Research suggests that dogs do indeed experience grief and mourning following the loss of a canine companion. They may exhibit various behavioral and emotional changes similar to those observed in humans during bereavement. Here are some signs that dogs might display when mourning the loss of another dog:

1. Reduced Activity: Dogs may become less energetic, playful, or interested in activities they once enjoyed.

2. Loss of Appetite: Some dogs may lose their appetite or eat less than usual.

3. Changes in Sleep Patterns: Dogs may experience disrupted sleep patterns, sleeping more or less than usual.

4. Seeking Comfort: Dogs may seek comfort from their owners or other familiar people, showing increased clinginess and attachment behavior.

5. Vocalizations: Dogs may vocalize more frequently, whining or howling, especially during the initial stages of mourning.

6. Searching Behavior: Dogs may search for their deceased companion in familiar places or areas where they used to spend time together.

7. Reduced Social Interaction: Dogs may withdraw from social interactions with other dogs or people, appearing less interested in play or interaction.

8. Increased Anxiety: Some dogs may display signs of anxiety, such as panting, pacing, or restless behavior.

9. Disinterest in Toys: Dogs might lose interest in their favorite toys or activities, showing a general lack of enthusiasm for things they previously enjoyed.

10. Grooming Changes: Dogs may neglect self-grooming, leading to a disheveled appearance.

11. Increased Sensitivity: Dogs might become more sensitive to touch, sound, or other stimuli, displaying a startled or anxious reaction.

12. Change in Body Language: Dogs may show changes in body language, appearing hunched, with their tail down, and avoiding eye contact.

13. Increased Vigilance: Some dogs may become more alert and watchful, constantly scanning their environment as if searching for their lost companion.

It's important to note that not all dogs will display all of these signs, and the intensity and duration of mourning can vary from dog to dog. If you suspect your dog is grieving, providing extra support, comfort, and their normal routine can help them through the process. It is also advisable to consult with a veterinarian or animal behaviorist for guidance on how to best support your dog during this difficult time.