See, Mom Did Like You Better


Siblicide. It’s the scientific name for what Cain did to Abel--and what many a younger child dreams of doing to an older sibling.

Now, a handful of siblicide researchers are discovering that when it comes to sibling rivalry, there are species out there far more bloodthirsty--or, in the case of plants, sap-thirsty--than any human.

From the siblicidal pods of certain trees in India to violence-prone baby mouse sisters to the newest study of sibling strife among birds, one conclusion is emerging: It is good to be born first.


University of Oklahoma zoologist Douglas W. Mock suspected as much. Mock, the youngest of four brothers, has studied siblicide for close to two decades. In a study of cattle egrets, a long-legged, sharp-billed species of white heron, he and Washington State University zoologist Hubert Schwabl report in the journal Nature that egret mothers favor their little early birds with more than the best worms: They give them an extra dose of male hormones.

Because the first baby egrets to break out of their pale blue shells also tend to be much larger than their siblings, the testosterone advantage gives the senior siblings the boost they need to get rid of their junior nest mates when push comes to shove.

Not only can that make the first-hatched more pushy and, therefore, successful at feeding time, it also may give them a lifelong competitive edge over any younger brothers and sisters that survive the early food fights.

This is good for the big brothers and sisters, of course. But the researchers say it is probably good for the species as well. If the older egrets don’t have to waste time and energy fighting equally large and aggressive siblings, then they--and, by extension, the whole cattle egret family--stand a better chance of survival.

Indeed, siblicide, suggest Mock and co-author Jennifer A. Gieg, may well be nature’s best survival-of-the-fittest solution when food shortages or other catastrophes strike. In “The Evolution of Sibling Rivalry” (Oxford University Press) due out this summer, Mock examines how hormone manipulations influence how animals as well as plants behave and survive.

In the case of the Dalbergia Sissoo tree in India, five of the six seed pods that develop around each tree flower are killed off by the seed pod closest to the flower. “It’s the first to receive the pollen and it uses it to produce a water soluble chemical that essentially drowns its little siblings on the stems below. Once it drenches the other seeds in the pod . . . the eldest seed floats away to start a life of its own,” Mock says.


Similar life-and-death dramas are being discovered among mammals. In certain mice families, for example, hormones give baby female mice that are sandwiched in utero between their big brothers extra doses of male hormone to keep up, at least physically, with the boys.

While scientists are still theorizing about any similar mechanisms in humans, Schwabl says he is intrigued by the notion that hormones from mother to offspring serve more than one function. “Certainly, there might be differences in the hormonal milieu in subsequent pregnancies as the mother grows older.”

But would such differences favor the older sibling or the younger one?

“It is an interesting question,” Schwabl says.