Pedro Reis, Sunghwan Jung, Jeffrey Aristoff, and Roman Stocker had the same impulse, with better equipment. Their 2010 paper in Science, “How Cats Lap: Water Uptake by Felis catus,” is one of the more elegant pieces of dimensional analysis in recent biology.

How Cats Do Not Drink

The simplest hypothesis — that cats curl the tongue into a spoon and scoop water into the mouth — is false. High-speed photography shows that the cat’s tongue does not form a cup shape. Instead, the cat extends the tongue tip downward toward the water surface and then rapidly retracts it. The motion is fast — too fast for normal video — and the tongue barely contacts the surface.

The contrast with dogs is instructive. Dogs do scoop: the tongue curls backward (not forward), forming a ladle shape that scoops water upward and backwards into the mouth. The mechanism is vigorous and inefficient — a significant fraction of the water misses the mouth entirely, which is why drinking dogs produce splashing and dogs often have wet chins. The mechanism works but is inelegant.

Cats produce almost no splash. The mechanism is different in kind.

The Physical Mechanism

Reis et al. (2010) used high-speed photography (1000 frames per second) to resolve the cat’s lapping motion. Their observations:

1. The cat extends the tongue tip downward until the dorsal surface (the top side) just touches the water surface. The ventral surface (the smooth underside) does not contact the water.

2. The cat then rapidly retracts the tongue upward. The tongue tip is moving at roughly $v \approx 0.7\,\mathrm{m/s}$ during this retraction.

3. As the tongue tip pulls away from the surface, a column of liquid is pulled upward by the adhesion between the liquid and the retreating tongue. The column rises against gravity.

4. The column eventually stalls — inertia is overcome by gravity — and begins to fall back. The cat closes its jaw at exactly the moment of maximum column height, capturing the peak volume of water.

5. The cat then extends the tongue for the next lap.

The cat closes its jaw before the tongue fully retracts. This is important: the jaw closure captures the water column, not the water adhering to the tongue. The tongue is the mechanism that creates the column; the jaw captures it.

Dimensional Analysis: The Froude Number

The relevant competition is between inertia (which drives the column upward) and gravity (which pulls it back down). Surface tension plays a role in stabilising the column but is not the primary factor governing the column height.

The balance between inertia and gravity for a fluid column moving at speed $v$ and of characteristic length scale $L$ (here, the diameter of the tongue tip, $L \approx 5\,\mathrm{mm}$ for a domestic cat) is captured by the Froude number:

where $g = 9.81\,\mathrm{m/s}^2$ is gravitational acceleration.

When $\mathrm{Fr} \ll 1$: gravity dominates, inertia is insufficient to pull a significant column of water upward. Very slow tongue motion would lift almost no water.

When $\mathrm{Fr} \gg 1$: inertia dominates, the column rises far above the surface but the jaw must be closed quickly before the large amount of water falls back. Very fast tongue motion wastes water and requires rapid jaw closure.

The optimal lapping frequency — maximising captured volume per lap — occurs near $\mathrm{Fr} \approx 1$, where inertial and gravitational forces are comparable and the column height is matched to the jaw closure dynamics.

Checking the Numbers for a Domestic Cat

For a domestic cat:

• Tongue tip diameter: $L \approx 5\,\mathrm{mm} = 5 \times 10^{-3}\,\mathrm{m}$
• Characteristic tongue tip speed: $v \approx 0.7\,\mathrm{m/s}$

Reis et al. found Fr of order unity — inertial and gravitational forces comparable — confirming that the lapping speed is tuned to the inertia-gravity balance. (The exact numerical value depends on the choice of characteristic length scale; using the tongue tip diameter as above gives Fr in the range 1–3, squarely in the regime where neither force dominates.)

Scaling Across Felids

The Froude number prediction yields a scaling law for lapping frequency across felid species of different sizes. If all felids lap at $\mathrm{Fr} \approx 1$, then the characteristic speed scales as $v \sim \sqrt{gL}$, and the lapping frequency scales as:

where $d$ is the distance the tongue travels per lap (roughly proportional to tongue length, which scales with body size). Since $L \sim d$ scales with body size, we get:

Larger cats have longer tongues and lap more slowly. The prediction is that lapping frequency scales as the square root of inverse tongue length — or, equivalently, as the inverse square root of body mass (since linear dimensions scale as mass$^{1/3}$):

Reis et al. tested this against high-speed footage of large felids. A domestic cat laps at approximately $4\,\mathrm{Hz}$; a lion laps at approximately $1.2\,\mathrm{Hz}$; a tiger at roughly $1\,\mathrm{Hz}$. The scaling is consistent with $f \propto m^{-1/6}$ across three orders of magnitude in body mass.

