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Research Roundup: Cowbirds, worms and oxygen

02 September 2004

The secret of cowbird success


This Eastern phoebe nest has a parasitic intruder. The larger gape belongs to the older parasitic brown-headed cowbird chick, while the smaller gapes are the phoebe's own young. (Mark Hauber photo)
The brown-headed cowbird is notorious as the nonpareil parasite among North American birds. A new study by biologists from Berkeley and the University of Cambridge shows that a cowbird chick, deposited in the nest of another bird species while still in the egg, survives after hatching by joining its nestmates in a chirping chorus that brings in more food than one noisy cowbird chick could demand from its host parents. By eating more than its share, the researchers found, the cowbird chick actually grows faster when sharing the nest and food with two host chicks than it does when all alone in the nest.

Said Mark E. Hauber, a Miller Research Fellow in the Museum of Vertebrate Zoology: “When a cowbird chick has nestmates, the whole nest brings in more parental care, because there is more begging altogether, and so the parents attend the nest more. But the cowbird monopolizes the feeding attempts by the parents. In these experiments, instead of getting 33 percent of the feedings that a brood of two host chicks and one cowbird chick gets, the cowbird actually got over 50 percent of the feeding. So, it grew better than when it was living alone.”

The strategy of sharing the nest to gain more resources appears to be successful generally among all the 100 or so species of birds parasitized by cowbirds, Hauber said: “We found in a comparative analysis of 18 different host species that, if you look at the growth of the cowbird chick, it does best in hosts who have about 1.8, or approximately two, nestmates growing up together with the cowbird chick.”

Hauber, along with University of Cambridge biologists Rebecca M. Kilner and Joah R. Madden, published their findings in the August 6 issue of Science.

Robert Sanders

When the worm turns (away from oxygen)

Researchers from Berkeley and UCSF have discovered how the nematode C. elegans senses oxygen levels in order to steer clear of surrounding areas that are too low or too high in oxygen.

In the process, the researchers also discovered that the worm doesn’t like as much fresh air as people thought. While nematodes grown in laboratory Petri dishes are kept at the same oxygen concentration humans breathe in ambient air — 21 percent — nematodes appear to prefer only 6-percent oxygen.

“It was totally unexpected that they would actually prefer 6 percent. We don’t know why, though it probably gives them some survival advantage,” said Michael A. Marletta, professor of chemistry and of molecular and cell biology at Berkeley, and a faculty scientist at Lawrence Berkeley National Laboratory. “And the bordering and clumping that worm experts refer to as social behavior is really the worms, in an artificial setting like a Petri dish, trying to get to an area of 6-percent oxygen, which they like. It’s a laboratory phenomenon.”

Bordering and clumping is a peculiar behavior in which worms cluster around the border of the Petri dish instead of spreading evenly around the surface. Marletta and his colleagues, members of the California Institute for Quantitative Biomedical Research (QB3), determined that the bacteria the worms feed on are at a higher density around the border of the dish, consuming oxygen along with the worms. Apparently, when oxygen levels are high, the worms pile onto the densest clumps of bacteria, because that’s where oxygen levels are lowest.

“The swarm of worms and density of bacteria together lower the oxygen concentration in that immediate environment,” he said. “We found that when we lower the oxygen concentration to 6 percent, the worms disperse in three minutes.”

At high concentrations, oxygen is toxic and corrosive. Worms avoid high oxygen presumably because it creates highly reactive chemicals that damage cells. By manipulating oxygen levels in Petri dishes filled with worms feeding on a lawn of bacteria, the researchers were able to show that bordering and clumping was actually a response to high oxygen levels.

Bargmann speculates that the oxygen-sensing system used by C. elegans may be used by other animals who must avoid low-oxygen environments, including fish. Humans may also have such a detector to trigger hyperventilation during exercise or exposure to anoxic environments.

“We are immersed in a 21-percent-oxygen atmosphere all the time, and our bloodstream and lungs maintain the optimum oxygen levels in our tissues. So, we take oxygen levels for granted,” Bargmann said. “But most other animals on the planet live in water or the soil, such as C. elegans. And since oxygen diffuses much more slowly in those environments, they must evolve ways to sense oxygen and react to changes in oxygen levels.”

Robert Sanders