What do you do if you don’t have fingers? Use your skull!

Channel catfish
(illustration by Aaron Olsen)

You’re lucky. You are the result of an evolutionary lineage that acquired moveable fingers and thumbs. This innovation allows you to grab and manipulate objects with your hands.

In addition to tasks such as posting pictures of your pandemic quarantine sourdough loaf to social media (we’re still doing that!), one of the most basic things you do with your hands is assist your mouth in acquiring and processing food.

Chances are, you wouldn’t dive right into that sourdough teeth-first. Instead, you would cut a slice, warm it in the microwave, spread some butter on it, pick it up and move it closer to your mouth, before letting your teeth take over. By employing your hands’ ability to manipulate the bread, you have made it tastier AND more manageable for your mouth.

This works great for us humans, but what about other animals in different evolutionary lineages, like fishes, that don’t have hands? As Drs. Olsen, Hernandez, and Brainerd show in a recent study using catfish, they use their skulls!

Dr. Aaron Olsen with a catfish in the XROMM filming tank
(image: Stephen Crocker, Brown University)

Most fishes have kinetic, or moveable skulls (to learn how this works, build a paper skull model here). Catfish, and probably many other fish, can use these movements to adjust water flow in their mouth cavity, and therefore, manipulate their food prior to swallowing it.

Using a combination of high-speed x-ray videos (or XROMM, X-ray reconstruction of moving morphology, watch a video here) and computational models, Olsen and his colleagues demonstrate that catfish skull bones are arranged in 5 moveable loops, with 17 links, 21 joints, and up to 19 degrees of freedom (DoF) of movement!

A map showing the major cranial components (color coded) and the links (connecting lines), joints (letters, corresponding to the next figure), and loops (roman numerals) connecting them.
(Figure 1F from Olsen et al. 2020)

This means that catfish aren’t simply opening and closing their mouth, they’re using their skulls to adjust water flow and prey movement in a way similar to how you would mold and shape a ball of clay with your fingers.

To understand how this is possible, we need to think about joint mobility, or degrees of freedom (DoF) of movement. There are 6 possible ways a joint can move – 3 of these are translational, or in a straight line, along the X, Y, and Z axes (up and down, side to side, and forward and backward). However most of the skeletal joints in our body use one or more of 3 rotational movements around the X, Y, and Z axes (roll, pitch, and yaw).

The 3 translational and 3 rotational degrees of freedom. If this was a catfish, it’s head would be pointed toward the left, in the forward direction.
(image: CC BY-SA 4.0, Wikkimedia Commons)

A joint that can do all 6 of these movements would have 6 DoF. That’s not usually possible for individual joints, which are restricted to 1, 2, or 3 types of movement depending on the kind of joint.

When multiple, jointed, skeletal elements work together in a structure, the total DoF are summed across each joint. This means that for a complex structure with multiple joints, the total number of ways it can move can be quite high!

For example, although the tip of your index finger bending to scratch your cat’s chin only has a single DoF, your whole hand holding onto a pencil to draw the #SundayFishSketch has 21 DoF! The greater the DoF, the more mobile and bendy the structure is.

Olsen and colleagues showed that catfish skulls have 19 DoF, which is almost as mobile as your hand!

A model of the primary catfish skull elements, their interconnecting joints, and their 19 possible degrees of freedom (lettered A through S).
(Figure 2A from Olsen et al. 2020)

However, having moveable joints is only the first step – they also have to use them in just the right way. Olsen and colleagues found that although 19 DoF are possible, only 7 DoF can explain 91% of the motion of the jaws, hyoid, and pectoral girdle in feeding catfish. This means that the biggest movements are not very mobile. In contrast, the rest of the skull movements are smaller in magnitude but have much greater mobility (the remaining 14 DoF).

This is like the difference between using your arm/hand to pick up a gallon of water from the grocery store shelf and performing a ballroom dance. To pick up the water jug, you would only use a few degrees of freedom because that sucker is 8 lbs and you really only need a few motions to move it up and down. However, during a dance, every movement of your arm and fingers is an artistic expression, and to get that just right, you would use a lot more degrees of freedom but each movement is relatively small.

Britt Stewart and Johnny Weir, Dancing with the Stars 2020 season
(image: Eric McCandless, ABC)

Similarly, when catfish manipulate their prey, they are basically ballroom dancing with their mouths.

This combination of using large, relatively immobile movements to suck in prey, and small, mobile movements to manipulate prey give fishes an extraordinary ability to capture and orient prey in their mouths, without having hands. Since water is dense, viscous, and incompressible, small movements of the skull can be used to fine-tune the pressure inside the mouth cavity, adjust the local flow of water, and change the position or orientation of prey so it goes down smoothly.

So even though catfish can’t bake a sourdough loaf (yet), they have solved the problem of making food more manageable to swallow using a skull that has similar mobility to our hands. Imagine if you didn’t have hands, and almost every bone in your skull moved while you were eating, instead of just your jaw! Even though fishes have solved the same problem using a different structure, they likely still play with their food in similar ways.

Fingers, schmingers. Who needs ‘em anyway? Not this guy/gal…

A catfish that ate an armadillo!
Image captured from youtube video by user Catfish and Carp.

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