The world's most loyal dogs

The oft-repeated adage is the dog is man's best friend, but some pooches have proven themselves especially worthy of that reputation.

For some pets it means marking a long vigil for a fallen comrade, like a dog in Japan that stayed beside its dead friend for days on end.

In happier circumstances, a dog's loyalty can be shown in its joy at a returning playmate.

We've compiled some of the most heartwarming (and heartbreaking) videos of the world's most loyal dogs.