Fathers with smaller testicles mean they are more involved in nurturing their children and have brains that are more responsive when seeing images of their children, according to a new study.
Evolutionary biological observations in male primates have manifested an inverse relationship between their capability to have babies, and the time they take to nurture their offsprings. Take the chimpanzees for example, they are said to be promiscuous and have testes two times the size of humans. On the other hand, gorilla males have smaller testicles but are known to protect their children. This recent study highly suggest that humans also show proof supporting these observations.
The team of researchers, led by anthropologist James Rilling, did an investigative study to find out the reason some fathers are more caring for their children than the others. They took 70 volunteer fathers who had one or two-year old children. An MRI or magnetic resonance machine was used to scan their testicles and brains. The fathers, as well as their wives, filled out survey forms to find out the commitment shown by the father’s involvement in taking care of their children.
Based from the survey results, men rated better fathers showed greater activity in the VTA (ventral tegmental area) part of their brain, which is the area involved in the reward system. Those fathers with relatively larger testicles however got lower scores in parental care and have noted low activity in their VTA. Sperm count is generally related with the size of the testicles and thus the team considered the size to indicate the mating levels of the subjects.
The team also had testosterone analysis for the same set of fathers and confirmed the findings that those active in parental care also have low levels of the said hormone.
Although they take this as a very significant advance in research, the team still considers other factors affecting levels of paternal child care. Experiences in taking care of siblings and the environment where they grew up are just some that may influence a male to be a lot more caring. The authors also say that the study suggest biological tendencies may direct a male to a particular style of parental care but nurturing fathers can also be made, not just born.
The study was published in the Sept. 10 issue the Proceedings of the National Academy of Sciences.