Wednesday, June 08, 2011

Tongues without cheek

It helps to have cheeks. We humans drink by forming a seal around the liquid (and often also the edge of a cup or bottle) with our lips, and then sucking inwards (with the help of our cheeks) to draw it into our mouths. This simple process is not available to other animals, such as dogs and cats, which don't really have cheeks to speak of.

I blogged some months ago about how cats lap up liquids. They don't use suction like we do, but as anyone who's watched a thirsty cat knows, it's all in the tongue. They touch their tongues to the surface of the water, and the liquid naturally adheres to the underside of the tongue. By pulling the tongue back quickly, the adhesion of tongue to water and cohesion between water molecules draws up a thin column of liquid into the mouth of the cat. This happens so quickly that it seems like they are 'flicking' water into their mouths - but no spooning or scooping is involved at all.

They're neat and classy, unlike dogs, right? Dogs seem to make a mess whenever they drink, and in the Times article that I linked to in my previous post, the scientists are quoted as saying that dogs scoop water up with the backs of their tongues, instead of elegantly lapping with the tips of their tongues. But this isn't really the case.

New research by a retired zoologist Alfred Crompton who used his 8-year-old dog Matilda shows that dogs do the same thing as cats do: adhesion followed by quick retraction of the tongue, except that where cats use the underside, dogs use mostly the dorsal surface. In neither case is any scooping of water involved. (via sarplus)

X-ray video of lapping in a dog from AW Crompton on Vimeo. (via Nature News blog)

So it's not true that dogs are dumb 'scoopers' while cats are sophisticated 'lappers'. As Crompton said, "Let's hear it for the dogs." But why are they still so messy? Probably because "the dog's tongue tip penetrates more deeply into the liquid than in cats, and consequently sprays more liquid around as the tongue rapidly withdraws".

Hummingbirds use a completely different mechanism to drink nectar from flowers. Because of their small size they are able to use a different realm of physical phenomena to aid in the drinking process: surface tension, instead of adhesion and bulk inertia. In this case, scientists were also debunking another long-held notion, that they drink nectar by capillary action up a tubular tongue (Steven Vogel's commentary is, as always, an informative read).

The hummingbird tongue is thin and forked at the ends. Two stiffening rods run down the length of the tongue, and each half is curled up like a straw that has been slit down lengthwise. So far so good. But what happens when the tongue is immersed in nectar is surprising: the 'lamellae' of the tongue unfurl themselves, and become flatter, no longer tube-like. The nectar sticks to the hydrophilic inner surface of the tongue.

As the tongue is withdrawn, the liquid surface of the nectar is now exposed to air; because of the surface tension of the air-nectar interface, the edges of the tongue are drawn inwards, causing it to re-furl passively. This limits the loss of nectar from the tongue. The bird brings the tongue back into its mouth, and then closes its bill leaving only a small gap. It squeezes its tongue through this narrowed opening, to wring the nectar off the tongue, starting a new cycle. This whole process is really quick, occurring over as little as 50 milliseconds, and is repeated over and over again.


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