They say a dog is a man's best friend, but a new study has found that our pet canines may be more sensitive to the higher-pitched voices of women.
An analysis of brain scans by researchers from the Institute of Cognitive Neuroscience and Psychology and Eötvös Loránd University in Hungry suggest dogs really do listen when we talk to them. What's more, they do so in a way that is oddly similar to human babies, with a preference for certain tones.
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Infants are highly sensitive to 'baby talk' – that high-pitched, sing-songy way of speaking that tends to exaggerate vowels – and studies show this exaggerated speech may actually help shape the child's developing brain for the better.
Unlike a baby, however, dogs aren't exposed to human female voices in utero, and in dog-to-dog interactions high-pitched communication is not typical.
So why have studies found dogs are sensitive to baby talk and high-pitched speech, too?
The research team set out to investigate. They trained 19 family dogs (Canis familiaris) consisting of eight different breeds, aged between 2 and 10, to climb into an fMRI machine and lie still long enough for the machine to scan their brains.
While each dog was inside the machine, they listened as the scientists played three different types of pre-recorded human speech: speech directed at dogs, speech directed at human infants, and speech directed at another human adult.
Sometimes the speaker on the recording was an adult male and sometimes they were an adult female.
Ultimately, the dogs showed greater activity in their auditory cortex when they heard the exaggerated voices we often use for pets or infants. And this was especially true when the speaker was female.
Neuroethologist Anna Gábor from Eötvös Loránd University thinks this "may be due to the fact that women more often speak to dogs with exaggerated prosody than men", referring to elements of speech like tone, stress, rhythm and emotion.
In the brain scans, human voices were processed in a secondary part of the dog's auditory cortex, known as the temporal pole, as well as an area between the temporal lobe and the frontal and parietal lobes called the Sylvian gyrus.
In past studies, neuroscientists have noticed a similar part of the brain 'light up' in human infants when they listened to baby talk.
As to how our pet dogs might have developed this human-like trait, scientists have two main hypotheses.
Either there is an ancient and universal sensitivity among mammals to sounds with higher pitch and greater variability in frequency, or this was a feature that humans selected for when domesticating wolves.
It's possible, Gábor and her colleagues explain, that pre-domesticated dogs with greater sensitivity to dog-directed speech "were more likely to stay close to humans and pay attention to their vocal cues."
Some experimental evidence does show wolves are more responsive to lower-pitched speech, while dogs are more responsive to higher-pitched speech.
Gábor's team was only able to include 19 dogs in their study, and they did not control for the sex of the pet's owners which could be an influencing factor in what types of speech the dogs were most sensitive to.
Further research is needed, but the authors say that the similarities they found between the way dogs and human babies respond to adult voices is worth exploring further.
Maybe our pets will turn out to be a really useful model for what we were like before we could speak.
The study was published in Communications Biology.