ITHACA, N.Y. -- Without enough estrogen-like hormone in their systems, female plainfin midshipman fish turn a deaf ear to the alluring love songs of the males. And, according to Cornell University biologists, a similar steroid-sensitive response could underlie changes in the hearing sensitivity of humans. The biologists experimentally boosted levels of the steroid hormone in non-reproductive females of the fish species Porichthys notatus, temporarily altering their inner-ear auditory mechanism so they could hear the males' hum-like advertisement call that says, essentially: "I have prepared a nest under a rock in shallow water, so deposit your eggs for me to fertilize, and together we'll make beautiful small fry."
Altering steroid hormone levels did not change the reproductive status of the females; the eggs they carried were still immature and were not ready to be deposited, even if the hearing-enhanced females had been attracted to the males' nests. Rather, the Cornell biologists found a hormonal trigger for a complex auditory system.
Andrew H. Bass, the Cornell professor of neurobiology and behavior who led the research team that reported the findings in the July 16, 2004, issue of Science ("Steroid-dependent Auditory Plasticity Leads to Adaptive Coupling of Sender and Receiver") comments: "We suspected that enhancing the sensitivity of the females' ears to the upper harmonics of the males' hums should improve detection of their vocalizations. Upper harmonics propagate farther in shallow water environments like those where midshipman males build nests and sing their love song to attract females.
"But the females can't process this vital information and respond appropriately if they can't hear it. Steroid hormones appear to provide a key molecule that leads to shifts in the hearing sensitivity of females," Bass said.
Paul M. Forlano, a graduate student in Bass's lab, and David L. Deitcher, one of Bass's
colleagues at Cornell, also identified the estrogen receptor in the female inner ear where the enhancement of the males' cues begins. As Forlano notes: "This was especially important since estrogen receptors are also present in the human inner ear but no one knows why they might be there. Our study now suggests a possible function."
The research, he notes, is of particular relevance to women with Turner's syndrome, a genetic aberration that results in loss of estrogen production in the ovary, that also shows an early onset in progressive high-frequency hearing loss.
Joseph A. Sisneros, a postdoctoral associate in Bass's lab at the time of the study and now a faculty member at the University of Washington, further explains: "Previous experiments in other laboratories also suggested that steroid hormones may play a role in causing some of the reported changes in hearing sensitivity of human females at differing stages of the menstrual cycle. Knowing how steroids can alter the hearing abilities of midshipman fish might give us important clues as to how these changes in hearing occur among humans."
Midshipman fish have long enjoyed a certain amount of local fame on the West Coast. During their seasonal mating time, when males move toward the shore from deep water and prepare to advertise their love nests, the humming chorus becomes so loud and persistent that houseboat residents often lie awake at night.
A long-time researcher on midshipman fish, Cornell's Bass notes: "Perhaps the remarkable hum of these fish will take on new meaning for the houseboaters when they discover that they may share some things in common with these vocal Lotharios of the sea. And we hope that others who study the human auditory system will be inspired by these experiments."
The fish study, which was supported, in part, by the National Institutes of Health, shows how steroid hormones can help shift the sensitivity of the auditory system to pay more attention to some social stimuli than to others, says Bass. "This system of hormone-based prioritization helps the brain focus on the most important stimuli, at any given time, in an ever-changing environment. Perhaps something similar is at work in humans."
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