Fathers with smaller testes are more involved in child care, and their brains are also more responsive when looking at photos of their own children, according to research published online today in the Proceedings of the National Academy of Sciences. Evolutionary biologists have long observed a trade-off in male primates between mating efforts to produce more offspring and the time males spend caring for their progeny. For instance, male chimpanzees, which are especially promiscuous, sport testes that are twice as big as those of humans, make a lot of sperm and generally do not provide paternal care. By contrast, male gorillas have relatively small testes and protect their young.Note the equation, made by all the headline writers, between being "involved in child care" and being "a good father." Not everybody sees things this way.
I am also skeptical of the evolutionary logic here. What I have read before suggests that testicle size is more related to female promiscuity than male. In a chimpanzee troop a female might mate with several males in one day, which gives an advantage to males who produce a lot of sperm, whereas gorillas kill any other adult male who gets near their harems and so don't face the same sort of post-coital competition. But this last bit is interesting:
The latest study suggests that humans, whose paternal care varies widely, show evidence of both approaches.Other animals are more limited in the different sorts of life strategies they can pursue, but we humans have built societies around many, many different ways of living. Including numerous societies in which a man who spent a lot of time caring for babies was considered a complete failure as a father.