Banerjee and Adkins-Regan then conducted pair-bond tests by moving groups of the birds into aviaries with biased sex ratios. In experiments where they wanted to test the partner preferences of males, they used aviaries that had twice as many males as females; the opposite was true for experiments looking at the preferences of females. The skewed sex ratios ensured that the birds had to compete — through songs and aggression — for their partners.
Overall, mother-deprived males overwhelmingly chose to pair with other mother-deprived males. Control males preferred opposite-sex partners, as did the females from both groups.
"Apart from the courtship and pairing behaviors, [the mother-deprived males] didn't seem to have any behavioral differences that were obvious," Banerjee said.
In 2000, Adkins-Regan studied the partner preferences of zebra finches that were raised without fathers, and found that both males and females had a greater preference for same-sex partners. Specifically, 38 percent of males and 25 percent of females that were raised without their fathers paired with same-sex partners, compared with 13 percent of males and 0 percent of females that were raised by both parents.
Without their dads, the male offspring didn't learn normal songs and were likely less attractive to females, so they paired with other males. The female offspring, on the other hand, may have imprinted on their mothers during development, Adkins-Regan reasoned.
Banerjee suspects the way males and females choose mates may explain why the same-sex partner preferences appear stronger for mother-deprived males than for father-deprived females. "It could be that females are less dependent on visual cues than males are," Banerjee said, meaning that even after imprinting on their mothers, females would still seek out mates that have the best songs.
Whatever the case, if sexual imprinting is behind the same-sex preferences in zebra finches, brain-imaging studies are necessary to really flesh out the link, Banerjee said.
Importantly, studies have also suggested that humans are susceptible to sexual imprinting. For instance, men often date women who resemble their mothers, and women date men who resemble their fathers, according to research published in 2009 in the journal Proceedings of the Royal Society B.
But the results of studies on sexual imprinting in animals, including the current one, don't have much to say on human sexuality, Banerjee said. "Human mate choice is so much more complex."
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