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  • Writer's pictureAndrei Schiller-Chan

What Crocodiles and Humans Have in Common

Updated: Jun 25, 2021

For a lizard to breathe, it must be stationary.

Why is this so? The reason is that the intercostal and thoracic muscles that are responsible for the dilation of its lungs are also required for its movement, the bending of the body from side to side, and, unfortunately for our lizard, breathing and moving are mutually exclusive!

Crocodiles, however, can move, swim, and breathe all at the same time — and so can humans. The ability for humans to run and speak with each other was vital for sustaining life not to mention the necessity of speech underlying play for healthy upbringing, ‘like other animals, we play at those things that are important to our survival, and social play promotes social cohesion’ (Heinrich, 2008). Stopping to catch a breath to say a word every second or so would not be practical but because of our evolutionary ties to lizards, this may shine some light into why we find it difficult to speak when nervous.

What You Can Learn From Crocodiles

Crocodiles, unlike the majority of their lizard counterparts, have a similar, but not homogenous, muscle to that of humans. The diaphragm. In crocodiles, it is called the Diaphragmaticus muscle and attaches to the hip joint, enveloping their ischium and pubis (pelvic girdle). Importantly, it is also attached to its liver and the liver is attached to its lungs. When a crocodile inhales, the contraction of the diaphragmatic muscle draws the liver causally (fancy word for ‘towards the tail’) which increases thoracic volume, creating negative air pressure, which aids inspiration. Indeed, crocodiles have three other primary muscle groups for respiration; the intercostal muscles which aid the cranial and caudal rotation of the ribs to increase volume, the rectus abdominis and transversus abdominis (core muscles) which rotate the pubic bone craniodorsally decreasing volume (‘towards the head and back’ [Figure A]) and the ischiopubis and ischiotruncus muscles rotate the pubic bone ventrally (towards the belly [Figure B]) to aid inhalation and also increase volume (Munns, 2011).

The piston-like movement of the liver towards the tail upon inhalation, and the elastic recoil which slams the liver back towards the lungs on exhalation, accompanied by the rotation of its pelvis, are the secrets why a crocodile can move, run and breathe at the same time!

It is able to create volume within its body and utilize a range of secondary respiratory muscles, whereas, the lizard cannot. Humans, like crocodiles, also make space to increase thoracic volume, and our ability to harness this synergy is the foundation of the rhythm inherent in our voice and why it is imperative to make sure this capacity is not defective…

Unfortunately, and all too often, this capacity is reduced, and it becomes problematic if your body decides to retreat to its lizard-like vestiges. Such a descent does not come without its consequences. The retreat is guided in particular by a neural vestige that we share with reptiles, which is triggered automatically by the autonomic nervous system when perceiving threat (for more information on how this system affects our mind, voice and body, read here). Losing this breathing capacity, we still have the ability to live, but it’s akin to firing an engine with only three cylinders when you actually have six at your disposal, and the first thing to suffer from this loss of capacity is your voice! A shaky voice, that breaks in the face of fear, is an example of diminished breath capacity, and a schism dividing the synergy between voice and respiratory musculature.


Underlying your voice is a complex tapestry of physics and biology

The Diaphragm

On every inhale approximately 500ml of air is taken in and this can be increased by nearly tenfold to 5 litres; you will always have on average 2 litres of air in your lungs and it is impossible to empty your lungs completely!

The phrenic nerve, which originates in the cervical spinal roots in your neck (C3, C4, C5), descends down into the thorax and innervates the movement of your diaphragm. It is also theonlysource of motor innervation to the diaphragm and is thus crucial to our survival. The diaphragm is a flat sheet of muscle, that looks like a dome, that sits below your heart and lungs, and above the abdominal viscera such as the stomach and liver. A membrane called thevisceral pleuraenvelops the surface of each lung and connects to the diaphragm so that any movement made by the diaphragm affects the lungs.

Why Your Belly Rises

When you inhale, the dome-like diaphragm flattens, and the abdominal muscles adjust to allow the position of your organs to move downward and outward to create space for the diaphragm to draw downwards. This is what is happening when you notice your belly rise; and, like the crocodile, it’s creating greater thoracic volume for breath capacity. In fact, the diaphragm has two domes which are connected by the central tendon where one dome is higher due to the underlying location of the liver. The diaphragm is also attached to the lower rib cage, the xiphoid process (the cartilaginous portion at the lower end of your sternum), and your lower lumbar vertebrae. It is surprising to most but your ability to breathe and speak efficiently is impacted by this very connection at the basis of your spine, and any shortcoming in this area, such as poor posture shakes the bedrock of your voice.

Working to free the voice’s potential begins by freeing the abdominal muscle groups from any redundant constriction, given that they intersect with the movement of the diaphragm. This “redundancy” alludes to the fact that muscles tend to tighten up when faced with a threat; and if one’s body does not recover homeostasis due to trauma, the measures the body put in place to survive can linger, such as a locked muscle.

The Ribs

The intercostal muscles*, supported by the lattisimus dorsi, lift the twelve sets of ribs upwards and outwards in every direction to allow for the lungs to expand. The ribs near the spine are bone but become more cartilaginous as they move closer towards the front of your body and where there is more cartilage there is more flexibility. The capacity to change the dimension of your thorax is designed to move downwards and outwards the lower you go in the body due to the dispersion of the cartilage’s trajectory in that direction.

The floating ribs, which are connected to the vertebrae and not your rib cage go overlooked when it comes to creating thoracic capacity. Unrestricted, they should swing outwardly, so it is important to keep this freedom near the location of the lumbar due to the diaphragm’s direct attachment to this area of the spine and the ribs need for freedom.

