Though men do seem to be getting better at playing stay-at-home dad, they still can’t claim to be the best fathers in the animal kingdom. Among primates, that honor may belong to marmosets, small tree-dwelling monkeys whose males spend 70 percent of their time caring for newborns. The result of all this baby time, according to new research, adds up to more than just a sensitive monkey. The nurturing actually boosts mental activity.