(Inside Science) -- Zebra finches, often studied for the male's deft ability to learn song, are social creatures. Their large colonies feature nuclear families with parents who often bond for life and young birds who get excited to see their parents. Now, scientists have discovered that key hormones influence at an early age how those bonds form. The research is the closest biochemical look at how young brains become affectionate in this iconic species.
Researchers have gleaned much of what we know about the role of these hormones, called nonapeptides, from studying mammals. In the monogamous prairie vole, for example, increasing the number of brain receptors of a hormone called oxytocin makes females more likely to choose a lifelong mate. In humans, oxytocin rises in blood plasma during orgasm, and it surges during and after childbirth. Scientists say the timing of these spikes could encourage us to bond.
But our social inclinations -- to be monogamous parents who both care for their young -- are rare in the mammalian world, said Nicole M. Baran, lead author of the new study, which appears in the journal Hormones and Behavior. While humans and prairie voles are evolutionary oddballs, she said, up to 90 percent of bird species are monogamous, pairing off for a period that could be a season or life, depending on the species. Experts also estimate that parents in more than four-fifths of bird species help each other raise their brood. To Baran, these strengths make birds a rich, largely untapped resource to better understand how a vertebrate brain forms strong relationships with others. The research could lend insight into how our own brains work, she said.
So Baran and her colleagues turned to the zebra finch, a songbird common in avian research labs. What would happen to the bird’s emerging social behavior, they asked, if they briefly tweaked its nonapeptide hormone signaling?
For seven days, the team either flooded a baby bird’s brain with one of these hormones, called vasotocin, or jammed nonapeptide signaling by blocking the brain’s receptors with a chemical called Manning compound. They then tested each bird’s behavior for about three months as it grew from infancy to young adulthood, just the beginning of its typical 5-year lifespan.
The team wanted to capture "that period of puberty when kids need to lose the attachment to their parents and start being interested in peers and opposite sex individuals," said Baran, who completed the research while at Cornell University in Ithaca, New York. She is now a researcher at the Georgia Institute of Technology in Atlanta.
Both male and female chicks with blocked hormones didn’t seem interested in their dads, the primary caregivers. The birds normally learn to save most of their "awkward little squawks" for their parents, Baran said. But hormone-blocked chicks were constantly vocal, suggesting they might have had trouble recognizing their father -- or that they didn’t care when dad was there.
The hormone-deprived birds displayed different bonding behavior as they grew up, depending on their sex. To gauge finch interest in a potential mate, the researchers measured how much time near the opposite sex the birds spent. In males, whose attentions usually spike around day 51, interest never piqued. But females took healthy notice of males, even without the influence of nonapeptide hormones.
Life also unfolded unusually for the chicks inundated with vasotocin. They saved more of their peeps until dad arrived, suggesting they were extra-sensitive to his presence. And as they matured, they showed "a prolonged attachment" to their parents, Baran said; both male and female birds had trouble fledging the emotional nest.
The results suggest that the nonapeptide hormones are crucial in shaping a bird’s social brain early in development, Baran said. Those effects linger into adulthood, even though the team manipulated hormone levels for just one week.
The study "extends the findings of mammals into birds to suggest that this [class of hormones] is a very widely used mechanism," said Larry Young, a social neurologist at Emory University in Atlanta who wasn’t involved in the study.
Young wonders which kinds of early experiences in a baby bird’s life actually rewire its brain, affecting its social personality later on. His own research in prairie voles, for instance, has shown that a mother’s licking and grooming activates her pups’ oxytocin-producing brain cells. As adults, voles with more oxytocin signaling spend more time huddling with their partners.
Baran is "very curious" about whether and how zebra finch parents might prime the brains of their offspring to bond.