Scientists have long believed that humans are the only animals that shed tears of emotions. Humans shed tears when they are sad or grieved. They also shed tears of joy. Until now, no other animal was known to express emotional tears.
Tear Volume Experiments
Eight researchers in Japan led by Kaori Murata conducted a set of experiments on 18 dogs in which they measured the tear volume in the dogs’ eyes under a range of emotional interactions between the dogs and humans the dogs knew.1 They performed the Schirmer tear test (STT), which measures the volume of teardrops with absorbent paper that’s placed against the dog’s eyeball. After 1 minute the researchers recorded how far up the paper the wetness from the tears had crept.
Among the 18 dogs, Murata’s team found a 10% average increase in tear volume when the dogs, after being separated from their owners for 24 hours, were reunited with them. When Murata and her colleagues performed the same test between the dogs and nonowners that the dogs were familiar with, they did not measure a statistically significant increase in tear volume.
In a separate experiment with 22 dogs, the researchers added the hormone oxytocin directly to the dogs’ eyes. Oxytocin, also known as the love hormone, stimulates the emotions of trust, empathy, and relationship bonding. The addition of oxytocin to the eyes caused even more tears to well up in the dogs’ eyes when they were reunited with their owners. The researchers concluded that an elevation in oxytocin generated by the happy reunion likely caused the extra tears.
Murata’s team anticipated that some scientists might presume the dogs’ eyes reacted to the added oxytocin as an irritant, which generated extra tears to flush it out from the eyes. Hence, in a control experiment, Murata and her colleagues added a substance of similar texture as oxytocin to the dogs’ eyes. In this case, they observed no extra tear production.
In a third set of experiments, 74 humans rated their impressions of 10 photographs of dogs. The human subjects consistently felt greater care and love for the dogs with the moister eyes.
When I read the research paper, I questioned the claim that dogs only shed tears of joy when they’re reunited with their owners. The researchers stated that they saw no such response when the dogs were greeted by another human that they knew. I reacted because a friend’s dog wells up with tears almost every time I greet the dog.
I think the sample sizes of dogs and humans in Murata’s team’s experiments were too small. I believe there are dogs that have just as strong, or nearly as strong, an emotional bond to a human friend of the owner as they do their owner. In the case of my friend’s dog, my friend says that I am their dog’s best friend. I get their dog’s most exuberant greeting.
Philosophical and Theological Conclusions
In my book Hidden Treasures in the Book of Job,2 I describe and explain the biblical doctrines that God created and designed the nephesh (soulish) animals to serve and please humans and to form relationships with humans. Dogs, more than any other nephesh animal, exhibit these characteristics. The research achieved by Murata’s team adds to the weight of scientific evidence for these biblical doctrines. Their research also suggests that additional experiments can be performed on human-dog interactions that could provide yet even more scientific evidence for the nephesh animal-human connection.
As I explain in my book, God designed the nephesh animals not only to help meet humans’ physical and emotional needs but also to teach humans several important spiritual lessons about themselves and about God. One such lesson affirmed by the experiments is that just as dogs express joy when they are reunited in relationship with a higher being, so, too, do humans when they are reunited in relationship with the higher Being. I think I speak for many Christians that when I sense I’m being reunited with my Creator-Savior-God to an extraordinary degree, I express tears of joy.
- Kaori Murata et al., “Increase of Tear Volume in Dogs after Reunion with Owners Is Mediated by Oxytocin,” Current Biology 32, no. 16 (August 26, 2022): PR869–R870, doi:10.1016/j.cub.2022.07.031.
- Hugh Ross, Hidden Treasures in the Book of Job: How the Oldest Book in the Bible Answers Today’s Scientific Questions (Grand Rapids, MI: Baker Books, 2011).