Mammals

Placental Mammals 101

Placental Mammals 101: How Reproduction In Mammals Works

Most mammals – except Monotremes and Marsupials – are placental mammals.

Yaks, rabbits, cows, hippopotamuses, bats and humans all fall within this category.

In all other vertebrates, the developing embryo is separated from its mother’s body by the amniotic membrane which surrounds the egg.

The reason for this is that after fertilization, the embryo takes on its own existence as a separate organism. This is because the mother’s auto-immune system (her internal defence mechanisms) will recognise the new embryo as a foreign body and attack it.

The amniotic membrane isolates the young embryo from all biological interaction with its parent, thus protecting it from Placental Mammals 101 attack.

This is good – without it, no reproduction except parthenogenetic would occur. The problem with it is that no nutrients can cross the barrier either. Thus once the young embryo is formed, its growth is limited to the amount of nutrients it has inside the egg with it.

In many species of vertebrates this means that the young are born very small. In the case of amphibians, fish and reptiles they must fend for themselves as miniature versions of the adult.

Birds get around this problem by using a system of intense parental care of the young, after they emerge from the externally incubated egg (except Megapodes whose young may never see their parents).

Placental Mammals vs Non Placental Mammals

The most primitive, non-placental mammals – the monotremes – also lay eggs.

The young which hatch from these are fed on milk – not on caught or collected food as in birds. Marsupials, the next evolutionary step, do not lay eggs. Instead, at the end of a short gestation period, the young are born live – but very immature to struggle to a teat and start feeding.

non placental mammal joey in pouch
Young Pademelon, a species of marsupial, in mother’s pouch

Marsupial mothers have a pouch covering the area where the teats are and it is in here that the young spend the next stage of their lives. Still, within the womb, embryonic marsupials (except for the Paramelidae) receive no nourishment from the mother after fertilisation.

Placental mammals are born in a much more advanced state than non placental mammals.

This is a result of a much longer gestation period. This longer gestation period is made possible by the placenta, which allows nutrients to travel from the mother’s system to the embryo’s – and for waste products to leave the embryo’s system so they can be disposed of by the mothers.

The Magic Of The Placenta

The placenta is the organ from which our group of mammals takes its name. It is the main reason we, the placental mammals, are so much more successful than other mammals.

The placenta is composed of several layers of material. It is very richly supplied with blood vessels and acts as an immigration barrier between the mother’s system and the developing embryo’s.

In this way, white blood cells and other immune system components (including blood) are kept within the boundaries of their own systems while nutrients (sugars, fats, minerals, etc) are allowed to pass in, and waste products to pass out, of the embryo’s environment.

This clever system allows young mammals to spend longer in their mother’s womb. They are then born, in most cases, ready to run with their parents after only a few hours or days of life.

There is however much variation between different mammals. For instance, herbivores are generally more capable of standing and moving on their own shortly after birth than carnivores.

Even within one order, there are great differences.

Thus young rats are born with closed eyes, naked and fairly helpless after a 21 day gestation period. While a baby guinea pig is born with open eyes, covered in hair and nearly able to feed itself after a 67 day gestation period.

baby rat born eyes closed
Just born baby rat has not yet opened its eyes.

A similar contrast occurs between the even more closely related Hare (eyes open, etc) and Rabbit (eyes closed).

Once born, young mammals are fed on milk and protected by one or more of their parents until they are able to fend for themselves. In the more social mammals, the young may then become part of the parents’ group.

There is much variation within this simple plan, but these particulars will be discussed in the individual pages for the various families.

Mammal Reproduction

In most mammals, fertilisation of the ovum (egg) takes place high in the fallopian tubes. Placental Mammals 101 How Reproduction In Mammals Works

The initial cell division occurs as the fertilised egg travels down the fallopian tubes. The egg becomes a ball of cells called a ‘morula’. This morula separates into an inner cell mass and an outer layer of cells. At this stage it is called a blastula.

The outer layer of cells is eventually called the chorion and the inner cell mass the zygote. While it travels down the fallopian tube, the developing cell growth is supplied with nutrition from stored food in the egg. But in mammals, this is in short supply and the developing zygote soon needs a new source of nourishment.

Finishing its journey down the fallopian tubes, the Blastula enters the uterine cavity (the inside of the uterus).

Within the uterus, the blastula connects with the endometrium of the uterine wall in a variety of ways, depending on which species of mammal we are looking at.

From this first contact, the placenta grows out of a complex of maternal material and embryonic tissues. There are at least 5 different forms the placenta can take in the different species of placental mammals. Though each species always takes the same form.

The differences involve changes in degree of contact and the number of layers of tissue between the maternal blood supply and the embryo’s. On the maternal side the possible layers are called the Endometrial epithelium, connective tissues and the endothelium of the blood vessels.

On the embryo’s side there is also the endothelium of the blood vessels, the mesenchymal tissues and the chorion. The differences are related to the different lifestyles of the various mammal species.

How Many Offspring Do Different Mammals Have?

