When a dog shakes water off its fur, the action is not just a random flurry of movements — nor a deliberate effort to drench anyone standing nearby.

This instinctive reflex is shared by many furry mammals including mice, cats, squirrels, lions, tigers and bears. The move helps animals to remove water, insects or other irritants from hard-to-reach places. But underlying the shakes is a complex — and previously mysterious — neurological mechanism.

Now, researchers have identified the neural circuit that triggers characteristic ‘wet dog’ shaking behaviour in mice — which involves a specific class of touch receptors, and neurons that connect the spinal cord to the brain. Their findings were published in Science on 7 November¹.

“The touch system is so complex and rich that [it] can distinguish a water droplet from a crawling insect from the gentle touch of a loved one,” says Kara Marshall, a neuroscientist at Baylor College of Medicine in Houston, Texas. “It’s really remarkable to be able to link a very specific subset of touch receptors to this familiar and understandable behaviour.”

Sensitive skin

The hairy skin of mammals is packed with more than 12 types of sensory neuron, each with a unique function to detect and interpret various sensations. A team led by Dawei Zhang, a neuroscientist then at Harvard University in Cambridge, Massachusetts, focused on a type of ultra-sensitive touch detecting receptors called C-fibre low-threshold mechanoreceptors (C-LTMRs), which wrap around hair follicles.

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In humans, these receptors are associated with pleasant touch sensations, such as a soft hug or a soothing stroke. But in mice and other animals, they serve a protective role: alerting them to the presence of something on their skin, whether it’s water, dirt or a parasite. When these stimuli cause hairs on the skin to bend it activates the C-LTMRs, says Marshall, “extending the sensibility of the skin beyond just the surface”.

To get laboratory mice to shake their fur like wet dogs, the researchers applied drops of sunflower oil to the backs of the mice’s necks. Nearly all the animals shook off these drops within ten seconds. The team then genetically modified some of the mice to remove most of their C-LTMRs. These animals showed a 50% reduction in shakes when oil droplets landed on their neck, compared with unmodified control mice.

The researchers also wanted to explore how signals from C-LTMRs travel through the nervous system to orchestrate the wet dog shakes. They traced the pathway to a group of neurons in the spinal cord; this connects to an area in the brain called the parabrachial nucleus, which is involved in processing pain, temperature and touch.

Using optogenetics, a technique that engineers neurons so that they can be switched on and off in response to light, the researchers blocked the activity of the spinal neurons. These mice showed a 58% reduction in shakes compared with control mice. Blocking activity in the parabrachial nucleus produced similar results. The mice still scratched, groomed and moved normally, suggesting that the neural circuit is specific to wet dog shakes.

Specialized circuit

The discovery opens up avenues for future research. “The wet dog shake is a very coordinated motor response,” says Thomas Knöpfel, a neuroscientist at Hong Kong Baptist University in Kowloon Tong, who adds that the study is a good starting point to study how the brain sends commands to control the movement. “Wet dog shake is triggered in many animals by psychedelic drugs,” he says. The response to psychedelics involves serotonin receptors, which also play a part in pleasurable touch. “That gives inspiration for some more work connecting the dots.”

Zhang says that future research could also investigate whether overactive C-LTMRs contribute to conditions such as twitch-skin syndrome in cats, which involves sudden rippling of the skin and excessive twitching, or to other kinds of skin hypersensitivity in humans.