Frogs use a viscoelastic tongue and non-Newtonian saliva to catch prey

Abstract

Frogs can capture insects, mice and even birds using only their tongue, with a speed and versatility unmatched in the world of synthetic materials. How can the frog tongue be so sticky? In this combined experimental and theoretical study, we perform a series of high-speed films, material tests on the tongue, and rheological tests of the frog saliva. We show that the tongue's unique stickiness results from a combination of a soft, viscoelastic tongue coupled with non-Newtonian saliva. The tongue acts like a car's shock absorber during insect capture, absorbing energy and so preventing separation from the insect. The shear-thinning saliva spreads over the insect during impact, grips it firmly during tongue retraction, and slides off during swallowing. This combination of properties gives the tongue 50 times greater work of adhesion than known synthetic polymer materials such as the sticky-hand toy. These principles may inspire the design of reversible adhesives for high-speed application.

The tongue as a gripper

“…The Stefan equation is only valid when the saliva layer is thin, the fluid is an incompressible Newtonian fluid and the flat plates are rigidthe last two assumptions do not hold for frog tongues. Nevertheless, in earlier work (Noel et al, 2017), we showed that Eqn 12 still gives a good estimate for the pull-off force of a plate from a frog tongue.…”

“…muscle activation from the arm's base to tip, likely to overcome the large drag forces in the water (Yekutieli et al, 2005). In contrast, the frog tongue stretches passively during unraveling, with the ability to extend in length from 36 to 49 mm, a stretch percentage of 130% (Noel et al, 2017). Other papers have noted stretch percentages of 180% for inertial elongators such as Bufo marinus (Nishikawa, 1999;Nishikawa and Gans, 1996).…”

“…The relationship between tongue resting length and body mass is shown in Fig. 3B using data gathered from 12 different literature sources (Anderson et al, 2012;Deban and Nishikawa, 1992;Emura et al, 2013;Igado, 2011;Meijaard, 1997;Muchhala, 2006;Naples, 1999;Nishikawa and Cannatella, 1991;Nishikawa and Roth, 1991;Noel et al, 2017;Pfeiffer et al, 2000;Reis et al, 2010) as well as our own measurements. Tongues of cow, coyote, Great Dane domestic dog, human, racoon, giant otter, fox, mink, rabbit, ring tail cat, hamster, rat and squirrel were gathered from Zoo Atlanta, a local dissection laboratory, and a local supermarket.…”

“…To measure saliva shear viscosity, a sample can be placed inside a cone-plate rheometer and viscosity measured across several orders of magnitude of shear rate. For both human and frog saliva, the viscosity is found to fit the Carreau-Yasuda model (Helton and Yager, 2007;Noel et al, 2017):…”

“…All biofluids decrease in shear viscosity η with increasing shear rate _ g. The Carreau-Yasuda model for shear-thinning fluids (dashed lines) is used to fit experimental data. Data have been replotted from the following sources: frog saliva (Noel et al, 2017), human saliva (Helton and Yager, 2007) and pitcher plant fluid (Erni et al, 2011).…”