Why Do Mammalian Spines Undulate Vertically? │ The Unique Evolution of Breaking Away from the Tail

1. Introduction

It has been a while since my last update due to various reasons. This may be sudden, but if you were asked to

list the characteristics of mammals, how would you answer?

• Fluffy body hair

• Viviparity, lactation

• High intelligence, etc...

Many people might think of things like these. Of course, all of these are correct and are important elements that characterize mammals.

However, here I would like to focus on
how the spine is used as a major feature that distinguishes mammals from other animals.

Amphibians like newts and reptiles like lizards move forward while undulating their spines from side to side. On the other hand, what about mammals?
Imagine a dog, cat, or rabbit sprinting.
They do not undulate side to side like a lizard; rather, their spines move in a way that bends vertically like a bow.

Considering the history of the evolution of terrestrial vertebrates (tetrapods), this seems very strange.
Tetrapods evolved from fish.
Fish swim in water by swinging their bodies and tails from side to side.
It is a very natural progression for their descendants, amphibians and reptiles, to walk while bending their bodies from side to side.
Thinking about it that way, doesn't the mammal, which uses its spine by bending it vertically, seem like quite an outlier?

When and how did mammals change the way they use their spines from side-to-side to vertical?
When I looked into it, I found that the history of mammalian evolution spanning about 300 million years was hidden there.

2. Mammals that run with their "spines"

As mentioned earlier, when mammals walk or run, they utilize not only their legs but also the flexibility of their spines.
As an easy-to-understand example, let's take the cheetah.
Let's look at a cheetah sprinting at full speed.

You can see that it is flexibly bending mainly around the lower back (lumbar vertebrae).
This movement has the effect of increasing the stride length, and at the same time, just like the high-rebound shoes often used in track and field in recent years, it functions to absorb the load and impact received upon landing with the lumbar vertebrae and the muscles of the whole body, and convert it into strong propulsion.

The cheetah is a rather extreme example, but to varying degrees, many mammals use their spines as springs in this way.
Even large mammals like giraffes, when threatening others, can sometimes show a fierce dash that is hard to imagine from their giant bodies while shaking their spines significantly up and down.

Mammals are animals that evolved in a direction where they use their spines as springs by bending them in the vertical direction.
It could also be said that by turning the lumbar vertebrae into a spring, mammals have made gravity their ally.

3. The history of mammalian evolution

What was the evolution of mammals that led to this point?

A common misconception: Are the ancestors of mammals reptiles?

In school textbooks from a while ago, it was stated that "fish → amphibians → reptiles → mammals," and that mammals evolved from reptiles, and I think that is still widely recognized today.
However, it is now known that this was not actually the case.

Mammals belong to a larger lineage called "Synapsids". (Other synapsids besides mammals went extinct in the past.)
In contrast, so-called reptiles such as living lizards, crocodiles, and dinosaurs belong to a lineage called "Diapsids".

Synapsids (mammals) did not evolve from diapsids (reptiles), but are thought to have branched off from amniotes (animals that lay shelled eggs, which evolved from amphibians) about 300 million years ago in the late Carboniferous period.

In other words, mammals and reptiles were born from a common ancestor and evolved in separate directions, like siblings.

Evolution of Synapsids

Below is a chronological list of representative synapsids.

1. Dimetrodon (Pelycosaur, Early Permian)

Dimetrodon is often mistaken for a dinosaur, but it is an early synapsid and an animal in a lineage close to mammals.
At first glance, it has almost no mammalian characteristics; reptilian features such as legs splayed to the sides and a thick, long tail are prominent.
When walking, it must have wiggled its spine from side to side like a lizard.

2. Gorgonops (Therapsid, Late Permian)

3. Thrinaxodon (Therapsid, Early Triassic)

In later eras, a group called therapsids appeared, and
as seen in 2. Gorgonops and 3. Thrinaxodon, changes in body shape can be observed, such as the legs beginning to move under the torso (erect posture) and the tail becoming smaller relative to the body.
Imagining from this body shape, the side-to-side wiggling during walking seems to have been significantly reduced.

