In scientific terms, “Tear volume increased significantly during reunion with the owner” according to a conclusion of a study carried out in Japan and published in the journal Current Biology. Yes, dogs have been found to cry tears of joy but their eyes well up. They don’t cry in the conventional sense and if they do, it could be a sign of eye irritation, a scratch or an infection.
The scientists decided that the welling up of the eyes due to an excess amount of tear fluid is “mediated by oxytocin”. What they mean is that oxytocin was behind the production of tear fluid. Oxytocin is a hormone that is produced in the hypothalamus and released into the bloodstream by the pituitary gland and it is called the love drug or love hormone. For example, when people kiss their pituitary glands release oxytocin.
The study concluded that:
“This is the first report demonstrating that positive emotion stimulates tear secretion in a non-human animal”.
Interestingly, it is a mutual arrangement with their owner because when the human participants were shown photographs of dogs with and without artificial tears added to their eyes the photographs with artificial tears “were ranked significantly higher than the normal, tearless dog photographs”. The ranking referred to was a measure of the desire to care for the dog.
Although it seems that the dog tears were tears of joy, they also benefited the relationship by improving the bond. This is one way dogs bond with their owners. They have the ability to make eye contact and have eyebrow muscles that can make their eyes look larger and more childlike. This can help to trigger “nurturing behaviour in humans” according to the researchers.
The researchers tested for the amount of tear fluid produced by using what’s called the Schirmer tear test. It is normally used to test whether a dog is producing sufficient tear fluid as a health check. Twelve female and eight male dogs participated at home with their owners. The breeds included miniature Dachshunds, Labradors and Poodles.
The dogs were taken from their owners and placed in a daycare centre for between 5-7 hours and then reunited with their owners. They had been previously checked for the amount of tear production and a further test was carried out a minute after the reunion. As mentioned, there was a significant increase. When the dogs were introduced to “familiar non-owners” there was no such increase in tear production.
The Schirmer test incorporates the use of absorbent paper applied to the edge of a dog’s eye. They measure how many millimetres of the paper become soaked in tear fluid. They found that in some dogs the measurement expanded from 13 mm to almost 20 mm.
The researchers did not test whether the same effect was experienced when dogs were reunited with other dogs or when they experienced negative emotions. On a personal level, I would welcome a similar study for cats! Cats get a raw deal when it comes to research such as the one described because they are harder to manage.
We need to know whether cats well up when they are reunited with their owners after a time apart. They are certainly extremely pleased to see their human caregiver after, for example, staying at a boarding cattery for a couple of weeks while their owner is on holiday. They will vocalise their pleasure and their body language clearly displays pleasure and relief. But do their eyes well up?
Dogs’ facial fast twitch muscles provides them with expressions which entice us
