Mourning in dogs

A wonderful eleven year-old Shih Tzu, owned by a very close family member, was struck recently by a car and killed. She left behind a mourning human couple and a lovely canine companion.
Her owner was reasonably, and rightfully concerned about how the living dog would fare after her companion's death. "I don't know," I replied honestly. "We'll have to see."
After more than three weeks since the accident, the surviving Shih Tzu has shown absolutely not a shred of evidence that she is missing her departed canine companion. It's paradoxically sad to report that she continues to thrive in the other's absence (obviously we don't want her to suffer, we just want or expect her to be "missing" the other dog).
Certainly, this is not the case for every dog who suffers the loss of a companion. According to a study done by the SPCA in the late nineties, nearly seventy percent of dogs manifest symptoms in cases like this: inappetence and lethargy are the most common. Are these dogs reacting to a change in their routine or are they literally depressed? I hate to anthropomorphosize but such prolonged symptoms in a person would be consistent with depression.
Wolves, our beloved pets' ancestors, mourn the loss of other wolves in many ways, such as vocalizing, refusing to eat, and futile searches for the deceased (I tried to find more on mourning in wolves but alas could not, so feel free to chime in, experts).
This post was introduced with direct, anecdotal evidence that not all dogs will mourn the loss of a companion. The status of both dogs must surely be taken into account, given that the surviving dog's status may change after the death of a companion. The consequent behavior of the owner(s) who lost the pet will also affect the behavior of the surviving dog.
I'd love to hear your personal stories.

Used with permission, taken from Beverley and Pack's photostream on Flickr.

 
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