A study of gorillas has shown that male gorillas who put energy into caring for infants - even infants that don't belong to them - earn a very valuable reward: lots and lots of sex. Male gorillas engaging in caretaking activity have five times as many children of their own as other gorillas. This contradicts some more basic evolutionary assumptions - that males should not waste energy caring for children when they could instead use that energy spreading their seed. In this case, the contradiction is solved by the fact that being willing to give care apparently makes the males much more likely to reproduce, either thanks to attractiveness or simply through more access to females. It could shed light on the evolution of fathership and the family unit in humans.
http://www.sciencemag.org/news/2018/10/male-gorillas-who-babysit-have-five-times-more-babies
http://www.sciencemag.org/news/2018/10/male-gorillas-who-babysit-have-five-times-more-babies