Group Common Name Scientific Name Weight in lbs Avg. Gestation Days Av. No. Offspring
Insectivores European Hedgehog Erinaceus europaes 3.4 42 5
  European Mole Talpa europea 1.5 35 4
  Eastern Mole Scalopus aquaticus   42 3
  Short-tailed Shrew Blarina brevicauda   18 5
  Common shrew Sorex araneus 0.02 25 7
Bats Large Mouse-eared Bat Myotis myotis   60 1
  Noctule Nyctalus noctula   42 44593
Tree Shrews Philippine Tree Shrew Urogale everetti 0.11 50 1
Primates Lesser Senegal Bush-baby Galago senegalensis 0.5 123 44593
  Slow Loris Nycticebus coucang 2.6 176 1
  Black Lemur Lemur macao 5.5 146 44593
  Indri Indri indri 19 60 1
  Lion Tamarin Loentopithecus rosalia 1.5 128 2
  Black Spider-monkey Ateles paniscus 19.3 225 1
  Red Howler-monkey Aloutta seniculs 10 185 1
  Rhesus Monkey Macao mulatta 11.8 163 44593
  Sacred Baboon Papio hamadryas 22 183 44593
  Lar Gibbon Hylobates lar 11.5 206 1
  Chimpanzee Pongo troglodytes 66 235 44593
Edentates Giant Anteater Myrmecophaga tridactyla   190 1
  Texas Armadillo Dasypus novemcinctus   260 44685
Rodents European Hare Lepus europeaus 7.7 42 13
  Snowshoe Hare Lepus americanus   38 5
  Alpins Marmot Marmota marmota   38 6
  Eastern Chipmunk Tamias striata   31 5
  European Red Squirrel Sciurus vulgaris 0.66 37 4
  Grey squirrel Sciurus carolinensis   44 4
  American Beaver Castor canadensis 48.4 105 4
  Hamster Cricetus cricetus   21 12
  Meadow Vole Microtus pennsylvanicus   21 5
  House Mouse Mus musculus   19 5
  Long-tailed Field Mouse Apodemus sylvaticus   24 5
  Harvest Mouse Micromys minutus 0.11 21 7
  Nutria Myocaster caypus   142 9
  Springhaas Pedetes capensis 7.7 77 1
Cetaceans Bottle-nosed Dolphin Tursiops truncatus 385 330 1
  Harbour Porpoise Phocaena phocaena 132 332 1
  Sperm Whale Physeter catodon 17 tonnes 425 1
  Fin Whale Balaenoptera physalus 80 tonnes 345 1
Carnivores American Black Bear Ursus americanus 200 212 2
  Grizzly Bear Ursus arctos 289 230 2
  European Otter Lutra lutra 17.6 62 3
  American Martin Martes americana 2.8 242 3
  Pine Martin Martes martes 3.5 278 2
  Long-tailed Weasel Mustela frenata 3.5 230 67
  American Mink Mustela vision 2.6 57 4
  Cape Clawless Otter Aonyx capensis   63 7
  Coyote Canis latrans 64 63 8
  Grey Wolf Canis lupus 100 63 4
  European Red Fox Vulpes vulpes 11.4 56 4
  Argentine Grey Fox Dusicyon griseus 9.6 53  
  Lion Panthera leo 285 110 3
  Tiger Panthera tigris 330 103 3
  Leopard Panthera pardus 110 98 44622
  Jaguar Panthera onca 165 97 3
  Puma Felis concolor 154 92 3
  Lynx Lynx canadensis 37 60 3
Pinnipeds S. American Fur Seal Arctocephalis australis 107 330 1
  Cape Fur Seal Arctocephalis pusillus 268 345 44593
  S. American Sea Lion Otaria flavescens 317 330 1
  Common or Harbour Seal Phoca vitulina 264 280 44593
Proboscideans Indian Elephant Elephas maximus 4 tonnes 625 44593
Sirenians West African Manatee Trichechus manatus 1.6 tonnes 152 1
Perrisodactyls Black Rhinocerus Diceros bicornis 1 tonne 540 1
  Mountain Zebra Equus zebra   365 1
  Malayan Tapir Tapirus indicus 605 392 1
Artiodactyls Hippopotamus Hippopotamus amphibius 1.5 tonnes 237 1
  Wild Boar Sus scrofa 352 113 8
  Camel Camelus bactrianus 1210 405 1
  Moose Alces alces 1320 245 2
  Red Deer Cervas elephas 616 234 44593
  Giraffe Giraffa camelopardalis 1760 435 44593
  Gaur Bos gaurus 1540 270 44593
  Blackbuck Antilopa cervicapra 77 180 44593
  Beisa Oryx Oryx gazella beisa 356 280 1

What Next?

I think that is probably enough science for now… I hope you have found introduction to reproduction in mammals interesting!

You may also want to check out how long do animals live.

Gordon Ramel

Gordon is an ecologist with two degrees from Exeter University. He's also a teacher, a poet and the owner of 1,152 books. Oh - and he wrote this website.

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