4. Morganucodon (Early Mammal, Late Triassic to Early Jurassic)

4. Morganucodon was one of the earliest mammals, and it had the appearance of a small mouse to escape from the dinosaurs that dominated the land at the time.
Its body structure was also much closer to modern mammals; its legs were in an almost vertical, upright posture, and its tail was thin and long, lacking the thickness of the former Dimetrodon.

When dinosaurs went extinct at the end of the Cretaceous period, mammals rapidly increased in size and diversified, leading to modern mammals.
And many modern mammals are completely upright, with a tendency for the tail to be small relative to the body.

Let's compare the tails.
The tails of reptiles and early synapsids (such as Dimetrodon) are 'conical' in shape, with a thick base that connects smoothly to the torso, looking quite powerful.

Compared to this, in many modern mammals, one gets the impression that a slender 'string-like' tail is growing abruptly from the lower back.

Looking at the history of evolution from early synapsids to modern mammals in this way, I get the feeling that there might be some relationship between the 'reduction of the tail' and the 'direction of spinal bending'.

4. What is the role of the tail?

What was the role of the tail in the first place?

One is its role as a 'counterbalance'.

Early synapsids had a posture where their legs were splayed to the sides of their bodies like reptiles (sprawling posture).
In this state, when the right leg kicks out, the body tries to rotate to the right, using the left front leg as an axis. Next, when the left leg kicks out, the body tries to rotate to the left.
In other words, every time a leg steps forward, a force acts on the body to sway it from side to side.

While it doesn't seem to be much of a problem when walking slowly, if you try to walk fast or run, the body is swayed even more significantly from side to side, and energy is wasted.
Therefore, by having a thick, long tail act as a counterbalance, the side-to-side sway of the body is canceled out, and the entire body can walk efficiently.

Another is its role as a 'propulsion engine'.

At the base of the tail, there is a large muscle called the caudofemoralis muscle, which connects from the tail to the femur. This is why the tails of reptiles and others look conical.
This caudofemoralis muscle pulls the hind legs backward and acts as a propulsion engine that generates powerful kicking force.

In other words, the tail was essential for suppressing the side-to-side swaying of the body and generating the force to move forward.

5. Why did the tail degenerate?

So, why did the tail degenerate in the lineage from early synapsids to mammals?
I have considered two reasons.

1. The influence of upright posture

Early synapsids had a sprawling posture, but their legs gradually moved under their torsos, changing to an upright posture. (Uprightness)

What changes when moving from a sprawling posture to an upright posture?
As mentioned earlier, a sprawling posture generates rotational force with every step, but an upright posture almost eliminates this problem, allowing leg power to be directed almost entirely forward, and significantly reducing the side-to-side swaying motion.

If that is the case, the need for a tail as a counterbalance would decrease, and it is thought that the tails of synapsids shortened as we saw earlier.

(Note: The cheetah used as an example actively uses its tail to balance when making sharp turns, but this is merely auxiliary; the tail does not directly generate the propulsion for turning. In other mammals as well, the tail is thought to have many roles such as balance, body maintenance, and communication.)

2. The influence of the diaphragm

In addition to uprightness, another important change was occurring in synapsids.
That is the acquisition of the "diaphragm."

Mammals use a diaphragm (a membrane-like muscle that separates the thoracic cavity from the abdominal cavity) to breathe, and it is thought that this was acquired by their synapsid ancestors around 250 million years ago.

As mammals, we humans use our diaphragms to breathe without thinking about it, but this is a special structure among animals.
In many reptiles and amphibians, the separation between the thoracic cavity (the space containing the lungs) and the abdominal cavity (the space containing the internal organs) is relatively weak.
Therefore, the movement of the trunk is transmitted directly to the lungs, making it easy to interfere with breathing.

When a lizard or similar animal dashes, the side-to-side swaying of the trunk compresses the lungs, making it difficult to breathe.
For this reason, they cannot continue to dash for a long time and must stop moving temporarily to breathe.

