that boosts competitive mating behaviors and traits falls when responsibilities for offspring begin, as the human body's evolved, advantageous hormone adjusting process activates to eliminate conflict between mating and rearing activities, an effect long observed in other species in which males share care for dependent offspring, Northwestern News Center reported
about the research team's findings that were published September 12 in the early online edition of the Proceedings of the National Academy of Sciences
Co-author anthropologist Christopher W. Kuzawa explained
, "Humans are unusual among mammals in that our offspring are dependent upon older individuals for feeding and protection for more than a decade. Raising human offspring is such an effort that it is cooperative by necessity, and our study shows that human fathers are biologically wired to help with the job."
To clarify the results of past studies that showed fathers tend to have lower testosterone levels, but did not indicate whether men who became fathers already had lower testosterone or whether becoming a father lowered their testosterone, the team followed 624 males, aged 21.5 to 26 years old living in the Philippines who were not fathers for four and a half years, and measured changes in their hormone levels after they fathered children.
The findings suggested fathers can experience a large, temporary drop in testosterone when they bring a newborn home, as their bodies' adjust hormone levels to help meet physical, psychological and emotional demands of helping care for a baby.
Because sustained high testosterone levels have been associated with increased risk for certain chronic diseases, according to the authors, they also speculated their findings could partially explain the poorer overall health single men often exhibit, compared to married men, who may be protected as they age by temporary testosterone reductions caused by intervals of fathering and parenting.
In related news: Digital Journal reported in March on testosterone's influence on mating behaviors
, and ScienceDaily reported in February 2010 on a study at Utrecht and Cambridge Universities that demonstrated administering extra testosterone diminished test subjects' capacity for empathy
Also according to ScienceDaily, mixed results have emerged from recent research into the relationship between testosterone levels and ongoing health: a multi-center U.S. study suggested high testosterone could raise elderly men's risk of heart disease
, but two other separate studies linked low testosterone with Alzheimer's disease
and increase risk of early death