Dogs experience a form of mourning when another dog in the household dies

Anecdotally I can agree with this. I recently had to euthanize one of my dogs who tragically suffered complications from anesthesia that caused him to waste away over a couple weeks.

This dog was only 2.5 years old and my older dog (6, both Great Danes) spent a couple months in a deep funk, not eating, sleeping all the time, always at my hip.

I'm a life long dog owner but I have never lost a young dog/puppy. It was very hard all around. Usually its the younger dogs experiencing the older ones passing and I like to think that they understand "the circle of life" in some intrinsic way.

Previous experience has shown me that older dogs tend to let us know when their time is near in some way or another. So we usually introduce a 3rd puppy around a year ahead of this time. I believe this helps survivors cope.

I wish there were some tools and techniques to learn to help my dogs cope in situations where we can't prepare and the loss is devastating and immediate.

Humans have tools for each other, what inter species tools do we have to cope with death?

We have a female stray dog on our street that had three pups[1] in one of its litters. One of the pups died one morning and we saw the mom and two siblings sitting near the dead body. While the body was disposed of that day, the family continued to visit and sit around the spot for the next couple of days.

This activity stopped after a while (don't know when). If this is not mourning, what is?

[1] The second one died when some idiot drove his vehicle over it. The third one disappeared a few months later.