In synapsids, the diaphragm structurally separated the chest and abdomen, and by functioning as a powerful pump, it became possible to continue breathing while moving.

The acquisition of the diaphragm may have also influenced the degeneration of the synapsid tail.

Looking at the mammalian skeleton, one notices something strange.
The ribs suddenly disappear around the chest, and the area around the abdomen, where the stomach, intestines, and other internal organs should be, is empty.
If you were given only the skeletal diagram without any hints, you might imagine a creature with an unusual outline with an extremely constricted waist.

It is so empty that you might worry if it is structurally sound, but rest assured. That area is supported by the tension of abdominal muscles and fascia instead of ribs.

This seemingly strange structure, where ribs remain only in the chest and the abdomen is empty, is a manifestation of the fact that respiratory function is concentrated mainly in the chest.
This also means that the muscles and skeleton that were on the waist side were freed from the role of "breathing" and could specialize in "propulsion."

In reptiles, the tail and the caudofemoralis muscles work together to generate propulsion, and the tail was truly the protagonist of propulsion.
In contrast, in the synapsids that led to mammals, the muscle groups around the waist were reorganized due to structural changes accompanying the appearance of the diaphragm, and perhaps a propulsion system utilizing the vertical flexing of the lumbar spine was reconstructed to replace the previous "tail-centered" one.

However, it is not easy to control body movement by linking the muscles of forward movement in an unstable upright posture. Behind this, advanced neural control, comparable to the "fly-by-wire" of modern fighter jets, is also essential.

The change in the movement of the mammalian spine from "side-to-side to up-and-down" could be called a major shift in the propulsion system, both in terms of hardware and software, from "tail to waist."

Conclusion

Perhaps because humans are also mammals, mammals are very familiar and relatable to us.
Maybe we rarely question their form simply because we have been too accustomed to seeing them since before we were old enough to know better.

This time, using the way the spine is used as a starting point, I was able to learn more about one aspect of the uniqueness of mammals.
Mammals are a family of synapsids with a long history of about 300 million years, and they are the descendants of survivors who overcame numerous mass extinctions on Earth.
Their capabilities are not just for show.
I would like to continue rediscovering the hidden powers of mammals like these.

If you found this interesting, I would be happy if you could like or follow.
A casual comment is also very welcome.

Click here for past articles on related themes:

List of Image Credits

1. Article header image
・AI-generated image

2. Restoration of Dimetrodon
・Title: Dimetrodon grandis.jpg
・Author: DiBgd
・Source: WikimediaCommons
・License: CC BY-SA 3.0

3. Restoration of Gorgonops
・Title: Gorgonops whaitsii1.jpg
・Author: Dmitry Bogdanov
・Source: WikimediaCommons
・License: CC BY-SA 3.0

4. Restoration of Thrinaxodon
・Title: Thrinaxodon BW.jpg
・Author: Nobu Tamura (http://spinops.blogspot.com)・Source:
WikimediaCommons・License: CC BY-SA 3.0

5. Restoration of Morganucodon
・Title: Morganucodon.jpg
・Author: FunkMonk
・Source: WikimediaCommons
・License: CC BY-SA 3.0

6. Alligator tail
・Title: American Alligator.jpg
・Author:
User:Postdlf
・Source: WikimediaCommons
・License: CC BY-SA 3.0

7. Giraffe tail
・Title: Namibie Etosha Girafe 03.jpg
・Author: GIRAUD Patrick
・Source: WikimediaCommons
・License: CC BY-SA 3.0

8. Image of crawling movement
・Title: Walk cycle of a tetrapod.gif
・Author: MaxxL
・Source: WikimediaCommons
・License: Public Domain

9. Diagram of human body cavities
・Title: Body Cavities Lateral view.jpg
・Author: OpenStax
・Source: WikimediaCommons
・License: CC BY-SA 3.0

10. Skeleton diagram of a cat
・Title: Felis silvestris restoration & skeleton.jpg
・Author: Hercule Straus-Durkheim
・Source: WikimediaCommons
・License: Public Domain