BioEd Online

Most animals look for the same things in a mate year after year: the peahen wants the peacock with the most extravagant tail, the doe opts for the stag with the biggest antlers, and the blue-footed booby looks for the bluest feet.

But things are more complicated for male lark buntings. Each year the females seem to be after a different sort of mate.

Evolutionary theory says that females can drive the evolution of some sexual characteristics in males by consistently selecting one characteristic in a mate. The peacock tail is a prime example, as the biggest and brightest feathers are indicative of the healthiest and 'best' males. Now a study by Alexis Chaine and Bruce Lyon at the University of California, Santa Cruz, has revealed a situation in which this theory does not apply.

Chop and change

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 in Science that the characteristics that were selected changed from year to year¹.

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," says Chaine. "A ‘good’ male is not necessarily always a ‘good’ male if environmental conditions are changing," he says.

A man for one season

The research community has suspected for a long time that a variety of different ornaments can have a role in sexual selection. But this is the first demonstration of females adaptively changing their mate preferences each year, explains Lauryn Benedict at the University of California, Berkeley. “It’s just amazing that different ornaments are selected each year, the research has huge implications for current sexual selection theory,” she says.

Which specific conditions trigger the difference in preferences is still something of a mystery. The authors suspect that dark colouration could indicate food-finding ability (since colour can often come from eating berries and fruits), and that large size could signal an ability to defend a nest well, but further research is needed to determine any links.

What the researchers can tell for certain is that females are not working by the simple single characteristic 'peacock' rules entrenched in biology for so long. Instead, they seem to be taking the environment into consideration and selecting characteristics that indicate which males will be best for the current circumstances.

Lark buntings are probably not alone. "This is just the beginning. I think we are going to find many more species that do the same thing," says Chaine.