MONDAY, Sept. 12 (HealthDay News) -- A new study suggests that testosterone levels drop after men become fathers, perhaps because they don't need to compete with other males for mates anymore and instead focus on bonding with their children.
The findings don't prove that fatherhood directly affects testosterone levels, and it's not clear how the hormonal systems in men may detect that they've become fathers in the first place.
Still, researchers found that men had "a really dramatic drop" in testosterone levels once they became fathers, said study author Lee T. Gettler, a graduate student at Northwestern University. "There's something that's going on in their first months that's helping them transition to their role as fathers."
Humans are unusual among mammals because the males actually assist with raising offspring, Gettler said. Only 5 percent of other mammals do that. In contrast, males help out in 90 percent of bird species, he said.
Gettler was inspired to launch the study in part because of his own childhood experiences. He and friends were scrawny kids while growing up in Minnesota but had strong fathers, he recalled. "We thought that when men became fathers, they got their man strength," he said.
The study, however, suggests something a bit different, at least when it comes to levels of testosterone.
The researchers examined results of blood tests of 624 young men in the Philippines who were followed over 4.5 years. About one-third of them got married or found long-term girlfriends and became fathers; those men were more likely to have higher testosterone levels before becoming fathers, perhaps because it helped them attract women.
However, once they became fathers, their testosterone levels dipped between 26 percent and 34 percent, depending on what time of day it was measured. That's roughly twice as much as testosterone levels dropped in the single men who didn't become fathers.
What do these lower testosterone levels mean for the men? It's hard to say. "I don't think it makes them less tough or less masculine," Gettler said. "It might make them more attuned to the needs of their kids and less oriented toward competing with other men outside of the family context, whether that means competing with other men for the attention of women or engaging in risky behavior."
Testosterone levels dipped even lower in men who directly helped take care of their kids. "Our assumption is that there's something about physically interacting with their kids, whether it's through sight or smell or physical touch, that activates something in the brain of men and has this trickle-down effect," Gettler said.
Richard G. Bribiescas, chair of anthropology at Yale University, said lower testosterone levels could "be a side effect of increasing bonding hormones such as oxytocin and prolactin. Lowering one's testosterone may also bolster a male's immune system and thereby decrease the possibility of passing along pathogens and infections to newborns whose immune systems are still developing."
Robert J. Quinlan, an associate professor of anthropology at Washington State University, said the study raises the question of whether testosterone levels don't drop in men who end up having poor relationships with their children. "One might manipulate the system by encouraging fathers to get the early experience with children that lowers testosterone levels, and then perhaps family stability and child outcomes would improve," he said.
The study appears in this week's online edition of the Proceedings of the National Academy of Sciences.
For more about testosterone, try the U.S. National Library of Medicine.
SOURCES: Lee T. Gettler, Ph.D., graduate student, Northwestern University, Evanston, Ill.; Robert J. Quinlan, Ph.D., associate professor, anthropology, Washington State University, Pullman, Wash.; Richard G. Bribiescas, Ph.D., professor and chair, anthropology, Yale University, New Haven, Conn.; Sept. 12-16, 2011, Proceedings of the National Academy of Sciences, online
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