant via artificial methods, and the science behind this has become increasingly impressive. In most cases, humans carry just one baby per pregnancy, although multiple births are common, even more so in artificially induced pregnancies.
The gestation period for humans is typically described as lasting for 40 weeks (although most people would say nine months). However, it’s completely normal for babies to be born anywhere between 38 and 42 weeks.
There are typically three stages of a human pregnancy which are called trimesters; the 1st, 2nd, and 3rd. The first trimester begins at the moment of conception and continues until around 12 weeks. By around the third week of pregnancy, the cluster of cells becomes an embryo although at this point, it is still not supported by the placenta, which doesn’t typically attach until around weeks 8 to 10. That said, by week three, the first signs of a brain are already beginning to form.
During the 2nd trimester, any dangers associated with early pregnancy loss are over, and many expectant mothers start to feel much better within themselves physically with things like morning sickness subsiding. It’s during this period that the baby grows significantly and can reach around 12 inches (30 cm) by the end of the trimester. Its organs continue to develop and the baby will begin moving around in the womb; this can be felt by the mother and, as the baby gets bigger, can even be seen from the outside!
The third trimester begins at around week 28 of pregnancy and the baby may almost double in size during this time. The baby will also practice expanding its lungs in order to be able to breathe in the outside world.
Around the 40th week of pregnancy, the mother will prepare to give birth, and her body will go into labor. This is a period where the uterus begins contracting, and the cervix (a muscular protective opening of the womb) starts dilating to allow the baby to pass through into the birth canal.
Over the course of several hours, the baby’s head will also push against the cervix, further encouraging it to open. This can be a very painful experience for the mother but humans have developed many medications that can be used to reduce pain or numb it entirely.
Once the cervix is fully open, the contractions within the uterus force the baby through the cervix and into the birth canal until it reaches the vaginal opening and is born. Following this, further contractions cause the placenta to break away from the uterine wall and this is also delivered through the vagina.
While human babies are fully developed at birth, they’re much more helpless than other types of placental mammals. This is to do with their brain development which isn’t as advanced as their other systems at birth. In order for their neurological development to match their other physical development, a baby would need to gestate for up to 21 months. However, their physical size at this point would be too large for the mother to cope with or indeed, give birth to.
They say that dogs are man’s best friend and many of you reading this will have had experience of dog pregnancy and birth as a pet owner or breeder. Dog pregnancies are similar to humans in that they also have a placenta but the gestation period is much shorter.
When a female dog first becomes pregnant, the fertilized eggs remain relatively high up in the womb. However, over about a week or so, they move down and sit in a uterine liquid much lower in the womb. They’ll stay in the uterine horn for up to 18 days, at which point, the eggs are embedded into the uterine wall.
During the first month, the young puppies will begin developing, and after around 30 days, it is possible to detect a fetal heartbeat from outside the womb. During the second month of pregnancy, the pups develop ready to be born, and their weight may double by up to 50%.
Generally speaking, dogs are pregnant for around 62 to 64 days. However, the size of the litter and the breed of the dog may impact the length of gestation.
Before the female gives birth, she may show several signs, such as a loss of appetite and becoming restless. Her body temperature may decrease, and she may become reclusive as labor draws ever closer. However, the first 12 to 24 hours of labor may go unnoticed, although uterine contractions will occur.
As the puppies start being delivered, it usually takes no more than 60 minutes per pup, and this stage of birth can last for up to 24 hours. Just like humans, the mother dog will deliver the placentas shortly after her young are born.
The blue whale is the largest animal on earth, and a fully grown adult measures up to 98 feet (30 meters) and weighs as much as 330,690 lbs (150,000 kg)! Even the calves are around 23 feet (7 meters) at the time of birth.
Being such large animals, it’ll come as no surprise that the blue whale requires a much longer gestation period and, on average, the females are pregnant for around 12 months. Because of this long gestation, females tend to only reproduce once every two to three years and they typically only give birth to a single baby each time. While it has been documented that blue whales may produce twins, it’s unlikely that both young will make it to full term.
Blue whales live for up to 80 years and don’t become sexually mature until between the ages of 5 and 15. When they breed, the young are usually born somewhere between December and February. But before this, they need to find a mate and this typically happens over the course of a few months when males will follow their chosen female and have to fight off other males that show an interest.
