"For me, the take-home message is, there is this tremendous potential for nurture in human males, albeit a potential that in different cultural and social situations is not always being tapped," Hrdy told LiveScience.
But the study also raises a number of questions, added Hrdy, who is professor emerita at the University of California, Davis; associate in the Peabody Museum of Archaeology and Ethnology at Harvard; and A.D. White professor-at-large at Cornell University.
For instance, no one knows how testicular volume changes over time (although testes do tend to shrink with age).
In addition, scientists aren't sure whether men who make more sperm are genetically wired to be detached dads, or whether early life experience or the act of caring for children leads men's bodies to invest less in sperm-making, thereby causing their testicles to shrink, she said.
Another possibility is that the trend reveals a trade-off that varies with men's mating strategy, said Robert Martin, a biological anthropologist and author of "How We Do It: The Evolution and Future of Human Reproduction," (Basic Books, 2013).