The fact that we can manipulate space in our body like the crocodile gives us almost endless possibilities to contort ourselves and therefore our environment; and indeed, we have been capitalizing on this biological trait ever since!


How the Breath Fuels the Voice: The Myoelastic-Aerodynamic Theory of Phonation

The air you breathe is the pilot light for your voice; for without a steady supply of air, the vocal folds cannot sustain the required passive vibration (caused by the Bernoulli Effect) for the demands of speech. In reality, your voice is made up of thousands of pulses of air, with each pulse filtered by the vocal tract to carve out the sound of your choice. To create these pulses, the air is forced up through the trachea by a controlled contraction of the respiratory muscles decreasing volume in your body and lungs; at the same time, your vocal folds are brought together, closing the glottis (the space between the folds), trapping air underneath them. Due to the diminishing volume in your body and lungs, this creates a build-up of positive air pressure below the folds and negative air pressure, or a vacuum, above the vocal folds. The imbalance of air pressure forces air through the folds so as to create equalization, but due to the velocity at which the air travels, this now creates an inverse in air pressure, which subsequently draws the bottom of the vocal folds back together again just as the top of the folds have opened. As the vocal folds come together and blow apart, this cycle creates vibration producing audible sound and will keep repeating until the balance is restored by the fact that you are no longer speaking and that the folds have opened.

The voice’s intensity is increased by increasing the airflow from the lungs and simultaneously increasing the resistance from the vocal folds. The vocal folds are blown further apart and remain so for a longer duration which increases the amplitude of the sound produced. On a last but general note, the rate at which they open and close gives you the pitch of your voice and if we take the pitch set for normal conversation, they vibrate approximately 110 times a second.


What Goes Wrong

The Myoelastic Theory of Phonation is an extremely energy-efficient means of speaking as the opening and closing of the vocal folds is driven by a passive mechanism, namely the physics underlying the movement of air between positive and negative air pressure states. So, how can it be that we get tired speaking or a sore throat from doing so?

Like the crocodile, we must maintain the ability to adjust and create volume within our body so as to have enough breath to speak what we mean. Our emotions and the ability to communicate them with honesty, clarity, and commitment depend on the breath we take and recycle. But we all know that in the heat of an argument, it can feel like we are choking, or that what we say doesn’t come out at all how we imagined. Stress, unfortunately, triggers an immediate nervous response in your body to survive the threat, most of the time without you knowing. Many of the biological features that aid efficient breathing and speech are thrown aside so that attention and energy can be directed towards the bodily requirements to keep you safe! And speech is not one of them.

If your shoulders rise when you inhale (typically when anxious), you may be using secondary breathing muscles in place of the primary respiratory muscles which requires more effort — especially if you are using these muscles to force air out when speaking. Another typical response to stress is a locked abdomen to protect the body’s organs; but the cost is that the contraction of this large muscle group denies the expansion of the gut, which impacts the diaphragm and leads to a decrease in rib expansion. To compensate, the shoulder and neck muscles, such as the sternocleidomastoid muscle, get involved to pick up the slack and create space, but if used with enough repetition the behaviour of utilizing these muscles to breathe can become a habit. So much so that letting the habit go feels unfamiliar and naked to breathe without them. The problem is that breathing in this way affects the tone and efficiency of your voice because it cannot keep up with the demands for speech; the voice tires out, which leads to pain, which leads to loss of inflection, spontaneity, and tone — all the key ingredients for social cohesion.

So, it is truly important that you inhale the right amount of breath for what you want to say in the most efficient way, and your body, like the crocodiles, has been doing this for you without cost. For many of us, living a life without anxiety is not an option, so it is important to study in-depth how your body reacts to stress, and, if you can, give your body a helping hand to teach it to breathe efficiently once again if some of these secondary breathing techniques feel familiar.

In closing, taking a breath should expand your rib cage in every direction, not just with a small rise in the chest, along with expansion on your sides and back. The rise of the belly means that the organs are making space for the diaphragm to descend so dropping a little of your vanity by not sucking your stomach in may go a long way in helping preserve your voice. The floating ribs and the area surrounding your lumbar should also feel a little expansion as they expand and contract. When these muscles contract to give you your voice, with a strong base of support to encourage the upward column of air through the vocal folds, you may find a confidence you never knew existed, and a shared divinity with that of the crocodile who has in all aspects mastered the art of breathing.



The Rib Cage & Intercostal Muscles

You have seven true ribs attached to the breast bone (sternum), three false ribs that are attached to the spine and combine at the bottom of the seventh rib, and two floating ribs.

The thoracic cage (rib cage) is anchored to twelve thoracic vertebrae (T1-T12) and two different sets of intercostal muscles, the internal and external, run between each rib. When the external intercostals contract they lift the lower rib up towards the rib preceding it, causing the rib cage to expand laterally, including some degree of anterior, posterior expansion.

The internal intercostals begin at the sternum, which means they collect at the cartilaginous portion of the ribs. They run around the body towards the back but don’t anchor at the spine. Their contraction causes the rib cage to decrease in size, and provide some degree of anterior and posterior movement of the interchondral portion of the ribs, where only cartilage is present.


Works Cited

Farmer, CG & Carrier, David. (2000). Pelvic aspiration in the American alligator (Alligator mississippiensis). The Journal of experimental biology. 203. 1679–87.

Heinrich, B. (2002). Why We Run: A Natural History. Ecco Press

Munns, L., Owerkowicz, T., Andrewartha, Sarah J., & Frappell, P, B. (2011). The accessory role of the diaphragmaticus muscle in lung ventilation in the estuarine crocodile Crocodylus porous. The Journal of Experimental Biology, 215, 845–852

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