Not much is known about the actual act of mating in blue whales, but it is thought that intercourse is very brief and doesn’t last much longer than a minute or so. After fertilization, the whale fetus develops at an alarming rate, growing as much as an inch (2.5 cm) per day!
The milk of the blue whale is incredibly high in fat and may contain up to 50% fat. This milk is what causes the calves to grow at such a rapid rate, especially when you consider that they might drink up to 59.4 gallons (225 liters) per day for the first six months of their lives. This causes the young to gain as many as 250 lbs (113 kg) every day! Interestingly, the nipples which are used to nourish the young are located on either side of the whale’s genitals.
Marsupials do have a placenta, although this is not considered a true placenta by scientists since it does not have the same advanced function as that of a placental mammal. Moreover, there are other differences that set these two groups apart, including a much shorter gestation period.
Marsupials give birth to very small young, in some cases, like the quoll, they weigh similar to a grain of rice. However, once they emerge from the uterus, the young stay inside their mother’s pouch, where they’re able to feed on milk as they develop.
The majority of the 334 types of marsupials are found on the continent of Australasia, but there are some American examples, including the opossum.
While these babies are born severely underdeveloped, there is the benefit of being able to be simultaneously pregnant and carrying a baby in the pouch. What’s more, where conditions are not optimal, some marsupials like the kangaroo, are even able to pause their pregnancies.
Kangaroos are perhaps one of the most well-known marsupials and are often used as a depiction for the group with a cute little joey inside its mother’s pouch. However, the life of a kangaroo starts off on a much smaller scale.
When kangaroos give birth, they produce a baby that is no bigger than a jelly bean. This tiny joey makes its way out of one of its mother’s three vaginas and into her pouch, where there are teats that produce milk for it to feed on. What’s super interesting is the joey doesn’t need to suckle as muscle contractions of the mother force milk out.
Aside from its clawed forearms, used to make their way into the pouch, the joey is vastly underdeveloped and unable to see.
While rabbits are usually seen as breeding machines, kangaroos aren’t far behind; it’s almost like they’re made for reproduction! Once the joey is established and comfortable in the pouch, the mother kangaroo can begin roasting her next fertilized egg. What’s really amazing is that kangaroos have the ability to do something called embryonic diapause which essentially means they can press pause on a pregnancy until the conditions are just right to sustain it. This usually happens in the very early stages while the pregnancy is still a cluster of cells.
However, once the second joey is born, it’s going to need to make use of the pouch but joey number one is still in residence. Never fear; the mother kangaroo has a solution. She has the ability to produce two different types of milk to nourish both of her babies despite their varying life stages.
Kangaroos are only pregnant for around 34 days, but once the joey is born, it’ll remain in the pouch for as long as 10 months. However, from around the age of 7 months, it will leave to explore, coming back to its mother afterward.
There are around 60 species of possum, but they all reproduce and give birth in a very similar manner. Being marsupials, they also have a very short gestation period, which usually lasts for around two weeks. Unlike the kangaroo, a female possum may give birth to up to 20 babies at once. The joeys must then make their way to the pouch, where there may or may not be teats that provide them with milk.
Sadly, while possum litters are quite large, it’s not uncommon for many of the babies to fall as they make their way to the pouch, resulting in their death. Out of a litter of 20, only around 13 will make it to the safety of the pouch.
Mating in possums takes place twice each year and for the longest time, people believed that the males, with their forked penises, mated with the females’ noses, who would then sneeze their babies into their pouches. Of course, this is a tall tale, and mating between possums is very similar to other mammals.
Possums are short-lived creatures that typically don’t live for longer than a couple of years. They stay in their mother’s pouch for around 100 days and are ready to mate as young as six to eight months. Just like the kangaroo, some species of possum are able to perform embryonic diapause and stop their pregnancies until they’re ready to continue.
The first stage of koala reproduction, as with any animal, is mating. However, this can come across as a pretty aggressive act, even when it’s consensual. In most cases, females will let males know if they’re not interested by hitting them, running away, and even peeing on them! But even when they’re receptive to the male’s advances, koala sex is anything but the cute and cuddly act we would imagine from them.
