Dogs turn out to be much more sophisticated drinkers than previously realized, since new research explains how they seemingly defy gravity with each gulp.
The findings, published in the latest Royal Society Biology Letters, put dogs in the same lapping category as cats. Both can lift liquids into the air before they swallow the near-freestanding vertical column of water or other drink.
But there is one important difference between cats and dogs: "Dogs make much more mess," co-author Alfred Crompton told Discovery News.
"Unlike cats that usually place their tongue tip on the liquid surface, dogs penetrate the liquid surface and consequently liquid is spread all around as the tongue is withdrawn," Crompton, a professor emeritus in the department of Natural History at Harvard University, told Discovery News.
He and colleague Catherine Musinsky used high-speed video recordings, including X-ray video, to record a domestic dog drinking both broth and a milk-barium mixture.
Analysis of the footage determined that a drinking dog curls its tongue tip backward, creating a scoop-like shape. While some liquid is collected in the "scoop," most adheres to the surface of the dog's tongue. The liquid rises in a column, against gravity, on the tongue surface as it is drawn upward. The dog's tongue then presses against ridges on the upper portion of its mouth interior, where liquid is trapped in recesses before the canine swallows.
"The adhesive property of liquid, its 'surface tension,' is what seems to defy gravity," Musinsky told Discovery News.
The researchers say that we, too, can test out water's adhesive nature.
"Place your palm on the surface of liquid in a bowl and rapidly lift your arm," Crompton said. "Liquid is drawn upwards."
"The trick about dog and cat lapping is to catch part of the liquid lifted in the mouth by closing the mouth to capture part of the column," he added. "Tongues are remarkable in the different shapes they can form."
Technically, humans could lap as dogs do, "but it would take a long time to get a decent drink," he said. "You can stick your tongue out, curl it down, place it on a liquid surface and then withdraw, but the tongue will immediately straighten out and spray most of the liquid before mouth closure."
The scientists were inspired to conduct the research, in part, after watching a Discovery Channel "Time Warp" video showing a dog drinking. In that case, the dog was lapping up water, so the liquid movement details captured by Crompton and Musinsky, with their broth and milk tests, may have originally been missed.
Last year, a research project led by Pedro Reis of MIT's Department of Civil and Environmental Engineering also looked at how animals lap up liquids, with a focus on cats.
In that paper, however, Reis and his team concluded that dogs rely more on scooping than cats do. "Although the dog's tongue also (like the tongue of cats) curls up caudally in a cuplike-shape," he said, "it penetrates the liquid surface and scoops up the water that fills its ventral side."
The new research, which involved Reis' assistance, places much less emphasis on this scooping, and more on how liquids adhere to a dog's tongue. The only exception is when dogs lap up thick gravy or another less watery substance.
"Once the consistency of porridge is reached, dogs move the tongue to get below the food rather than sticking the dorsal surface of the tongue tip against the food," Crompton explained.