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Mom Likes Me Best: Egg Size Predicts Offspring Survival

While comedian Tommy Smothers always told his brother, “Mom always liked you best,” a study by biology researchers gives new meaning to the classic phrase. Researchers Wendy Reed and Mark Clark at North Dakota State University, Fargo, and Carol Vleck at Iowa State University, Ames, have found that female American coots favor their largest offspring, even before they hatch.

Reported in the November issue of The American Naturalist, the study was conducted over two breeding seasons on small wetlands in the prairie-parkland region south of Minnedosa, Manitoba, Canada. Reed and colleagues placed newly-hatched chicks in foster nests. Their findings show that chicks from a female’s largest eggs survived better than their smaller genetic siblings. This was true even though siblings were not raised together, nor raised by their genetic parents.

Egg size and other maternal investments vary even more among different mothers, but that did not matter for survival of the offspring. “What matters is that an offspring is the largest among its genetic siblings,” says Reed, associate professor of biology at NDSU. “It doesn’t mean that bigger is always better, but it does mean that being bigger than your brothers and sisters is important for survival.”

Often called marsh hens or mud hens, the 16-inch long American coots are known for their territorial and noisy, cantankerous habits. Females lay 5 to 16 eggs per clutch and movement of broods among ponds is rare, which facilitates monitoring the survival of the young. Researchers located 66 nests during the egg-laying stage or during egg incubation and marked them with a unique code, measuring their length and breadth. Chicks were hatched through incubation, allowing researchers to identify which chick hatched from which egg. Chicks used in the fostering study were assigned to foster families, with chicks fostered into nests in three visits. Adult coots readily accepted the foster chicks who were observed weekly by researchers.

Instead of creating offspring that are more similar to one another, the study suggests that maternal effects, such as egg size, can result in greater differences rather than similarities among siblings. “This can affect evolutionary dynamics and provide new explanations for why so much diversity is seen in egg size and other important life history traits,” says Reed.

Like many mothers, American coots seem to face the classic mothering paradox—is it quality or quantity that matters? Results of the study suggest that differences in the quality of maternal resources provided to offspring are more important than the absolute quantity of resources. These differences in quality ultimately determine whether the young offspring survive. When researchers placed the large eggs into a foster-brood, they found little support for large eggs within foster-broods having higher survival probability than small eggs.

So while size matters—for American coots, it appears you don’t have to be the biggest—just the biggest among your own genetic siblings to increase your chances of survival.

For more information:
“Maternal Effects Increase Within-Family Variation in Offspring Survival”
The American Naturalist, Nov. 2009, Vol. 175, No. 5, pp.685-695
Funding for this research study was provided by the Delta Waterfowl Foundation, National Science Foundation grant IBN-98-01503, a Sigma Xi Grant-in-Aid of Research and the American Museum of Natural History, Frank Chapman Fund.

Carol Renner | Newswise Science News
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