Among the canid family, there were some prior indications: some wild wolves have been reported burying the carcasses of two-week-old pups, and a dingo mother had been observed transporting its deceased pup to different locations in the days following its death.
But the evidence was overall sparse, and, when it came to domestic dogs, confined to anecdotal reports from owners, which run the risk of anthropomorphism and over-stating the case.
The new study, published in the Nature journal Scientific Reports, involved a survey completed by 426 Italian adults who owned at least two dogs, one of whom had died while the other was alive.
Negative changes were reported by 86% of owners, with a quarter saying these lasted longer than six months.
These behaviours included more attention-seeking (67%), reduced playfulness (57%), and reduced overall activity (46%).
Surviving dogs also slept more, became more fearful, ate less, and whined or barked more.
The researchers found that the length of time the two dogs had lived together was not an important factor in determining grief – rather it was the quality of the relationship the pair had shared that mattered.
How much the owner felt the loss also played a significant role, suggesting that the surviving dog was also responding to the human’s emotional cues.