Massive over-simplification. Altruism is found all over the animal kingdom, and is a product of natural selection like anything else.
John Maynard Smith in particular has some excellent work on this subject.

A person doesn't need 'logical justification' nor is it a 'belief'. It would be normal behavior for most.

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I thought that was supposed to be some kind of stupid pun.

And why would you save the adult chimp?

Because the other 8 people are going for the baby, someone has to save the poor chimp.

oh i know this. but altruism for other species is rare. minus symbiotic relationships.

that is, don't expect much of the animal kingdom to come to the babies aid, even if they didn't have their own specie to choose from.

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What can be asserted without evidence can be dismissed without evidence.

Honestly speaking, and I avoid speciesism, I would save the baby. The death of the baby would cause a great deal of human suffering, which is deeper in many profound ways than that experienced by other animals. So in the interest of choosing the action that would cause the least suffering, I would save the baby.

Yes, that is an extremely important point to note.

There are lots of examples of outer-species altruism. Dogs, for example, have been known to put themselves at risk to save humans quite often.

Darwin himself has many such stories in "Descent of Man", including several examples of chimps defending humans from other (even larger) chimps.

You can say that it's not common, but that's by no means certain, and the reality is that an animal is more likely to defend a member of another species that it has an established friendly relationship with than a member of it's own species that is hostile.

So a strong case can (and has) been made that species plays less of a part in altruistic behavior than does familiarity and friendship between the two animals.

We see this with human beings also. Humans will defend their pets against strangers of their own species who attempt to harm the animal.