Three Kinds of Mammal Love

There’s nothing like a little David Attenborough to remind you of just how awesome this planet – and your Linnaean class – really is. (I’m watching Life of Mammals and made it through only the first episode before turning it off to write this.)


Mammals made a quiet entrance into the world of dinosaurs as tiny, mouse-sized creatures more than 200 million years ago. Since then, mammals have branched off into hoofed animals, primates like us, felines, canines, and more.

All mammals share a few common traits:
1. Skin glands. These can include sweat, sebaceous, scent, musk, and most well-known, the mammary glands that produce milk.
2. Pelage (pronounced, “PEL-edge”), also known as hair or fur. Even mammals like whales, dolphins, and the pangolin can have some sparse hair.
3. Our red blood cells don’t have nuclei, and we are warm-blooded (meaning we produce our own energy from our food, rather than soaking up the sun for energy.)
4. Three middle ear bones, which help us to hear better.
5. Backbones.
6. A four-chambered heart.
7. A lower jaw comprised of only one bone, rather than several.

Other traits, however, like live birth or placentas, depend on the sub-class. Cross the jump to get a quick introduction to the three sub-classes of mammals and their awesome unique traits!

1. The Monotremes

The most primitive present-day mammals, platypus and echidna, are strange beings that look like an amalgam of other creatures glued together. Like other mammals, they have hair and are warm-blooded. They also produce milk, but not through a nipple; instead, monotreme moms have an area on their bodies where milk ducts come together and secrete through the skin. Unlike other mammals, they lay eggs and possess only a single opening for excretory and birthing purposes (called the “cloaca”). These last two traits are remnants of a lineage partnered with reptiles (which typically lay eggs and have cloacas). How cool is it to see these traits come together in one animal that is still around today? Let me answer that for you: it’s really cool.

Platypus have a sensitive, fleshy “bill” that can detect electric impulses from their prey on the muddy river bottom where visibility is low. They have spectacularly webbed feet and thick fur that keeps them warm in cold water. Echidnas have long, fleshy snouts and long tongues for probing for insects. Like porcupines (a placental mammal), they’re covered in long, modified hairs, some of which have hardened and hollowed out to become spines.

There are only five total species of Monotremata (4 echidna and 1 platypus), and all reside in Australia and New Guinea. Male platypus are one of the only mammals to have venom! (You can read more about that here.)

Long-Beaked Echidna, via Wiki.

Platypus. Credit: Nicole Duplaix via National Geographic 

2. Marsupials

Found only in Australia and Central and South America (with the one exception of the North American opposum), these mammals give birth to tiny, barely-developed fetuses, which use their forelimbs to climb through mom’s fur and into a warm, cozy pouch. Much in the same way a placental mammal baby develops inside the womb for months, this little pink blob of flesh and organ will develop in mom’s external pouch. Within the pouch, however, there is no placenta; only the nipple and milk to provide nourishment. Marsupial moms do not gestate for as long as placental moms.

Marsupials come in a variety of styles, and the most amazing ones to me are those that evolved to hop as their method of locomotion. Kangaroos, Wallaroos, and Wallabies possess tendons in their legs that act like springs, storing energy as the animal presses down with its feet and releasing that energy as forward motion. In this way, kangaroos and their relatives can cover incredible distance while remaining upright and expending less energy in the dry heat of Australia. Rock wallabies agilely move around rocks and cliffs like mountain goats thanks to corrugated padding on the bottoms of their feet.

Yellow-footed Rock Wallaby via Australian Wildlife Conservancy. Notice the huge hind feet.

We’re all familiar with marsupial koalas, but are you aware of a little creature called a numbat? Besides having one of the best names in the animal kingdom, these insectivores are so squirrely and cute I want to put a little sweater on it and take it everywhere with me. You know, so I can educate people about marsupials. Obviously.

Nummy numbat. Credit: Martin Pot via Wiki

3. Placentals

We placentals feature a special gelatinous, sack of blood vessels and cords called a (surprise!) placenta whilst in utero. This magical feat of evolution siphons nutrients (and a variety of other things, including toxins and antibodies) from mom’s bloodstream into baby’s, nourishing baby as she grows. Since mammals have fairly long gestations, some species can give birth to very developed babies. Many ungulate (hoofed) species, like deer, horses, and wildebeests, have babies that are on their feet within minutes or days of being born. Since these types of critters are prey for a range of predators, baby needs to be ready to run with those gangly legs as soon as she can.

Other mammals, like humans, felines, and canines, for example, give birth to helpless babies that require protection and continued development outside of the womb. For mammals as a whole, the young stay with mom for a substantial period of time compared to other groups of animals. What we’ve traded evolutionarily for the convenience of the lay-it-and-leave-it egg system is a more intensive method of prolonged parental care. Instead of having a high number of young that are programmed to be self-sufficient upon birth or hatching, mammals produce far fewer offspring and tutor them for a long period in an effort to ensure their survival.

The most fascinating part of being a mammal, to me, is our females’ ability to produce milk, a perfect nutritive substance for our young. (And when I say “females,” I mean every mammal EXCEPT the Dayak fruit bat, the only male mammal known to produce milk!) The mammary glands are stimulated to produce milk by lactogenic hormones pumped into the blood from the magical pituitary gland at the time of birth. Milk possess water, fats, protein, vitamins, and minerals. It also provides antibodies from mom, supporting baby’s newly developing immune system.

The human mammary system via Hormones like prolactin stimulate the alveoli (found in the lobules) to create milk, which is then moved through ducts to the nipple. Boobs rule!

All in all, I’m proud to say I’m a mammal. Although it would be pretty cool to be a marsupial mammal. I’m always misplacing my keys and wallet.

Thanks for reading and if you’re feeling your “mammalian pride,” check out these t-shirts and stickers at the RedBubble shop!


A personal sidenote on breastfeeding:

Sadly, breastfeeding has become a taboo issue in American society. Women are not taught how to properly breastfeed, or even that it’s okay and good and natural to breastfeed. We are reinforced to believe that it’s a shameful act, and certainly not appropriate to do in front of other people. Again, we’ve managed to disassociate ourselves from the fact that we are, in fact, animals, and that we have a natural instinct and ability to nourish our young. I can’t in good faith talk about mammals and milk production without guiding you to the International Lactation Consultant Association. These people are the experts on lactation and have done wonders for moms that have been abandoned or misguided (including by their own OB/GYNs, many of whom are truly undereducated in lactation) by the healthcare system. Check them out for more information and help stop the hurtful myths and misinformation surrounding breastfeeding.

Posted on May 7, 2011, in Fauna and tagged , , , , , , , , , , , , , , , , , , , , , , , , , , , , , . Bookmark the permalink. Comments Off on Three Kinds of Mammal Love.

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