Let's start here, with a "are they comfortable/happy"-- "uncomfortable/unhappy" Standard

The photo is captioned in 1937: "An Irish Terrier has volunteered to act as foster mother to a litter of Indian wild dogs cubs (dheles) at the London Zoo. The experiment is being made to see if the terriers domesticity will have its affect on the wild pups."

I may be wrong, but I think they are "pups" and not "cubs" That said, looking at this dog's wagging tail, bright alert eyes, upright ears and and smiling face "volunteering" to be the foster mother with out being restrained by a cable, rope, or chain, I can only assume her "complacent/resigned" behavior might have more of an effect then her "domesticity." LOL This look is similar to what you "do not" want to see in a "happy performing, I like what I am doing animal act." When watching an animal act, eyes closed, animals laying down or staring blankly ahead, or afraid to take a deep breath while performing, is the same resigned/ subjugated behavior you do not want to see, as in the Irish Terrier "volunteering." Regardless of how many pieces of meat or "kiss 0 deaths" you may witness. We have seen enough of this "happy" behavior in the old days of animal keeping when they were captured and put in barren cages, surrounded by people, instead of being acclimated to their new life to know the difference if it is explained, and not just answering for the animals.