When it comes to interpreting dog growls, some humans are surprisingly good at taking the hint, a new study shows. Scientists testing how people categorized different types of natural growls found that people could largely tell playful vocalizations from threatening ones – though women and dog owners seemed to do better than their peers.
The findings, described in Royal Society Open Science, shed light on the relationship between dogs and humans – as well as on underlying vocal behaviors that might be shared across mammalian species.
Plenty of research in recent years has delved into dogs' ability to understand humans. But relatively little seems to have focused on whether humans are any good at understanding dogs – even though communication is a two-way street, especially in two species that have developed in such close proximity to each other.
"We know relatively little about the vocal communication system of dogs, and the most studied vocalization (not surprisingly) are the different barks," lead author Tamas Farago, an ethologist at Eotvos Lorand University in Budapest, Hungary, said in an email. As dogs were domesticated by humans, barks likely changed significantly and became the main way dogs communicate. Growls, however, may not have changed as much since the point that dogs diverged from wolves.
"On one hand, this makes them interesting to study how information is encoded in their acoustic structure," he added, "but on the other hand, it is also fascinating for us that dogs use them in strikingly different social contexts: during play and agonistic interactions."
For this paper, Farago and his colleagues tested humans using natural dog growls gathered in three scenarios: while playing tug of war with their owner, while guarding their food from another dog and while they felt threatened by a stranger.
The scientists found that overall, humans were pretty good at differentiating the growl types, classifying them correctly about 63 percent of the time (well above the chance level of 33 percent). They correctly identified 81 percent of the play growls, but were less accurate when it came to food-guarding (60 percent) and threatening (50 percent) growls. On some level, this makes sense; the latter two, both meant to ward off a competitor or a threat, might share some of the same sound qualities.
"What (was) surprising is that the listeners rated the threatening growls to be more fearful and less aggressive than the food guarding ones, as these growls were acoustically very similar," he said.
Still, there certainly were key acoustic differences in the three types.
"We found that playful growl bouts are built up from short, quickly repeated growls, while the aggressive ones were more elongated," he explained. "The food-guarding growls differed from the threatening growls in their formant dispersion, a parameter that gives a size impression of the vocalizing individual for the listeners."
(Formants are concentrations of acoustic energy at different frequencies. Humans use them to make different vowels; in dogs' growls, the formants' dispersion – how close they are to each other – can signal a dog's size.)
Dog owners were much better than other humans at correctly identifying a growl's meaning – which was actually surprising because previous research didn't find such a strong advantage when people rated dog barks. It's possible that this is because barks are loud and easily heard by bystanders, while growls are more likely to be heard regularly only by people who spend a lot of time near dogs. Humans, it seems, can be trained to understand their canine companions.
Women also were better than their peers at distinguishing between the growls, scientists said.
"This is a common pattern in emotion recognition studies," Farago said. "Probably women are more empathic and sensitive to others' emotions, and this helps them to better associate the contexts with the emotional content of the growls."
Dogs may not be the only mammals to pack information into sound this way, Farago said. The few findings on how humans perceive other animals' vocalizations (including macaques, pigs and cats) suggest that among mammals, there are simple rules rooted in biology that may define how emotional states get translated into sound structure.
"Along with our recent fMRI findings about the similar processing of emotional vocalization between humans and dogs, our results further emphasize how much we share with other mammalian species," he said.
The findings, Farago added, might help humans better understand and improve our relationships with dogs. This could be especially important for children, who may have trouble telling the difference between a playful and aggressive dog, and aren't yet fully aware of the consequences.