It's necessary to distinguish between kin and non-kin altruism in these cases. Kin altruism follows easily from the perspective of gene replication. Genes that

It's necessary to distinguish between kin and non-kin altruism in these cases. Kin altruism follows easily from the perspective of gene replication. Genes that lead parents to ignore infants that require care tend not to get replicated; genes that encourage nurturing helpless infants tend to survive on to subsequent generations. Non-kin altruism is much rarer, but not impossible. There's a lot of cool math and game theory revolving around reciprocality rates and cost-benefit analyses in demonstrating rates of non-kin altruism in non-human species. (E.g., vampire bat blood-sharing, prey warning calls.) Of course, humans are the major outlier, since we will frequently hold a door open for a complete stranger with no realistic expectation of reciprocation. Lastly, some cases of cross-species altruism represent, not real altruism, but instinctive nurturing and bonding activities that have been "hijacked" by circumstances. E.g., a lioness whose cubs died will allow piglets to feed, which will placate motherly instincts.