How Cats Drink: Inertia Against Gravity

Biology lens: a tongue for drinking and grooming

Cats and dogs share the problem of incomplete cheeks, and both lap 3. For a while it was thought that dogs, unlike cats, scoop water with a spoon-shaped tongue. High-speed light and X-ray videos showed that dogs, too, rely on water sticking to the tongue. Inside the mouth, the liquid is moved backwards towards the throat on the surface of the tongue; ridges on the palate hold it in place while the tongue moves forward again. A dog needs at least three lapping cycles to move a portion of water through its mouth 4.

The details still differ. Dogs accelerate the tongue more strongly than cats and curl it backwards, which pulls a larger column of water from the bowl. Sean Gart and colleagues filmed 19 dogs: their tongues left the water with accelerations of 1–4 times gravity and speeds of 0.7–1.8 m/s. Dogs time their bite to the moment when the column breaks up – and the large, splashing column explains why dogs are such messy drinkers 3.

Within the cat family the mechanism appears to be the same. Reis and colleagues measured lapping frequencies across the cat family, Felidae, and found them consistent with one physical model – which suggests that the mechanism is conserved among cats 1.

The cat’s tongue has a second specialty. It is covered with sharp, backward-pointing spines, the papillae. Alexis Noel and David Hu studied the tongues of six cat species, from the domestic cat to the lion – a 30-fold range in body weight. Each papilla has a hollow cavity at its tip that soaks up saliva on its own and releases it onto the hairs during grooming 5.

Physics lens: inertia against gravity

When the tongue pulls away from the surface, the water stuck to it is set in motion. A moving body of water tends to keep moving – that is its inertia. At the same time, gravity pulls the water back towards the bowl. As long as inertia dominates, the column rises; when gravity takes over, it slows down, stops and falls back. Reis and colleagues showed that the cat exploits exactly this competition: it closes its mouth before gravity can win 1.

Physicists compare the two effects with a dimensionless number, the Froude number:

Here U is the speed of the tongue, g the acceleration of gravity and R the size of the tongue tip. If Fr is much larger than 1, inertia dominates; if it is much smaller, gravity does. The balance between the two, Fr of about 1, sets the rhythm of lapping 1.

Two other forces play only a minor role at the scale of a cat or a dog. In experiments that mimicked a dog’s tongue, Gart and colleagues estimated Reynolds numbers of about 10³ to 10⁴ and Weber numbers of 10 to 1,000: viscosity and surface tension are small compared with inertia and gravity 3. For very small animals this changes – they drink using subtle effects at the water surface, such as capillary forces 2.

Cats and dogs differ in one more respect. For cats, the water’s motion is governed by steady inertia: what counts is mainly how fast the tongue moves. Dogs accelerate their tongue so strongly that the acceleration itself matters – an unsteady inertial regime, a kind of open pump driven by acceleration 3.

Mathematics lens: why big cats lap only a little slower

A scaling argument predicts how the lapping rate depends on body size. Start with the balance of inertia and gravity, Fr ≈ 1:

One lap takes roughly as long as the tongue needs to travel a distance of the order of its own size R at speed U. The lapping frequency f is the inverse of that time:

Animals of similar shape grow in all directions alike: their volume and mass M grow as R³, so R grows as M^1/3. Inserting this gives

Measurements across the cat family support this prediction 1, and dogs follow the same trend 3. The exponent −1/6 is small: a lion that is about 47.5 times heavier than a 4 kg house cat should lap only 47.5^1/6 ≈ 1.9 times more slowly.

Without gravity in the argument the result would be very different. If all cats moved their tongues at the same speed, the frequency would fall as 1/R, that is as M^−1/3 – a lion would lap 47.5^1/3 ≈ 3.6 times more slowly. Bigger tongues move faster, because a bigger column needs more speed to keep gravity at bay.

Computer science lens: a scaling law in code

How fast do different members of the cat family lap compared with a house cat? The program below applies the scaling law of the mathematics lens, f ∝ M^−1/6, to six cats and compares it with the law that would hold if tongue speed did not grow with size. The body masses are typical values for illustration, not measurements from the cited studies, and the program gives frequencies relative to a house cat rather than in laps per second.

import numpy as np
import matplotlib.pyplot as plt

# Typical body masses (kg) – rough values for illustration, not measurements
CATS = {"domestic cat": 4, "ocelot": 11, "cheetah": 45, "jaguar": 80, "lion": 190, "tiger": 220}
EXPONENT = -1 / 6     # frequency ∝ mass^EXPONENT: inertia balancing gravity (see the maths lens)


def relative_frequency(mass, reference=CATS["domestic cat"]):
    """Lapping frequency relative to a domestic cat.

