The human-dog relationship is unique among animals: having co-evolved for so long, most dog owners will attest to the uncanny ability that their canines have when it comes to reading and responding to emotions. But a new study suggests that the dog-human connection is so profound that our dogs actually cry for us, or with us.
Jason Bittel, a contributing writer at National Geographic, covers animals for a living — and he's felt this kind of canine connection firsthand. Speaking with Salon by email, he recalled a powerful experience he felt when his ten-pound Pomeranian, Marla, was swarmed by yellow jackets.
"I found her flattened to the ground and shivering, so I scooped her up and ran to the bathroom, turned on the shower, and stood there pulling the stinging insects out of her thick black fur with my bare hands," Bittel recalled. "I'll never forget the look in her eyes, first of sheer terror, and then, after the dirty work was done, teary-eyed gratitude."
"After tens of thousands of years of dogging our footsteps, it seems pretty reasonable that hormones, tear-ducts, and whatever else is associated with emotional interactions would co-evolve right along with us, too."
This memory did not return to Bittel out of nowhere. It was prompted by a new study published in the scientific journal Current Biology which proves that, when dogs are reunited with humans who they love and have not seen in a while, their eyes produce tears of joy.
In the study, scientists took advantage of knowledge of a key hormone shared by humans and canines alike — oxytocin, the so-called "happy hormone." When an oxytocin solution was applied to dogs' eyes, their tear volume increased, suggesting a link between feelings of happiness and the quantity of tears their eyes produce. They also measured the tear volume produces by dogs before and after they reunited with different types of humans (one type being their owner; the other type, a familiar person who was not their owner). When dogs were reunited with humans in the former group, their tear volume significantly increased; when it was with the latter, it did not. That suggests a teary-eyed response occurs when reunited with someone they were fond of.
Finally, to show that humans bond with dogs through canine tears as much as vice versa, the scientists asked human participants to rank their sense of emotional attachment to various photographed dogs. Humans generally gave more positive scores to dogs whose eyes had produced more tears.
"Dog[s] shed emotional tears associated with positive situations," Takefumi Kikusui, PhD, DVM, a professor at Azabu University and corresponding author on the study, told Salon by email. "This is the first report, as far as we know, to show emotional tears in animals."
Kikusui added, when asked for an explanation for this behavior, that a "dog's teary eyes can facilitate human caregiving behavior to dog —. and this enhances the bond." The study indicates that "dogs' emotions are expressed in a similar way to human emotions" and that this "helps humans understand canine emotions."
Renee Alsarraf, a veterinarian and author of "Sit, Stay, Heal: What Dogs Can Teach Us About Living Well," told Salon by email that the study is a "good start" in terms of illuminating the phenomenon that many veterinarians and pet parents suspected — that dogs can "cry."
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The specific type of canine crying in question is what Alsarraf called "psychogenic tears," meaning tears that derive from a psychological rather than physical origin.
"We know for a fact that when dogs and their pet parents look into each other's eyes, it feels good to the canine as their oxytocin levels rise — a hormone that makes us feel good," Alsarraf explained. "We also know that dogs produce basal and reflexive tears but typically there has been the scientific belief that they are not capable of psychogenic tears. However, this study shows that there is an increase in some tear production within the eye during a happy, emotional period."
"I think this study is an incredible confirmation of something we've always known – that eye contact fosters a powerful, emotional connection between two living things."
Alsarraf also had some criticisms of the study. For one thing, Alsarraf observes that they do not state if the test for measuring tear volume was "positioned in the eye for a specific time period. The standard duration of placement for which we have a tremendous amount of data is [one] minute."
Alsarraf also expressed an interest in seeing statistics that could "assess if the increase in tear production is statistically significant."
"An increase in tear production does not necessarily mean all dogs will have tears flowing down their faces as we humans often do when crying," Alsarraf noted.
For the questions raised by these observations to be addressed, other scientists will need to try to replicate the research in Current Biology. Even so, the paper remains a major step forward in demonstrating that dogs' tears are connected to their emotions — and that, in the case of their fortunate human companions, those emotions are associated with affection.
"I think this study is an incredible confirmation of something we've always known – that eye contact fosters a powerful, emotional connection between two living things," Joel Sartore, a National Geographic Explorer and photographer wrote to Salon. Sartore has been specifically photographing images of animals for over a decade, taking care to emphasize their eyes. "I photograph every animal on plain black and white backgrounds so that each one is the same size, and thus has an equal chance of being noticed. When people can look into their eyes, it moves people, and often engages their feelings of compassion and a desire to help," Sartore says.
Bittel, for his part, reacted to the study — and to his reflections on his experience with his canine companion, Marla the Pomeranian — by suggesting that maybe humans have co-evolved with their furry best friends.
"Maybe I let my own emotions get the best of me in that interpretation, but I also think that we've seen sled dogs adjust their microbiomes to eat what we eat and dog brains reorganize their structure to better perform the jobs we breed them," Bittel explained. "After tens of thousands of years of dogging our footsteps, it seems pretty reasonable that hormones, tear-ducts, and whatever else is associated with emotional interactions would co-evolve right along with us, too."
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