# A Wet Mouse Shakes 30 Times a Second

A Labrador needs about 4.5 Hz and a brown bear 4 Hz. The adult mouse measured 29 Hz, rounded to 30 in the title; the physical model explains the rhythm through the acceleration needed to release water from fur.

The study by Andrew K. Dickerson, Zachary G. Mills and David L. Hu, published in 2012 in the *Journal of the Royal Society Interface*, filmed 33 animals from 16 species at 500–1,000 frames per second. The relationship was fitted across 25 representative points.

## Size sets the rhythm

The sample runs from 0.01 kg to 260 kg, a ratio of 26,000 to 1 and more than four orders of magnitude. The measured fit is *f* ∝ *M*<sup>−0.22</sup>, with R² = 0.95. The physical model predicts the exponent −3/16, or −0.19 when rounded.

The adult mouse in the table shakes about 6.4 times as fast as Labrador 2. One cycle lasts 34 ms in the mouse, 222 ms in the Labrador and 250 ms in the brown bear.

## The drop leaves when inertia beats capillarity

Surface tension holds water between clumps of hair. A rapid change of direction produces the acceleration *Rω*<sup>2</sup>, which increases the drop's apparent weight until release.

Published values range from 12 to 72 times gravitational acceleration. If the adult mouse copied a dog's 4 Hz rhythm, the calculation gives about 1 × *g*, leaving water attached to the fur.

## Loose skin amplifies the movement

On the Labrador, the skin marker had an amplitude of about 90°, while estimated spine movement was 30°. Three times the amplitude produces a ninefold gain in speed-related force.

## Shaking removes about 70% of the water

Fur retains about 30% of accumulated water after shaking. The energy model estimates a cost between one ten-thousandth and one thousandth of the energy required to evaporate the removed share, equivalent to 0.01%–0.1%.

## Limits of the rule

Most species are represented by a single animal. Some large-animal dimensions came from published estimates, and the largest or nearly hairless mammals may depart from the observed relationship. The study explains drying mechanics after the gesture begins.

## Source

- [Andrew K. Dickerson, Zachary G. Mills and David L. Hu, “Wet mammals shake at tuned frequencies to dry”, *Journal of the Royal Society Interface*, 2012](https://doi.org/10.1098/rsif.2012.0429)
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