    Inertia balances gravity when the tongue speed U is about √(g·R) (Froude number ≈ 1),
    so one lap takes a time of order R/U = √(R/g). The tongue radius R grows like mass^(1/3)
    for animals of similar shape, hence frequency ∝ √(g/R) ∝ mass^(−1/6).
    """
    return (mass / reference) ** EXPONENT


print("animal          mass    laps relative to a domestic cat")
for name, mass in CATS.items():
    print(f"{name:13s} {mass:5d} kg   {relative_frequency(mass):5.2f}")
lion = relative_frequency(CATS["lion"])
print(f"a lion is {CATS['lion'] / CATS['domestic cat']:.1f} times heavier "
      f"but laps only {1 / lion:.1f} times slower")

masses = np.logspace(0, 2.6, 100)
plt.rcParams["font.size"] = 13      # large type: the chart stays readable on a phone
fig, ax = plt.subplots(figsize=(6.4, 5.2), layout="constrained")
ax.loglog(masses, relative_frequency(masses), lw=2.5, label="inertia vs. gravity: mass^(−1/6)")
ax.loglog(masses, (masses / CATS["domestic cat"]) ** (-1 / 3), "--", lw=1.8,
          label="if speed did not grow: mass^(−1/3)")
for name, mass in CATS.items():
    below = name == "tiger"             # lion and tiger are close: one label above, one below
    ax.plot(mass, relative_frequency(mass), "o", ms=7, color="black")
    ax.annotate(name, (mass, relative_frequency(mass)), textcoords="offset points",
                xytext=(-10, -20) if below else (6, 6), fontsize=11)
ax.set_xlabel("body mass (kg)")
ax.set_ylabel("laps per second (domestic cat = 1)")
ax.set_title("Bigger cats lap only a little slower")
ax.set_ylim(0.15, 2.5)
ax.set_yticks([0.2, 0.3, 0.5, 1, 2])
ax.xaxis.set_major_formatter("{x:g}")   # plain numbers: 1, 10, 100
ax.yaxis.set_major_formatter("{x:g}")
ax.legend(loc="lower left")
plt.show()

• Open in Colab
• Download .py
• How to run this code

Output · what the code prints, re-run at every build

animal          mass    laps relative to a domestic cat
domestic cat      4 kg    1.00
ocelot           11 kg    0.84
cheetah          45 kg    0.67
jaguar           80 kg    0.61
lion            190 kg    0.53
tiger           220 kg    0.51
a lion is 47.5 times heavier but laps only 1.9 times slower

What the result teaches:

• Size matters, but weakly. A lion is 47.5 times heavier than a house cat but laps only 1.9 times slower (relative frequency 0.53). Across the whole range from house cat to tiger, the frequency changes by less than a factor of two.
• The exponent carries the physics. The dashed line shows what would happen if every cat moved its tongue at the same speed: the lion would lap at only about a quarter of the house cat’s rate. Because gravity sets the tongue speed, U ≈ √(gR), bigger cats move their tongues faster and the frequency falls much more gently.
• A model, not a measurement. The program uses a proportionality, not measured laps per second; the body masses are rough typical values. It shows the shape of the law that Reis and colleagues tested against measurements 1.

Two programming ideas are worth noticing:

• Relative values remove unknown constants. The scaling law only says that frequency is proportional to M^−1/6. Dividing by the value for a house cat cancels the unknown constant of proportionality – a common trick when only a scaling law is known.
• Log–log plots turn power laws into straight lines. On logarithmic axes, f ∝ M^−1/6 becomes a straight line with slope −1/6, so different exponents can be compared at a glance.

Try it yourself – each change takes one line:

• Set EXPONENT = -1 / 3: the law for a constant tongue speed. The lion now laps at 0.28 of the house cat’s rate, 3.6 times slower.
• Set EXPONENT = 0: if size did not matter at all, every cat would lap at the same rate, 1.00 – and the lion only 1.0 times slower.

animal          mass    laps relative to a domestic cat
domestic cat      4 kg    1.00
ocelot           11 kg    0.71
cheetah          45 kg    0.45
jaguar           80 kg    0.37
lion            190 kg    0.28
tiger           220 kg    0.26
a lion is 47.5 times heavier but laps only 3.6 times slower

animal          mass    laps relative to a domestic cat
domestic cat      4 kg    1.00
ocelot           11 kg    1.00
cheetah          45 kg    1.00
jaguar           80 kg    1.00
lion            190 kg    1.00
tiger           220 kg    1.00
a lion is 47.5 times heavier but laps only 1.0 times slower