Positive antioxidant balance may help in better reproduction, barn swallow study shows

Mar 1 2010

A balanced diet, exercise and reduced stress not only can lead to a longer life, but also better reproduction, according to a new study by a team of researchers, including one from Arizona State University, on barn swallow nutrition and mating habits.

The study shows that swallows who maintained a positive antioxidant balance over the course of their breeding season were those who produced the most young.

The results of the study are presented in the February 25, 2010 issue of PLoS One, a journal of the Public Library of Science, in the article "Positive carotenoid balance correlates with greater reproductive performance in a wild bird." The study was led by Rebecca Safran, an assistant professor in ecology and evolutionary biology at University of Colorado in collaboration with Kevin McGraw, an associate professor in the School of Life Sciences at Arizona State University. Co-authors include Colorado doctoral students Matthew Wilkins and Joanna Hubbard, and project volunteer Julie Marling.

Little seems to be easy for the North American barn swallow. The pint-sized bird travels thousands of miles to its nesting grounds and then almost immediately upon arrival engages in its courtship and mating rituals. If successful in these activities, the barn swallows then need to feed, warm and protect their offspring.

While working at a seemingly exhausting pace, it turns out that the strongest of the barn swallows are not only up to the task, but they excel at it. The reason: The leaders of the pack "have a prime 'management system' for antioxidants," said McGraw. "Even after completing the arduous tasks of migration and reproduction, these intense breeders still find themselves carrying a surplus of antioxidants to combat additional challenges."

"Our results indicate these top-of-the-line barn swallows are less stressed and have higher functioning immune systems," added Safran.

The antioxidants that McGraw and his colleagues studied are carotenoids - plant compounds like lutein and beta-carotene found in fruits and vegetables. These compounds often are sold as human nutrition supplements.

In a variety of animals, these carotenoids can have potent free-radical-quenching and immune-boosting activity. "Whether the stud birds are acquiring more carotenoids from food or having to use less to keep their bodies healthy, they're clearly successful at keeping levels high while out-reproducing their competitors."

While several other studies have examined how carotenoid levels in animals are linked to health and other aspects of fitness at single points in time, the new study is the first to consider how an individual's temporal change in carotenoid levels is associated with its evolutionary fitness.

Conventional wisdom, McGraw explained, is that vigorous activities such as migrating thousands of miles, courtship, nesting and reproduction should deteriorate the physiological state of animals like swallows.

"Among a variety of animals, reproduction can compromise health and decreased health can inhibit reproduction," McGraw said. "But here we show that, among the best barn swallows, they're able to both keep carotenoid levels high and breed the most. Thus, we don't find clear support for a health/reproduction trade-off among wild animals."

CU's Safran and her team, which included dozens of CU students and volunteers, trapped scores of barn swallows with mist nets in rural sites around Boulder County, Colo., measuring and weighing them and taking blood and feather samples before releasing them back into the wild. Each bird was sampled between two and four times over the breeding season. Blood analysis work was done in McGraw's lab at ASU.

Three carotenoids were measured by McGraw - lutein, zeaxanthin, and beta-cryptoxanthin - all of which are sold in health food stores around the world. The swallows acquire carotenoids from the bodies of insects they eat, which get the antioxidants from feeding on plants, McGraw said.

"The biggest contribution of this study is the fresh perspective on assessing each animal's dynamic changes in antioxidant status over time, as opposed to just before or after a given task in life," McGraw explained. "So many variables like diet, genetics and exercise contribute to how good or healthy an individual is at any snapshot in time, and we believe we took a more holistic view of a swallow's condition by longitudinally tracking their own performance over the season."

Interestingly, McGraw equated this to how humans are coming to view personal wellness programs.

"Humans also are incredibly variable in their diet, genetics and health, so rather than focus on a universal goal and set-point to achieve for all people, a more reasonable assessment might be to focus on the fluxes and improvements of individuals," McGraw explained.

He added that these findings dovetail nicely with previous work on barn swallows. Safran and McGraw have shown that barn swallows with more intensely colored feathers circulate more carotenoids through their bodies and reproduce the most in a year.