The same could be said about humans' behavior toward their domestic pets, which are to a large degree bred to retain the traits of infant wild animals.

Not sure what you mean. I didn't mean to undermine dolphins; they are highly intelligent species and definitely altruistic. The point is simply that they don't save a drowning human because they care about a random drowning mammal, but because they mistake it to a drowning dolphin because we both have lungs. Not sure that has anything to do with what you said.

Oh I just meant domestic pets appeal to the same parts of the human brain that human infants do, in part because humans have bred domestic pets to retain infantile traits into adulthood. For example, an adult dog retains most of the personality traits of a baby wolf, whereas an adult wolf does not.

This is somewhat analogous to your explanation of dolphins acting altruistically toward humans because we resemble their young when struggling in the water.

In the same way, it would be very hard for most of us to ignore a lost crying puppy wandering down the street.

No, it is not that simple. Humpback whales have been observed to interfere and mess up with orcas protecting gray whale calves from being killed. It was observed several times and researchers think that humpbacks just really cannot stand orcas harming other whales, read more here: https://www.smithsonianmag.com/science-nature/what-humpback-...

Humpback also have spindle (mirror) neurons in their brains, those are thought to be related to cooperation, empathy, etc. Humans have them too, but not many other species (https://www.sciencedaily.com/releases/2006/11/061127111607.h...).

It is good to not anthropomorphise, but we really should finally move away from Descartes' view on animals being simple automatons.