Large Broods Drive Bird Parents to an Early Grave

By Elizabeth Preston
Apr 4, 2014 6:38 PMNov 20, 2019 12:04 AM


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If your parents ever groaned that you and your siblings were aging them rapidly with your bickering or loud music, they may have been right—especially if they were jackdaws. Scientists who artificially increased or decreased the size of these birds' broods found that extra-large families make parents die sooner. One theory of aging says that it comes from accumulated damage to a body's cells, and that animals have a limited amount of resources to spend fixing that damage. Using more resources on something like having and raising offspring may leave an animal with a smaller reserve for keeping its cells in good shape. This means reproducing might make it age sooner. Some long-term studies of wild animals have found evidence that this is true. But, writes Jelle Boonekamp, a biologist at the University of Groningen in the Netherlands, these studies compared animals that naturally had larger or smaller numbers of young. So there might have been an unseen third variable at work—some trait that gives parents more young while also shortening their lives. To address this problem, Boonekamp and his coauthors stole babies from some birds and tricked others into adopting them. The scientists studied jackdaws, a type of crow, living in colonies around the university. Jackdaws live in monogamous pairs. Couples have an average of 4.5 eggs per year, and both parents take care of their young. Each nest was randomly assigned to be either reduced or increased by two baby birds. The scientists shuffled four-day-old nestlings between nests to shrink or grow the families as needed. The parents, apparently not bothered by this, kept on feeding whoever now lived in their nest. There were 186 parents in the study, and for each year that the parents survived and returned to the nesting site, the scientists continued enlarging or reducing their broods—the idea being that if a mother jackdaw were genetically inclined to have big or small broods, she'd do it every year. Jackdaws aren't especially long-lived. Only 72 percent of parents with reduced broods survived into the second year of the study. But they fared better than birds with enlarged broods—only 59 percent of these beleaguered parents lived. The difference in survival grew more extreme with each year. After the third year of the experiment, only 20 percent of large-brood parents were still alive, compared to 61 percent of parents with small broods. The effect was the same for moms and dads. Having more young might kill parents sooner for reasons besides aging. For example, spending more time out gathering food for the nest could make a parent more vulnerable to predators. But Boonekamp says there didn't appear to be a difference between the two groups of birds in these kinds of deaths. He suspects that when parents had more mouths to feed, they simply aged faster. "Producing and rearing many offspring [may leave you with] a smaller energy resource budget for maintaining your health," Boonekamp says. The real difference may be even more extreme than what his study showed, since the jackdaws that adopted extra young didn't actually have to produce or incubate the extra eggs. But he says that what this faster aging means biologically, on the level of real bodies and cells, is unknown. "The exact mechanism is basically a black box." One possible mechanism of aging has to do with telomeres, little protective DNA caps at the ends of chromosomes. A cell's telomeres grow shorter whenever it replicates, which limits the cell's lifespan. Boonekamp is now studying telomeres in birds and hoping to learn what role they play in aging. He's not sure what he'll find if he pries open the black box, though it's probably not a stereo system blasting music.

Image: by Joe Dunckley (via Flickr)

Boonekamp, J., Salomons, M., Bouwhuis, S., Dijkstra, C., & Verhulst, S. (2014). Reproductive effort accelerates actuarial senescence in wild birds: an experimental study Ecology Letters, 17 (5), 599-605 DOI: 10.1111/ele.12263

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