Once copulation has taken place, the female will release her egg, and the beginning of pregnancy can start. Just like other marsupials, koalas don’t have long gestation periods and are typically pregnant for around 35 days. When the joey is born, it must make its way to the pouch where the teats are located and it can feed on its mother’s milk. Since the baby weighs just 0.02 oz (0.5 grams) at birth, it has a lot of growing to do and will remain inside the pouch for up to seven months.
However, after this, the baby still won’t leave its mother and is carried around on her back until it reaches around one year of age.
Generally, koalas will reproduce between October and March, but there are records of koala births occurring in all months across Australia, depending on the area.
If I told you that some mammals lay eggs, you might question whether they are mammals at all. But they are. Monotremes have all of the characteristics of mammals but they don’t give birth to live young. Instead, they lay soft-shelled eggs out of a single hole used for mating, excretion, and egg laying.
There used to be hundreds of types of monotremes, but today only five species remain, and four of these are types of echidnas.
They lay their eggs in a pouch in their belly, and once the young are born, they feed on their mother’s milk. However, unlike placental mammals and marsupials, monotremes have no nipples or teats. Instead, their milk comes out of their pores!
The duck-billed platypus is one of the world’s most curious creatures. It has many features that are more commonly seen in birds such as a beak and webbed feet but it’s actually a mammal. The males have a venomous barb on their feet and they’re one of the new types of venomous mammals. Others include the slow loris.
Being a monotreme, the duck-billed platypus lays eggs as opposed to giving birth to live young. The species usually become sexually active at around the age of four, and breeding has been noted to occur at different times depending on the location. For example, individuals in Queensland may mate earlier in the year than those in Victoria.
There isn’t a lot of information on the breeding habits of the duck-billed platypus, as much of the action takes place below the water. However, it is known that the female must be receptive to the male’s advances. After copulation occurs, the female enters into a gestation period that lasts for around 27 days.
At this point, she will lay her eggs which require a further 10 days of incubation before the young emerge. Females typically lay between one and three eggs each time, and breeding normally takes place around once per year.
The eggs are laid in burrows on dry land, despite the platypus being an aquatic mammal. When they hatch, the babies are no bigger than a lima bean and are essentially helpless. They are sustained by their mother’s milk for the first three to four months of life and are then developed enough to swim and feed on their own.
There are four species of echidna that complete the monotreme group. These animals are also found in Australia and live in woodlands or forests where they can often be found hiding in rock crevices and caves.
When it comes to mating, the echidna has some pretty weird ways but they’re certainly interesting. Since the number of actively reproductive males far outweighs the number of females, these creatures are known to engage in group sex with one female and several males. On top of this, in order to ensure he reproduces, it’s not uncommon for males to attempt to mate with a hibernating female!
What’s most interesting about echidna reproduction is the male’s penis! It has four heads, but only two become erect at any one time. While this baffled scientists for some time, it’s now been observed that there is a septum inside the penis, essentially splitting it in two and allowing blood to flow to one half or the other. The male can even control which side he wishes to become erect.
Female echidnas do not mate every year, but when they do, they typically produce just one egg. This remains within her body for up to 28 days when it’s around the size of a grape. At this point, the egg needs to be incubated inside the mother’s pouch for a period of ten days.
When the young echidna, known as a puggle (how cute) is ready to be born, it uses a special tooth to chip its way out of the egg. Inside the pouch, on the abdomen of the mother are several pores which secrete milk. Unlike other mammals, echidnas do not have nipples or teats.
When Did Marsupials & Placental Mammals Diverge?
It’s no secret that, over millions of years, animals have evolved, and the creatures we see today are vastly different from their prehistoric ancestors. When it comes to mammals, it’s thought that the marsupial way of giving birth was the original and that placental mammals have only existed for around 65 million years.
It has recently been discovered that the common ancestor of modern placental mammals was a shrew-like creature from the Jurassic period. Around the time of the mass extinction was when these mammals were first thought to have developed, and there’s the suggestion that there was some serious diversity between species. However, after this, the evolution of mammals appeared to slow down.
In terms of when marsupials split from other mammals, it’s thought that this happened as many as 90 million years ago. It’s thought that they evolved in North America, gradually moving south to Australia during the Cretaceous period.