A friend recently asked me a question about mammals. He knew that mammals have certain distinguishing characters, like the specialized ear bones (and accompanying jaw joint), mammary glands, hair, etc. His question was, why do these things go together, i.e., why aren't there mammals with hair but no mammary glands. A good question, and to answer it, we need to look at the big picture.
Mammals have a suite of adaptations that all related to being 'warm-blooded'. Technically, the better term for their physiology is 'endothermic', because they generate their heat internally. This is costly in terms energy - it takes a lot of food to keep those fires burning. So mammals have a number of mechanisms to do this.
First is that specialized jaw joint. This give mammals a powerful bite. Along with that joint came precisely occluding teeth, which I talked about in my last entry.
These two things together make for efficient food processing. As I said before, mammals chew, which is different than most vertebrates, who swallow chunks of food whole. Chewing allows for more complete physical breakdown of food, and therefore more energy is obtained from the food, and as we've said, mammals need lots of energy.
As a consequence of the precise occlusion, there was another change in the dentition of mammals - diphyodonty. This is a fancy word for having two generations of teeth. Just like you, all mammals have baby (aka deciduous) teeth and adult teeth. Some mammals have modified this pattern, but the underlying diphyodonty is always there. Having two generations of teeth allows for far more precise occlusion than if teeth are constantly being replaced and regrown.
Perhaps strangely, this way of growing teeth probably led directly to another mammalian characteristic - mammary glands. Deciduous teeth are often less functional than adult teeth, so there is a time when juvenile mammals are at a disadvantage for processing food. Having a readily available, and easily digestible food source, in the form of mother's milk, helps alleviate this. Mammary glands are modified sweat glands, and in most mammals they are collected together into teats. We can see a bit of the transitional form in those primitive mammals, monotremes (platypuses and echidnas). They don't have teats, but instead have 'milk fields' where the modified glands aren't consolidated.
Not only do mammals needs lots of food to power their high energy physiologies, they also need oxygen, and there are a couple adaptations that they have to help in this regard as well. The first is up their noses. Mammals have unique structures called conchae or turbinates inside their noses. These thin, bony plates, are covered in mucosal epithelium (i.e., snot producing tissue) and are well supplied with blood. Their job is two-fold. First, when incoming air passes over them, they warm and moisten the air, making oxygen exchange in the lungs easier. Second, on exhalation, they recover heat and moisture from the outgoing air. With the high ventilation rates in mammals, this helps them avoid dehydration and heat loss. Interestingly, birds, the only other endothermic vertebrate group, have turbinates as well, although theirs aren't as complex and are cartilagenous, not bony.
CT scan of human nasal turbinates. They are the comma-shaped structures in the nasal chamber in the middle of the image.
A final adaptation to endothermy in mammals it the way they run, which again may sound like a stretch of a connection, but this is also involved with their oxygen demands and breathing rates. Many terrestrial vertebrates have a sprawling stance, with their limbs held out to the sides of their bodies, and the distal parts of their limbs at a 90 degree angle (think of doing a push-up with your arms to your side and you get the picture). This causes them to move with a side-to-side motion, that is their torso is alternately bent from one side to the other. This is because they move opposite limbs at the same time - front left and hind right move together, then front right and hind left. This makes breathing while running very hard to do. The lung on one side gets compressed, while the lung on the other side expands, so while running air just moves between the lungs, and not in and out. Mammals have developed an up-and-down (or dorso-ventral) way of flexing their torso. While running, both hind limbs move forward, then both front limbs. This flexes and extends the entire trunk, so both lungs are compressed and expanded at the same time. So mammals are able to couple their breathing with their strides, so they can keep those metabolic fires burning, even while running. (Note: telling this story makes a thousand teachers of comparative anatomy do a funny dance every year…)
A diagram showing movement of air in running reptiles (with sprawling gaits) and mammals (with upright posture)
So mammals have a related group of characters that all relate to the energy demands of endothermy:
- a new jaw and jaw joint
- specialized teeth that are replaced only once
- mammary glands
- nasal turbinates
- upright stance and coupling breathing and running
- hair - which I didn’t get into, but the insulation it provides helps retain heat.
After all that, I can answer my friend’s question. These characters tend to all go together, but there are exceptions that have arisen over the course of evolution:
- Cetaceans (whales, porpoises and dolphins) have all lost their hair (but they have blubber)
- Many mammals that don’t need to chew have lost or modified their lower jaw and/or teeth (like anteaters).
- Some mammals have modified the diphyodont pattern, like elephants and manatees
- There is a huge amount of variation in the size and shape of nasal turbinates across mammals
What’s interesting is the parallels with the other endothermic vertebrate group, birds. For instance, feathers do the job of hair.
So mammals have a number of characters that make them mammals, all relating to their peculiar physiology. They all came together at the origin of mammals, bit by bit over millions of years, each making their energy generation and use more efficient. Today we can see these in most modern mammals, and the exceptions nicely prove the rules.