High hormone levels in seabird chicks make them slay their siblings

June 19th, 2008 - 1:13 pm ICT by ANI  

Washington , June 19 (ANI): A Galapagos Island seabird, called the Nazca booby, has a tendency to kill its own siblings from the very moment it comes out of its shell. Scientists have now attributed this murderous behaviour to high levels of testosterone and other male hormones found in the hatchlings.

Led by David J. Anderson, professor of biology, Wake Forest University, the study found that its is the elevated levels of male hormones, called androgens, which increase aggression in both male and female chicks and prepare the birds to fight to the death as soon as they hatch.

“The older of two Nazca booby hatchlings unconditionally attacks and ejects the younger from the nest within days of hatching,” said Anderson .

The fact that Nazca boobies have difficulty in raising more than one chick, thus it is vital for the older chick to vanquish the younger for increasing its own chances of survival.

The study suggests that the high hormone levels also cause the surviving chicks to behave like bullies after growing up, making them frequently seek out nestlings in their colony. In those visits they often bite and push around the defenseless youngsters.

In the study, the researchers took blood samples from Nazca booby chicks within 24 hours of hatching. In 15 nests with two eggs, blood samples were taken from both hatchlings. Samples were also taken from 15 hatchlings in one-egg nests. This was followed by analysing the hormones by researchers at the University of Maryland . For comparison, the researchers did the same for blue-footed boobies, a closely related species.

The researchers thought that the Nazca booby hatchlings experience the high level of aggression-related hormone during a “sensitive period” in their growth, when long-term growth patterns are easily affected. Some Nazca booby nestlings experience a one-two hormonal punch, raising their aggression hormones even higher when they actually have a nest mate. The nestlings that fight siblings become bigger bullies as adults than the Nazca booby nestlings who never fight.

“The hormones that are part of this epic battle early in life seem to permanently change some aspects of their social personality,” said Anderson .

Aggression-related hormone levels are three times higher in Nazca booby chicks than their less aggressive cousins, the blue-footed boobies, who do not have the same lethal fights right after hatching and do not go on to bully their fellow birds as adults.

The study appears in the latest edition of the online journal PLoS ONE. (ANI)

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