Do cats really think they are liquid? They can get into anything.

Produced by: Science Popularization China

Author: Su Chengyu

Producer: China Science Expo

"Cats are liquids" is not a joke. In 2017, MA Fardin, winner of the Ig Nobel Prize in Physics, discussed the physical state of cats from the perspective of rheology. That is, a substance can behave as a solid or a liquid depending on time. The key is the length of observation time, which is measured by the "Deborah Number (De)". This parameter was first proposed by Professor Marcus Reiner of the Israel Institute of Technology and can be used as one of the parameters for measuring the viscoelasticity of a fluid.

Cats are "liquid"

(Image source: Treplus Communities)

In the short term, a cat is like a solid because it retains its shape. In the long term, a cat is like a liquid that can slowly flow and adapt to the shape of its container, such as a cat can "flow" into a bowl.

Kittens love to crawl into boxes

(Photo source: veer photo gallery)

Scientists also observed cats' liquid-like behavior on different surfaces, such as "spreading" on rough surfaces but "sliding" on smooth surfaces. In general, cats are an "active fluid" that can move and adjust its shape on its own, unlike ordinary passive fluids that require external forces to push them, which makes their flow behavior more complex.

Why can't people drill freely?

The premise of a cat being liquid is that its body is soft enough, but for humans, the head can get through but not the body (here is a reminder not to get through iron fences casually). For a cat, if the head can get through, the body can definitely get through. The main reason is that humans have clavicles, and the width of the shoulders is determined by the length and position of the clavicle, and this structure is fixed in humans.

Cat's collarbone

(Image source: Reference 2)

However, cats' collarbones are basically degenerate, unlike humans who have obvious collarbones. Without the restriction of the collarbone, cats can compress their bodies to a certain extent and show "mobility", which is why they can "squeeze" through narrow gaps. In addition, the cat's spine contains 30 vertebrae, which is a few less than the human spine.

Cat's spine under X-ray (Image source: Reference 4)

Through X-ray analysis and angle measurement of cats, it was found that the cat's spine has extremely high flexibility in torsion (rotational movement around the longitudinal axis of the spine), especially the torsion range of the middle thoracic spine (T4 to T11) is very wide. The experiment measured the angle change of the cat's spine at maximum torsion and found that the torsion angle was close to 180 degrees.

The large spaces between the vertebrae of the spine and the very flexible discs of cats contribute to the great mobility of the spine. This structure allows the cat's spine to have a greater arc when bending forward and backward, providing a "spring-like" effect, allowing them to twist or jump quickly and squeeze through smaller spaces.

In addition, the flexibility of muscles and ligaments is also important. In 1981, scientists conducted an experiment to let cats jump as high as possible to observe how their muscles work. It was found that in exercises such as jumping, the cat's hind limb muscles such as the semitendinosus, gastrocnemius and extensor digitorum longus will show highly coordinated activities. The activity of these muscles is recorded by electromyography (EMG) during jumping, showing their different reactions in the preparation stage and the force stage.

The experiment also measured the ground reaction forces of the cats' hind limbs during jumping and analyzed the angle changes of the hip, knee and ankle joints. These data show that cats' joints maintain a wide range of motion throughout the movement, helping them achieve their maximum jumping height. Cats' muscles and ligaments are very flexible, which helps them adjust their body posture in confined spaces.

Does a cat know that it is a "liquid"?

The cat came out of the drawer

(Photo source: veer photo gallery)

In an experiment in 2024, Péter Pongrácz built a special experimental device that resembled a gradually narrowing door frame and recruited 30 domestic cats of different ages, sizes, and breeds.

The cats were about to face a series of increasingly narrow openings, and the scientists were to observe whether the cats had an accurate awareness of their own body size - that is, whether the cats judged in advance whether they could fit through the increasingly smaller openings, or whether they relied on a "try and see" approach.

Opening examples

(Image source: Reference 1)

The experiment was not smooth sailing, as cats are notoriously difficult to deal with in experiments. In one experiment, Pongrácz found that the cat escaped into the ventilation duct. Imagine that the scientists prepared everything and carefully designed a narrow opening. The cat's owner stood on the other side and tempted it with its favorite snacks, but the cat was indifferent and started licking its paws instead, as if laughing at these "stupid" humans. After all, the only rule for cats to participate in the experiment is-unless they want to!

At first, the opening was wide enough for almost all the cats to pass through easily. However, as the opening gradually narrowed, some cats began to stop. The cats carefully observed the narrow passage, some chose to try, while others turned around to look for other routes.

Scientists have noticed that when cats are faced with a narrow but high enough opening, they do not make an immediate judgment, but approach the opening to "test". Their noses and whiskers gently touch the edge of the opening, as if to assess whether they can pass through. The researchers observed that when the width was 12 cm, about 70% of the cats chose to try to pass through, but when the width was reduced to 7 cm, this proportion dropped rapidly to 15%. Obviously, cats become more cautious when the width of the opening is less than 10 cm. As the experiment progressed, the height of the opening was gradually reduced.

When they think they have a chance to get through, they move forward boldly, compressing their bodies slightly and nimbly squeezing through the opening. However, when the opening becomes too narrow, larger cats are more likely to stop and give up trying. At this point, cats begin to show awareness of their own size.

Surprisingly, cats showed more hesitation when faced with openings less than 15 cm high. Scientists recorded their behavior: when the opening was higher than 20 cm, almost all cats continued to try to pass through, but when the height was reduced to 15 cm, more than 80% of the cats stopped and showed significant hesitation. Some cats even chose to bypass the obstacle or give up trying and wait for further instructions.

During the observation process, the scientists also found some individual differences. Younger cats and smaller cats appeared to be more bold when passing through narrow openings, while older or larger cats were more cautious.

On average, cats weighing more than 5 kg are more likely to give up trying when faced with an opening less than 9 cm wide. For cats weighing less than 4 kg, more than half will try to pass through even if the opening is reduced to 6 cm. In other words, in some cases, cats really think of themselves as liquids that can pass through any opening at will.

Simply put, if cats are a liquid, then they also have only one dimension, a liquid with limited height but unlimited width.

The kitten emerged from the gap (Image source: Screenshot of content posted by a Youtube user)

Conclusion

Dogs handle the same problem in completely different ways. A few years ago, Pongrácz recruited some dogs to participate in the same experiment. The study showed that when the dogs encountered a small opening that was not suitable for passing, they would slow down and hesitate to pass through, and sometimes choose to take a detour. It is clear that the dogs show obvious body awareness and make decisions before approaching based on the size of the opening.

This contrast illustrates the difference in size awareness between the two animals, with dogs tending to rely on stronger size cognition, especially when faced with a small environment, while cats rely more on flexibility and heuristic strategies to deal with confined spaces. In Pongrácz's words, "Cats may be liquid, but they also know when to stop flowing."

References:

[1]Pongrácz P. Cats are (almost) liquid!—Cats selectively rely on body size awareness when negotiating short openings[J]. iScience, 2024.

[2]Zajac FE, Zomlefer MR, Levine W S. Hindlimb muscular activity, kinetics and kinematics of cats jumping to their maximum achievable heights[J]. Journal of Experimental Biology, 1981, 91(1): 73-86.

[3]Fardin M A. On the rheology of cats[J]. Rheology Bulletin, 2014, 83(2): 16-17.

[4]Macpherson JM, Ye Y. The cat vertebral column: stance configuration and range of motion[J]. Experimental brain research, 1998, 119: 324-332.