An ornithologist at the University of Rhode Island who studies the physiological changes that birds undergo to migrate has found that the capacity of a bird's gut to change with environmental conditions is a primary limiting factor in their ability to adapt to the rapidly changing climate. And he believes that most other animals are also limited in a similar way.
Scott McWilliams, URI professor of natural resources science, says that spare capacity -- the extent to which animals can modify their physiology to deal with ecological changes -- varies from species to species, with some having great capacity to change while others do not.
"It's all about the time scale over which evolution occurs in relation to the timing of the changes now occurring in the environment, because there are likely to be mismatches," he said. "Our rapid climate change is happening too quickly for most animals to evolve a response."
His research, funded by the National Science Foundation, was published last week in The Proceedings of the Royal Society.
McWilliams and a colleague at the University of Wisconsin measured the spare capacity of white-throated sparrows, a common migratory songbird in eastern North America. He found that birds acclimated to a very cold environment (-29C) were able to eat 2 to 4 times as much food as sparrows acclimated to summer temperatures, although the sparrows could not eat enough to live at temperatures colder than -29C.
"They dramatically increase the size of their gut to accommodate the greater amount of food they must eat to meet their energy needs in the cold, yet they are able to just as efficiently digest their food when they eat much more," said McWilliams. "That tells us something about their ability to flexibly respond to climate change. Plenty of birds migrate south because they have too limited a capacity to respond in this way. But white-throated sparrows have the spare capacity to modify their physiology to deal with substantial environmental change."
However, when the birds were given no time to acclimate to the cold temperatures, they were only able to increase their food intake by about 50 percent. The researchers found that the birds needed at least two days to acclimate to the new conditions before they were able to eat more.
One implication of this finding is that birds that fly long distances in migration -- an activity that causes their gut size to decrease because they do not eat while flying -- need a day or two to reconstitute their gut before they can resume the maximum food intake required to continue their migration.
McWilliams says that his study has defined the ultimate limits of the gut capacity of white-throated sparrows. If similar limits could be established for other species, that data could be incorporated into climate models to better understand which species will likely be able to survive the coming environmental changes.
"All organisms have some level of spare capacity," McWilliams said. "The animals that live in constant environments haven't had to evolve much capacity, so those animals are probably going to have the greatest challenge adapting to changing conditions."
According to the researchers, the limits of spare capacity have been studied in very few other species, with most work focused on several varieties of snakes. But they say that the limitations all animals face are in their ability to convert food into usable energy.
"The gut limits the overall design of the animal," McWilliams said.
The URI researcher's next step is to do similar studies of how the fat composition of birds changes with environmental conditions and in response to their energy needs. Since certain polyunsaturated fats are known to improve flight performance, McWilliams said that this study will have relevance to the type of food birds should strive to eat in preparation for migration and thus what type of foods should be provided to birds in the landscape.
- S. R. McWilliams, W. H. Karasov. Spare capacity and phenotypic flexibility in the digestive system of a migratory bird: defining the limits of animal design. Proceedings of the Royal Society B: Biological Sciences, 2014; 281 (1783): 20140308 DOI: 10.1098/rspb.2014.0308
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