An international team of animal behaviorists surveyed 426 dog owners in Italy, all of whom owned at least two dogs -- one of which died while the other was still alive.
Nearly 90% reported a change in the surviving canine's behavior, according to the research, published last week in the journal Scientific Reports.
Lead author Stefania Uccheddu, a veterinarian at San Marco Veterinary Clinic in Veggiano, Italy, told CNET the most common behaviors reported were increased attention-seeking and less interest in food and play. In some cases, the change lasted a few months. In others, it continued for years.
Asked whether the owners were perhaps projecting their own grief onto their pets, Uccheddu said her team included psychologists who worked to separate the owner's feelings from the change in their dog's behavior.
The main factor influencing whether a surviving pet showed grief, according to Uccheddu, was "the quality of the bond between the two dogs."
"We observed the change whether the relationship was parent and child or just a close connection," she said.
The pets' human companion did have an influence on their response in one regard, Uccheddu said: If their owner exhibited more anger, depression or trauma, the surviving dog acted more fearful. "Dogs are social animals," she said. "Any time they don't have a member of the pack anymore, their entire routine changes."
To help them process the loss, dog owners "need to ensure there's predictability in their days and continue to share activities with them."
Because their study relied on self-reported data, Uccheddu and her colleagues hope to add new research that can address practical and ethical concerns. Mourning responses have been reported in a variety of animals, including whales, elephants, birds and other animals. Primates, for example, have been known to carry their deceased young for weeks.
"If seeing the corpse is part of the death ritual," the authors write, "considering that domestic dogs have no access or only a brief access to the corpse, then only the reaction to separation from the bonded individual might be evaluated."