There was a time when both dogs played together, and herded people and other dogs as a team. They haven't done this for a few years. It’s hard to punish the alpha for doing what is instinctual. Also, unlike the average chihuahua, a collie could survive as part of a dog pack in the wild. The old collie and the previous alpha had stood up to and fought off a pit bull, both loyally defending me. Now I must defend the old collie from the current alpha.

With dogs and cats, there are no real guidelines about when it is permissible to put down a loyal friend. As noted in the PA article, unlike human beings,

Animals don't make diagnosis easy. Their instincts remain rooted in the wild, where survival requires covering up weaknesses and infirmities. But keepers who spend years watching these animals sense when something's wrong.

Like responsible pet owners, zoos feel obliged to take care of animals that have grown too old to be exhibited, yet for both the challenge is to decide when the quality of life has become so poor that one should euthanize.

Even in old animals that appear healthy, examination after death often finds they "suffer from a range of health problems that may not have been apparent when they were alive," a group of mostly Swiss veterinarians wrote in an article published last year in the journal Animal Welfare.

"Zoos often unwittingly condemn their animals to long painful lives," wrote the authors, calling on zoos to use a scoring system to evaluate geriatric animals' quality of life in order to make more informed decisions about euthanasia.

My rabbits died in their sleep, one after living to the age of 13. Of my dogs, two of them stopped eating. Both turned out to be suffering from cancer and must have been in pain for at least a year, maybe more, before their deaths - one dying on the operating table, the other euthanized because she was inoperable.

My current old dog has bad teeth. Before he came to me, his tail was badly broken and the bones fused, causing him pain that during his youth he easily bore without complaint. Yet with the onset of arthritis, this old injury has been a major problem. He falls. On bad days, he might fall twice or three times going around the block, and stumble going up or down the curb.