A new study on a group of female songbirds has shed new light on the female prerogative to change her mind when choosing mates.
Scientists studying the mating habits of lark buntings, a migratory songbird that breeds in the prairies of Colorado, found that females consistently showed strong preferences toward males with certain traits.
But rather than consistently choosing one particular trait, such as beak size, as a sign of male fitness, the females vary the traits from year to year, depending on their prevailing tastes.
For example, while beak size was the top trait for female lark buntings in 1999, body feather colour was all the rage in 2003.
Until now, researchers had generally assumed that factors driving the evolution of what females look for in partners remain unchanged over time.
But the study of the lark bunting, by Alexis Chaine and Bruce Lyon at the University of California, Santa Cruz, challenges the idea, showing that there are changing fashions in what traits females think are attractive.
Chaine said that many other species would also select their ideal mate depending on the existing trends.
During their 5-year study, Chaine and Lyon noted the plumage and size characteristics of 384 lark buntings (an average of about 80 per year) in Colorado.
They tracked how these physical characteristics related to male reproductive success by monitoring the male's offspring each year, with genetic tests to confirm paternity. They expected to find only one or two characteristics that were selected above all others, but they report that the characteristics that were selected changed from year to year.
In 1999, males with big beaks had the most offspring. In 2000 it was dark body-colouration that did the trick. In 2002, small body size was best and then the following year the trend reversed, with large body size selected instead.
"All biologists are taught that female preferences are static, but our findings really made a lot of sense once we started thinking about it," Nature quoted Chaine, as saying.
"A 'good' male is not necessarily always a 'good' male if environmental conditions are changing," he added.
However, Lyon cautioned that additional experiments are needed to provide definitive evidence of female preferences. The study was based on statistical correlations, which provide clear evidence that the traits of mated males differ from those of males without mates.
"We suspect it's because females are choosing males with particular traits. Ideally, we would like to test that with field experiments," Lyon said.
He added that the dynamic sexual selection seen in lark buntings is probably occurring in other species as well. This has implications for theoretical models of how sexual selection influences the evolution of male traits.
"The assumption that sexual selection is static is something we've all taken for granted. This study might cause some people to rethink their systems and take another look at their data," Chaine said.
The study is published in the journal Science.