Youth Beats Experience: Older parents hatch weaker offspring (Reproduction)
Authors: Leslie Mitch 
Published: August 2003

If you want to raise a strapping, healthy youngster, it helps to be young yourself--at least if you're a swallow. Older parents produce smaller chicks with less aggressive immune systems, according to new research. Although the cause is unknown, the findings jibe with the results of lab studies and surveys of human families and might elucidate how organisms budget their resources for reproduction.

Every spring the swallows return to San Juan Capistrano in California, and they also return to the less celebrated stables around Milan, Italy, where since 1993 Saino and colleagues have been capturing and banding nearly all these birds--more than 3000 in total. A swallow almost always nests in the same location, and it often pairs up with the same mate, so the researchers could readily track individuals from year to year. Focusing on 138 broods, the team weighed the chicks and gauged their developmental rate based on the growth of a particular tail feather. To assess the strength of the chicks' immune systems, the researchers injected a T-cell-stimulating chemical into the wing and measured the resulting swelling: The bigger the lump, the more energetic the T cells. The test indicates the birds' likelihood of survival, according to the group's previous results.

Compared with the offspring of youthful birds, the chicks of older parents were smaller and their feathers developed more slowly. The hatchlings from older mothers also had weaker immune systems. The researchers aren't sure how to explain the findings. Having splurged on their early brood, older parents might lack the physiological wherewithal to continue to produce such robust offspring. The mother's eggs might no longer be as nutritious, for instance. Another possibility is that the birds grow weaker with age independent of reproduction: Perhaps the grizzled swallows are tuckered out and don't nab as much food. The results provide some circumstantial support for the first hypothesis. When the researchers compared the strength of the T cell reactions in successive clutches in the same family, they found an inverse relation. According to Saino, this analysis suggests that parents that invest heavily in one brood are diverting resources from subsequent offspring.

Playing favorites makes evolutionary sense because of the steep mortality rates in this species, the barn swallow, says biodemographer James Carey of the University of California, Davis. Only about one-third of the parents survive to the next breeding season, so they should pump as many resources as possible into the current clutch. "They've got one shot, maybe two shots," to raise a family, he says.

The work is important because it reveals that parental age effects occur in the wild, says evolutionary geneticist Daniel Promislow of the University of Georgia in Athens. He adds that similar results have emerged from several recent investigations of human families as well as an older one by Alexander Graham Bell (yes,
that
Alexander Graham Bell), which found shortened life-spans among the children of older parents. By putting off childbirth until they are wealthy and emotionally stable, human parents might be bequeathing poor health.

--Mitch Leslie; suggested by Donna Holmes

N. Saino, R. Ambrosini, R. Martinelli, A. P. Moller, Mate fidelity, senescence in breeding performance, and reproductive trade-offs in the barn swallow.
J. Anim. Ecol.
71
, 309-391 (2002). (The abstract and full text are not available online.)