The table below shows the predicted versus observed scaling:

The $m^{-1/6}$ scaling captures the correct trend — larger cats lap more slowly — though the predicted frequencies for the largest cats somewhat overestimate the observed values. The discrepancy may reflect the limitations of the simple allometric assumption (that all linear dimensions scale as $m^{1/3}$) and the fact that tongue geometry does not scale isometrically across the full range of felid body sizes.

Why Not Just Lick?

A natural question: why not simply allow the tongue to fully submerge and absorb water through the papillae, as the tongue already contacts water when lapping? Several answers:

1. Papillae are not sponges. Feline papillae are hollow and scoop-shaped (filiform papillae with hollow tips), optimised for grooming and food manipulation, not passive absorption. Active wicking is limited.

2. The cat cannot breathe with its mouth submerged. A lapping mechanism that keeps the mouth mostly closed except for the brief jaw-closure moment allows continuous breathing through the nose during drinking.

3. Speed and efficiency. The inertial column mechanism delivers significantly more water per jaw movement than surface tension adhesion alone. At 4 laps per second, a domestic cat takes in roughly $0.14\,\mathrm{mL}$ per lap, for a total of roughly $34\,\mathrm{mL/min}$ — comparable to sipping rates in animals that use more direct intake mechanisms.

The cat has converged on a hydrodynamically optimal strategy under the constraint of keeping the oral cavity mostly sealed during the intake cycle.

The Robotic Tongue

Reis et al. constructed a robotic cat tongue to verify the mechanism: a smooth glass disc lowered to the water surface and retracted at controlled speeds. The column height as a function of speed followed the predicted inertia-gravity balance, confirming that the mechanism does not depend on any specifically biological property of the tongue — it is a fluid dynamics result that applies to any surface moving away from a water interface at the right speed.

The robot lapped at the same Froude number as the cat.

Dogs, Horses, and the Comparison

Dogs cup the tongue caudally (backwards) rather than ventrally, forming a ladle. The mechanism is faster and delivers more water per stroke but is messy — the ladle is formed outside the mouth, and water sloshes freely. Dogs lap at roughly $3\,\mathrm{Hz}$ with a tongue tip speed significantly higher than cats, producing Fr well above unity. The excess inertia is why dog drinking generates splashing.

Horses, by contrast, create a near-seal with their lips and use suction — a fundamentally different mechanism that requires no tongue projection at all. The lapping mechanism of felids is phylogenetically specific and appears to have evolved under selection pressure for both efficiency and noise suppression, consistent with the ambush-predator lifestyle. A cat that splashed while drinking would alert prey at a water source. A cat that laps near-silently does not.

A Note on the Measurement

Getting reliable high-speed footage of a cat drinking is harder than it sounds. Our cats drink at different times of day, in different moods, and the presence of a camera tripod next to the water bowl is regarded as grounds for drinking elsewhere. Pedro Reis et al. solved this by filming their laboratory cat, Cutta Cutta, in a controlled setting. Their footage is available online and is genuinely beautiful: a slow-motion waterfall in miniature, rising improbably from the tongue tip and held there by the balance between upward momentum and downward gravity, until the jaw swings shut.

The physics is in the timing.

References

• Reis, P.M., Jung, S., Aristoff, J.M., & Stocker, R. (2010). How cats lap: Water uptake by Felis catus. Science, 330(6008), 1231–1234. https://doi.org/10.1126/science.1195421

• Aristoff, J.M., Stocker, R., Jung, S., & Reis, P.M. (2011). On the water lapping of felines and the water running of lizards. Communicative & Integrative Biology, 4(2), 213–215.

• Vogel, S. (1994). Life in Moving Fluids: The Physical Biology of Flow (2nd ed.). Princeton University Press.

Changelog

• 2025-12-15: Updated water intake per lap from 0.04 mL to 0.14 mL (Reis et al. report ~0.14 +/- 0.04 mL per lap; the previous value was the standard deviation), and updated the intake rate accordingly (~34 mL/min). Updated the papillae location from ventral to dorsal surface. Updated the Aristoff et al. reference to the correct 2011 Communicative & Integrative Biology article. Removed the Jung & Kim (2012) PRL reference (article number 034501 resolves to a different paper).