When climate is shifty and unpredictable, birds are more likely to sleep around.
The findings, which suggest that birds may seek out diverse genes for their offspring when they are unsure what the future will hold, might help predict what will happen as climate changes in the coming decades. If weather conditions become more variable in certain places, as some models predict, birds might adapt by becoming more unfaithful. And that’s not necessarily a bad thing.
“The overall message of the paper is that there is a lot of hope because females can still employ all of these mechanisms they use to find the best partner available,” said Carlos Botero, an evolutionary ecologist at North Carolina State University in Raleigh.
The research could also offer insight into why people sometimes stray from their mates.
“Humans have been able to transform the environment to such a level that basic processes like rainfall and temperature affect us very little,” said Botero. But, he said, changes in the stock market or other economic indicators could serve as the human equivalent of a variable environment.
“You might think this is the guy of your dreams based on the world you think is occurring,” Botero said. “But if the world changes, your idea might change, too.”
Many species of birds practice social monogamy, which means that they pair up and stay together for at least one breeding season, and many come back together year after year. When chicks are born, monogamous birds work together as a couple to take care of the babies until they leave the nest.
But studies have also shown that certain birds are prone to infidelity. When scientists do genetic tests on chicks, they find that many are actually sired by birds other than the ones who act like their fathers. Birds also get divorced, according to painstaking studies that tag and follow each bird in a colony from year to year. In cases of divorce, birds mate successfully one year but choose other partners the next.
To figure out what drives birds to cheat and stray, Botero and colleague Dustin Rubenstein collected more than 400 studies of infidelity and divorce on more than 200 species of birds from around the world. They put all of the information into one big data set, and they started looking for patterns.
When climate becomes variable and unpredictable, the scientists report today in the journal PLoS ONE, birds are more likely to seek out new partners.
The strategy makes sense, Botero explained, offering this example: Birds with big heavy bills are good at cracking thick, hard seeds, which is a very useful trait in a place that is usually very dry. But those bills become unwieldy when heavy rainfall leads to more tiny seeds, which birds with smaller beaks are better at manipulating.
If the weather is consistently wet or dry, a female’s ideal strategy is obvious. If the weather starts changing all the time, on the other hand, cheating will improve a female’s chances of mixing with the best genes for whatever the environment might offer when her chicks are born. And it goes both ways — males might stray for similar reasons.
The findings point out how subtle the effects of climate can be on animal behavior, said Mike Webster, an ornithologist at Cornell University in Ithaca, N.Y. Instead of considering shifts in rainfall, melting of glaciers or other one-way changes in climatic conditions, as many studies do, the new work points out the importance of looking at the more complicated phenomenon of how changeable climate is.
“A really important next step will be to look at whether birds are responding directly to weather variation or whether this is more of a long-term effect so that over time, they evolve to have certain behaviors in variable versus less variable climates,” Webster said. “That will tell us a lot about how they will respond to climate change in the short-term versus the long term.”
It’s not yet clear, in other words, how quickly birds will be able to respond to changes in climate variability.
“One thing we know is that current climate change is happening at a rate that we haven’t been able to document in recent times,” said Rebecca Safran, an evolutionary biologist at the University of Colorado, Boulder. “Selection can only respond generation by generation. I would not cast this as good news.”