Study: Hormones cause birds to kill siblings

The Nazca booby, a seabird found only in the eastern tropical Pacific, is so eager to be an only child that it emerges from the egg ready to kill its sibling on the spot.

A team of biologists led by professor David J. Anderson of Wake Forest University has shown that elevated levels of male hormones are responsible for this violent tendency. Their study, published this week, also found that those birds who kill a sibling in their infancy are scarred by the event for life, seeking out interactions with younger chicks in a way we might call molestation.

Unlike their cousins the blue-footed boobies, Nazca parents cannot bring in enough fish to feed two healthy chicks, so their optimal brood size is one. Killing the rival is the baby booby's way of securing a healthy childhood.

"It is not common for boobies to be born in pairs," Anderson said, "but even though most of the chicks do not end up killing a sibling, they are all born with high levels of male hormones, just in case they need to."

Anderson, who has been monitoring more than 16,000 Nazca booby nests in the Galápagos Islands since 1984, was particularly excited about the results of the study that tie siblicide in newborns to permanent personality changes.

"We found that those boobies that engaged in obligate siblicide when they were younger tend to seek out social interactions with unguarded chicks as adults," Anderson said. "Sometimes it's a nice interaction, but in general the interactions are negative for the chicks. The adult could beat up a chick or even engage in sexual behavior toward it."

So how does killing a sibling affect a bird's adult personality? The answer lies in hormone activity during the killing.

"Those who do commit siblicide experience a temporary increase in testosterone during the act," Anderson said. "It's like a one-two punch of more testosterone on top of already elevated testosterone levels from birth. Even though the hormone changes are temporary, they occur during a very sensitive stage in the booby's development and have an effect on personality."

Early exposure to hormones affecting personality is not a phenomenon limited to boobies.

"We know pretty well that it happens in rodents," Anderson said.

Mouse embryos are arranged in the womb like peas in a pod. If a male is sandwiched between two other male embryos, the testosterone leaking out of those embryos can affect its disposition, making it more dominant and aggressive. On the other hand, if a male embryo is flanked by two females, it is more likely to be docile.

What about in humans?

"There have been studies in humans linking traits like aggression or sexuality to early hormone exposure, like in the womb," Anderson said.

"But we don't know enough right now to say anything definite about that."