Nothing is more profound than grief that can follow the loss of a family member and a pet companion.  But did it come about as an evolutionary function?  A British psychiatrist and an evolutionary psychologist believed that grief did not originally evolve as an emotion response to the death of a loved one, but to motivate a searching response for living companions who are gone, perhaps from a relationship breakup or forced separation.  This is referred to as the “reunion theory.”  It is believed that grief motivates searching behaviors that could help locate and reunite with a lost partner.  Therefore, the grief we experience when a human loved one dies is an evolutionary side-effect of the search-promotion function, according to the reunion theory.
An innovative way to test the reunion theory was developed as a comprehensive account of grief that included elements of the theory.  It was reasoned that if grief evolved as a means to motivate attempts to locate missing loved ones, the unconscious guest to find lost partners would be hyper-vigilant, leading too mistaking unrelated sights and sounds for missing companions.  These perceptual errors are referred to as “false recognitions.”  They are not hallucinations but misperceptions of real external stimuli such as the resemblance of a missing loved one in a complete stranger.
The following are predictions about false recognitions of grieving individuals based on the reunion theory:

• Bereaved people who are more attached to their lost companions will be more likely to experience false recognitions
• Bereaved people who frequently look at photos of lost companions will be hyper-vigilant
• Bereaved people will experience less frequent false recognitions as time since the loss passes

The study examined false recognitions in pet owners whose dogs or cats have died within the previous two years and the subjects had to have owned the pet for at least six years.  The final study included 107 bereaved dog owners and 57 cat owners.
The online survey the participants took included a standardized 23-item index of how much they loved their pets, a measure of how often they looked at photos of their dead pet, and the False Recognition Questionnaire containing six items related to the frequency that the subjects false recognition experiences in the last two weeks (hearing or seeing their dead pet).  The following are the results of the vast majority of the bereaved pet owners that had false recognition experiences within the previous 14 days:

• 84% believed they had seen or heard their dead pet
• 93% found pet related habits hard to break (feeding the dead pet; call its name)
• 70% had mistaken a sound for their dead pet
• 77% had mistaken another animal for their dead pet
• 32% had felt the urge to look for their dead pet
• 21% sometimes felt an image of the pet so strong, they felt the animal was still alive

As always, there are limitations to this research.  The subjects were self-selected on the basis of grief for a deceased pet, and it is unclear how often false recognitions occur among typical pet owners.  However, the study did provide limited support for the reunion theory of grief.
This study does exemplify our relationships with animals and insights into the aspects of human psychology but the reunion theory of the evolution of grief still sounds a little far-fetched to me.