system
Given the following post, respond as if you're commenting on Reddit:
human
This is a silly question, I know, but... what causes animals to realize that an animal of another species is only a baby and in need of help?

Most recently I began wondering it after seeing a video here on reddit where a crow was apparently caring for (feeding, protecting, etc.) an abandoned kitten.  I wondered how did the crow know that A) that the cat is a baby, not just a small cat, B) that the cat was not a danger to it, and C) that the cat was unable to care for itself and needed help?  I see how a crow would recognize an abandoned baby of its own species, but the cat is so different from it, I was pretty surprised.  It happens a lot - cat adopts squirrel, tortoise adopts hippo, dog adopts deer, etc.

Maybe I'm just not giving animals enough credit and it's merely a case of observation.  They realize it just as we recognize babies of other species.  But I wondered if there was something specific that triggers the "I must care for this baby" thing.  Is it just a matter of mother-instinct not making distinctions between species?  Are there any cases of male animals doing this?
gpt
First there's the question of how animals detect  and measure "cuteness". As Fuco1337 said, it's mostly big head and sad eyes and many other clues.

Then there's a more interesting question: why some animals recognize "cute"?

Not all animals react to cuteness. Fish, Amphibians and most reptiles will  when confronted with a cute thing, ignore it or eat it. This is mostly common with mammals (althought there's a video on youtube of a chicken nurturing a cat). 

Mammals are different from other animals in that they generally care a lot more for their young – it's an evolutionary strategy, you either do like the fish and crap as many little fishes you can, or you have only a few per litter but invest a lot of time making sure they reach adulthood.

This reflects in many aspects of mammalian life: longer gestation, often a very dependent baby, breastfeeding – and detection of "cuteness". This is of course not a universal characteristic, but a evolved one. For example suppose this scenario: babies have big heads, so mammal mothers will learn to take more care of their young relatively to the size of their heads, therefore caring less when they become adults. Babies, with bigger heads have more chance of having more attention so big-head-baby genes will spread and more babies will be born with bigger head. So whatever characteristic used to define "babiness" tends to be exageratted overtime, because it's good for both the mother and the son. There, that's how cuteness evolves.

Now, why would an animal take care of another one that's not it's child? It's a question many have asked and googling for altruistic gene will get you a good lecture, but in a nutshell it's like this: if you have a gene that makes you take care of cute, specially if it that cute something reminds you of you, then it's probable that those genes will spread. Similarly the "guilt" gene could be something that errs on the side of caution – it's better to take care of a baby that is probably unrelated to you than to risk letting one die.

I would imagine that taking care of other animals is mostly a bug in the system. If most mammals err on the side of caution, then it's probable that their instinct will be to take care of anything that seems anything like a cute baby. Note that the stories you hear are more often than not a mammal adopting another mammal, and that humans will prefer an animal that seems to mimic human expressions.

Then of course this bug can be exploited by other animals. Cats and dogs started their relationship with humans as a cooperative one – humans give dog protection and dogs help with the hunting etc. But over the last centuries both animals have evolved into look more and more baby-like, even if that means the total loss of the hability to find food by themselves.

So, you probably have read somewhere about a fungus that evolved to control a cockroach brain. Cute puppies have evolved into controlling a bug in our human brains.