Science: Cats Are Delicate Drinkers, Physics Shows

An informal conversation with Pedro Reis and Roman Stocker.

Cat-and dog-owners already know that their beloved animals are completely different from each other, but scientists now have more evidence that relates to how our furry friends lap up liquids. While dogs plunk their tongues in water to scoop it up and fill their cheeks, cats use two physical forces—gravity and inertia—to delicately suck up liquids with just the tip of their tongue. Much like elephant trunks and octopus arms, cat’s tongues are remarkably agile given the lack of skeletal support. Therefore, the findings may provide new inspiration for soft robotics technology.

“The starting point of this research was one morning in the spring of 2007, during breakfast, as [co-author] Roman was watching his cat lap. Within the context of trying to understand the physical and natural world around us, the question of ‘How does a cat drink?’ arose naturally,” said Pedro Reis, assistant professor at MIT and co-author of the study.

The new research was published 11 November at the Science Express Web site.

In the study, Reis, along with Roman Stocker, Sunghwan Jung, and Jeffrey Aristoff, used high-speed imaging to capture the balance of forces that underpin cat lapping and the mechanics of the water being lapped.

“Our curiosity centered on the question of what action the cat must perform to get liquid to overcome gravity and get into their mouth,” said Reis. “From the observational point of view, we knew we would enjoy and be able to tackle the challenge of capturing a process that is too fast for the naked eye and needs high-speed imaging to be resolved, then explained.”

It turns out that cats curve their tongue backward so that the top surface lightly touches the liquid. When cats raise their tongues rapidly, water is drawn up into a liquid column that grows by inertia. The cat then closes its jaw to capture the liquid before gravity breaks up this column.

To get a better idea of the mechanism behind lapping, the team performed physical experiments in which a glass disk placed on a water surface (mimicking a cat’s tongue as it leaves the water) was pulled upwards. High-speed imaging showed water being stretched upward to form a column, just like in lapping cats.

Experiments with different lapping speeds allowed the researchers to quantify the competing roles of gravity and inertia in setting the optimal lapping frequency. This sequence of events revealed a few surprises about a cat’s lapping—in particular the discovery that lapping is actually very different from the way dogs lap, since a dog’s tongue penetrates the liquid surface and scoops up water.

This movie shows three lapping events of an adult domestic cat, Felis catus, during its normal drinking process. Tap water was mixed with a small amount of liquid yogurt to aid visualization. The video was recorded at 120 frames/s and is here shown slowed down four times. As the tip of the tongue comes in contact with the liquid surface, water adheres to the dorsal side of the tongue's tip. A liquid column forms when the tongue is rapidly lifted. The liquid column grows by inertia, until gravity induces its break-up through pinch-off. Jaw closure results in the capture and ingestion of part of this column.

[Video courtesy of Roman Stocker, Sunghwan Jung, Jeffrey M. Aristoff and Pedro M. Reis]

In contrast, cat’s tongues do not dip into the liquid, and only the tip of the tongue is used for lapping. Another surprise was the rapid speed at which at cat’s tongue moves—nearly one meter per second. This implies that the surface tension of the liquid plays no role in the lapping, which is instead dominated by inertial forces and gravity.

The authors also discovered that you can actually use the balance of inertia and gravity to predict lapping frequency, or how fast a cat is lapping. They tested this hypothesis by measuring the lapping frequency for eight species of felines, from videos acquired at the Zoo New England or on YouTube. In agreement with their formula, the lapping frequency decreased with increasing animal size.

In other words, the larger the feline, the smaller the lapping frequency. This observation suggests that the domestic cat's inertia/gravity-controlled lapping mechanism is the same in wild felines.

Nadia Ramlagan

11 November 2010