Ever been caught in the crossfire of a wet dog firing droplets of water away from their fur with a mad shake? Well, they can't help it. Scientists have discovered the underlying mechanism that drives dogs – and many other hairy mammals – to vigorously shake water out of their fur, in a fascinating look at the genetics that trigger this involuntary behavior.
Neurobiology researchers at Harvard Medical School's Howard Hughes Medical Institute have identified the complex but rather efficient sensory mechanism that drives the "wet dog shake," proving that your pet isn't trying to make you pay for that bath by also soaking you.
"Wet dog shakes are an evolutionary conserved behavior observed across mammals that serves to remove water and other irritants from back and neck hairy skin, a skin region that is largely unreachable by self-grooming or -licking," the researchers noted in the paper's preprint. "We propose that C-LTMRs detect the lightest forces acting on hairy skin, including water, movements of insects and parasites, and other stimuli that deflect vellus hairs, triggering motor behaviors that have evolved to remove water, mechanical irritants, and potential threats."
Through a series of complex experiments on mice, the scientists tested a number of stimuli including oil and puffs of air, applied to the back or neck area of the animals – places difficult for them to groom. They tracked the initiation of the shaking action with high-speed video, to note the onset time, the frequency and the duration.
Since neurobiology is much more about identifying the mechanisms in the brain that trigger behaviors, than just observation of said behaviors, the team employed optogenetics, genetic manipulations to suppress certain pathways, and in-vivo calcium imaging to identify active neurons. Then, they bred mice without C-LTMRs neurons and saw a significant reduction in the "wet dog shake" behavior in response to mechanical stimuli such as oil and water.
C-LTMRs – or C-fiber low-threshold mechanoreceptors – are sensory neurons that fire up in response to light mechanical stimuli on fur or hairy skin in mammals. While they've been known to activate due to stimuli involving touch, it was not clear that they were key to triggering these almost comical whole-body shakes in animals.
Through its work, the team identified how the stimuli opens the Piezo2 mechanosensory ion channel, which regulates the C-LTMRs, which then connect with spinoparabrachial (SPB) neurons, triggering excitatory postsynaptic currents (EPSCs) along a specific pathway that controls this neuron-motor response mechanism.
The researchers were also able to stimulate the neurons with light (optogenetics) to trigger the shake response even without any physical stimuli on the fur, showing that activating C-LTMRs alone could initiate the behavior.
"The finding that C-LTMRs contribute to stimulus-evoked wet dog shakes afforded an opportunity to ask how these enigmatic mechanosensory neurons engage central circuits to mediate somatosensory behaviors," the team noted. "We found that C-LTMRs are indeed synaptically coupled to SPNs, since optogenetic activation of C-LTMR terminals evoked excitatory postsynaptic currents (EPSCs) in both SPN populations."
"Collectively, these findings demonstrate the contribution of a C-LTMR–spinoparabrachial pathway in mechanically evoked wet dog shakes," the team added in the paper.
While it may be more than we ever want to know about why water ends up dripping from the ceiling after Fido's bath, it's a fascinating look at just how complex sensory communication and responsive behaviors are. And until quite recently, technology limitations made this sort of discovery remain in its hypothesis stage.
So, next time your dog gives you a spray, remember they're not trying to annoy you – it's simply all in their genes.
The study was published in the journal Science.
Source: Howard Hughes Medical Institute via Phys.org
