In the News
Cats lap liquids
with a flick of
the tongue and
Imbibing a delicate interplay
between inertia and gravity
By Susan Milius
Sorry, Fido. A paper in the jour- nal Science has just ascribed “elegance and complexity” to the way cats drink.
A dog drinks by forming its tongue
into a little cup that merely ladles liquid into its mouth, says coauthor Pedro
Reis of MIT. “Cats are much more
sophisticated in the knowledge of fluid
dynamics,” he deadpans.
Instead of scooping, cats use what the
researchers describe as a “subtle mechanism” in which water sticks to the tip
of the tongue and is pulled up into the
mouth, taking advantage of the water’s
inertia. Though cats have been lapping
liquids in public for millennia and early
high-speed photography revealed some
basic aspects of their drinking, Reis says
he knows of no detailed analysis of the
phenomenon preceding the one he and
his colleagues published online November 11. “It’s amazing how you look at
something and think, somebody must
have studied that before. But as happens
with many things in everyday life, that is
not the case,” Reis says.
He and his colleagues do cite scenes
of cats drinking from Quicker ’n a Wink,
a 1940 film featuring MIT professor and
high-speed photography pioneer Harold
“Doc” Edgerton. The film won an Oscar,
but Edgerton never completely elucidated how cats drink.
The gaps in lapping analysis do not surprise functional morphologist Rebecca Z.
German of Johns Hopkins School of Medicine in Baltimore. “ What we know about
mammalian feeding is woefully incomplete,” she says. It’s hard to observe, and
scientists have studied other feats such as
locomotion in much more detail.
Unlike people and pigs and sheep,
cats and most other adult carnivores
don’t have the right kind of cheeks to
suck in a liquid. If a bowl or a puddle is
shallow enough, a cat plunges its tongue
to the bottom and essentially licks the
liquid off. But the motion known as
High-speed movies of house pet Cutta
Cutta drinking helped owner Roman
Stocker and his colleagues explain how
cats get liquids from bowl to mouth.
lapping works quite differently.
Figuring out the mechanics of cat lapping took high-speed photography; the
project’s lappers-in-residence typically
managed 3.5 dips of the tongue per second. What the camera revealed is that a
cat shoots its tongue down in a J shape,
with the tongue tip curled under to touch
the liquid’s surface, explains coauthor
Roman Stocker, also of MIT. The tip
does not scoop into the liquid but pulls
directly back into the mouth, reaching
peak speeds of 78 centimeters per second.
Liquid adheres to the tip of the tongue
and rises in a column as the tip retracts.
Just before gravity can overcome the
rising liquid’s inertia and collapse the
column in a splash, the cat catches the top
portion of the liquid column in its mouth.
Along with watching movies of cats
drinking, researchers tested their ideas
about the process with a device that
tapped a disk against a liquid and pulled
upward at whatever speed the researchers wanted. With this device and a lot of
calculation, the researchers were able to
confirm or collapse various hypotheses.
At first researchers thought that the
distinctive roughness of a cat’s tongue
would play a role in pulling up the liquid
column, Stocker says. Wrong. Cat tongues
have smooth tips, and, as it turns out, a
smooth, wet surface works well for lapping. To get similar wetting properties in
their experimental setup, the researchers
used a glass disk in their test device.
The viscosity of the liquid did not make
a difference in the process, at least within