https://langint.pri.kyoto-u.ac.jp/

An interesting paper about altruism in monkeys: Masserman (1964)

https://greatergood.berkeley.edu/article/item/the_evolution_of_empathy

Rationing ravens and merciful monkeys: can animals be altruistic?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3295314/pdf/pnas.201108517.pdf/?tool=EBI

Click to access pnas.201108517SI.pdf