August 1, 2024, 3:21 pm | Read time: 4 minutes
Most dogs are happy when they see their owners after a few hours of separation and wag their tails happily. However, researchers from Japan have discovered that dogs can also cry with joy.
It has long been thought that humans are the only living beings whose emotions can trigger crying. Japanese researchers have now conducted the world’s first independent study, which has come to a different conclusion. In the study, dogs that saw their owners again showed a measurable increase in tears. The scientists attributed this increase to the “cuddle hormone” oxytocin production.
How did the researchers proceed?
A group of eight Japanese scientists led by Takefumi Kikusui from Azabu University in Sagamihara investigated the topic in several experiments. The scientists examined the overarching question of whether emotions can lead to increased tearing in dogs. They presented the results of their study in the journal “Current Biology“.1
To this end, the research team examined several groups of dogs in different situations. First, the volume of dogs’ tears was measured at home with their owners and then again after five to seven hours of the owner’s absence.
The scientists carried out the Schirmer tear test on the animals. Vets usually use this test to diagnose conjunctivitis or corneal diseases in dogs. A value of 15 millimeters per minute is considered normal.
The values were within the normal range if the dogs were at home with the owners. The dogs were then dropped off at a dog daycare center, and the value was measured again when they were picked up. The experiment showed significantly increased tear levels during the first five minutes after collection.
In another experiment, the dogs were picked up by people they knew but not by their owners. In this experiment, the effect was not observed, and the tear levels remained within the normal range.
Do dogs cry because of oxytocin?
Professor Takefumi Kikusui told the Guardian that he discovered the link between the hormone oxytocin and tears in dogs when his own dog had puppies and nursed them. The team then decided to carry out an experiment to investigate the reunion between dogs and their owners.
The researchers then also analyzed whether there was a direct link between oxytocin and the flow of tears in dogs. To this end, they tested two solutions, one with and one without oxytocin, which were dripped into the dogs’ eyes. The samples with the hormone led to a significantly greater flow of tears.
How do dogs’ tears affect humans?
The study also investigated how it affected humans to see dogs cry. To do this, photos were taken at different times and then presented to a test group. The test subjects were asked to indicate on a scale – ranging from “I want to care” to “I am afraid” – their emotional response to the images of the dogs. The results indicated that the willingness to provide care increased by as much as 15 percent when the animals were depicted with moist eyes.
The researchers suggested that dogs have developed the ability to communicate with humans through eye contact and that this behavior, coupled with crying, could potentially influence human responses to them. The owner’s impulses to protect or care for the animal were specifically mentioned.
Further research needed
According to the study, it is not yet clear whether the observed reaction can also be transferred to encounters between other dogs. It also needs to be investigated whether dogs cry and tear up as a reaction directed towards humans.
Subsequent research is needed to determine whether dogs also use emotional expression and tear production to communicate with one another. The social role of tears in canine behavior remains to be elucidated. Nevertheless, the researchers hypothesize that the canine practice of sniffing or even licking each other’s eyes upon meeting may serve as a form of social signaling.
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Sources
- 1 Murata, K., Nagasawa, M., Onaka, T., Kanemaki, N., Nakamura, S., Tsubota, K., Mogi, K. and Kikusui, T., (2022). Oxytocin and Emotional Tear in Dogs. bioRxiv.