Now, researchers report evidence that in African starlings such altruistic tendencies are most common among species that live in savannas, where the rainfall in any given year is virtually impossible to predict.
"When the unpredictability of your environment is high, you don't know in advance what conditions you will be facing when the next breeding season rolls around," said Dustin Rubenstein of the University of California, Berkeley and the Cornell Laboratory of Ornithology.
"Faced with this uncertainty, it pays, evolutionarily speaking, to live and breed in social groups that will help you weather the bad times and make the most of the good times. Living in cooperative family groups may be like a form of insurance against the unpredictable nature of the environment that allows individuals to maximize their reproductive success over the course of their lifetimes."
Over the past few decades, mounting evidence has shown that many species of cooperatively breeding birds live in semi-arid tropical and sub-tropical environments, particularly in sub-Saharan Africa and Australia.
Although that pattern suggested that the environment might explain the behavior, few studies showed a strong relationship between the incidence of cooperative breeding among species and any particular environmental feature, the researchers noted.
In the new study, the researchers examined the pattern of breeding behavior exhibited by 45 species of African starlings in relation to the environments in which they live—including savannas, deserts and tropical forests—while controlling for the evolutionary relationships among them. They also examined the rainfall patterns typical of each of those environments in 47 African countries as far back as 147 years ago.
They found evidence that different starling species have independently evolved the cooperative breeding system upon moving to savannas, where they showed that the amount of year-to-year variation in rainfall is greatest.
Rubenstein's group suggested that cooperative breeding is probably advantageous in environments that vary over time because it allows for reproduction in harsh years, as well as sustained breeding during benign years.
Such a strategy might become more widespread in an increasingly uncertain future, the researchers suggested.
"Some researchers have argued that climate change will lead to greater temporal environmental variability," Rubenstein said. "That is, we might expect more extreme weather like severe droughts and devastating floods, and ultimately more variable environments than we see now.
If this increase in variability occurs, many species of animals—including humans—will have to adapt to the increasing unpredictability of their environments. By studying how animals have already adapted to temporally variable and unpredictable environments, we can gain insights into how social behavior may change in the wake of climate change."