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Fall 2001

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Good breeding?
Scientist studies hows and whys of mating choices

     On a small Canadian island lives a population where extramarital sex and illegitimate offspring are not just common, but more prevalent than fidelity and “family values.” It’s a population of Savannah sparrows, and there’s nothing extraordinary about their behavior: many species of songbirds, while socially monogamous, are known to mate more or less promiscuously, with females often laying clutches of eggs of mixed paternity. Scientists assume that such behavior must be adaptive—must help the birds compete and survive—but how exactly? The National Science Foundation is paying a Skidmore professor to investigate.

Biologist Corey Freeman-Gallant, at right, frees a netted bird during an outing with high-school students enrolled in a summer program at Skidmore.

     Biologist Corey Freeman-Gallant has studied the sparrows on Kent Island, New Brunswick, for five years; this three-year, $650,000 grant will take his research several steps further. While Freeman-Gallant specializes in DNA “fingerprinting” to study mates and offspring, his co-investigator in the NSF project, Bowdoin College’s Nathaniel Wheelwright, is a fieldwork specialist and director of the Bowdoin Scientific Station on Kent Island. In close collaboration with students from both colleges, they’ll monitor nests and measure, leg-band, and draw blood from large numbers of Kent Island sparrows in the summers and then process and compare DNA data in the Skidmore lab in the winters. “The students will gain a rich perspective on the power—and limitations— of both field and lab methodologies,” says Freeman-Gallant.

     At the core of the research is how Savannah sparrows pair up and mate. Freeman-Gallant has shown that females alter their fidelity according to how well their social mates—their husbands, so to speak—perform as care-giving parents to the first batch of offspring; “negligent dads,” he says, “lose paternity in subsequent clutches.” But how females choose their extramarital partners is in question: they may prefer males whose size, plumage, or song indicates superior health and fitness, or they may choose males who are simply genetically different from themselves. While the more general “good genes” theory seems logical, there’s also evidence that females can perceive genetic relatedness and will avoid inbreeding with males who are too closely related to them.

     Such discrimination has been observed in at least two other species: lab mice and college students. In studies with college students, subliminal cues from body odors prompted stronger attractions for potential mates who were more distantly related; and female mice paired with genetically similar males sought to mate instead with less-related males. In the mouse research, females gravitated to those males in whom one particular genetic marker, a component of the immune system called Mhc, was significantly different from their own. Freeman-Gallant’s project includes analyzing Mhc from the Kent Island sparrows to determine whether females prefer mates with high degrees of Mhc unrelatedness.

     “Eventually,” Freeman-Gallant says, “we’d like to determine whether the ‘illegitimate’ offspring do in fact have stronger immune systems or better survival rates—as a study on European bluethroats has shown. But for now, our focus is on detailed tracking of mating choices.”

     Along with special funding for student research and travel, the NSF grant underwrites a full-time Skidmore lab technician and new DNA sequencing equipment, which “promises to be transformative, giving students experience with important technology and allowing faculty to be more creative in designing new research and courses,” according to Freeman-Gallant.

     Genetic analyses are already under way this semester, and Bowdoin-Skidmore research teams are preparing for an in-depth investigation into the intimate affairs of Kent Island’s Savannah sparrows next summer. —SR

 


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