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Seabirds Give New Meaning to Sibling Rivalry

June 1997

It's spring, the season when nesting birds peacefully raise families of young in shaded nests high above the fray below.

Unless those birds are masked boobies, that is. The tall seabirds live and breed in tropical oceans, where they survive by making high-velocity fishing dives into the sea. In an NSF-funded study, biologist David Anderson found another, more startling, survival technique: Masked boobies kill their own siblings, with their parents' help.

Anderson, of Wake Forest University in Winston-Salem, North Carolina, used this example to subject Darwin's evolutionary theory to some of the most rigorous testing to date, and found an answer to the question: How does a species in which siblings kill each other fit into Darwin's theory of natural selection?

This research shows that evolution sometimes works in surprising ways, says NSF's Mark Courtney. "Anderson's research is a good example of how evolutionary theory can provide explanations of behavioral traits that superficially may seem aberrant," says Courtney, Program Director in NSF's Population Biology Program, which funds Anderson's research. "And, as with Anderson's research, rigorous testing of a theory requires careful experimentation and observation, often over several generations, before the full evolutionary context becomes clear."

Understanding masked boobies requires taking a long-term look at the effect of their behavior on their reproductive success, says Anderson. "The common view of Darwinian natural selection is that it favors the evolution of parental interest in offspring," he says. Parents that care for their kids raise the most kids and spread their genes the most rapidly. However, in masked boobies and some other animals, siblicide is an integral part of reproductive success. Siblings engage in lethal battles shortly after hatching from their eggs, with only one making it." And it turns out that parent boobies are far from disinterested observers of the family conflict.

Strictly A Family Battle

Masked booby parents actually facilitate and encourage this early siblicide, according to Anderson, setting up a lethal contest between the older and younger chick, and stacking the deck in favor of their elder. "In masked boobies, reproductive success is not about being caring and nurturing, but making short-term sacrifices of your own offspring for long-term gain." Anderson acknowledges that boobies may stretch many people's understanding of reproductive evolution, but these birds do fit into Darwin's theory. "The bottom line isn't how many babies you have now, but the total number of healthy offspring over the long term," says Anderson. When you have more young in a brood than you can care for well, it may pay to pare off some of them, says the scientist.

Masked boobies are an extreme example of the point. They lay two eggs. If both hatch, typically about six days apart, siblicide occurs about a day after the second egg hatches. The older chick forces the younger one out, leaving its tiny sibling prey to certain death from heat or patrolling mockingbirds.

Understanding the Strategy

The question for Anderson is: Why?

He first began studying siblicide in 1984 among boobies that nest on the island Española in the southern Galapágos. He asked why these animals engage in behavior that would seemingly condemn the species to extinction. Why lay two eggs if the goal is only one offspring? "Siblicide demands an explanation because it's close relatives killing each other. Going back to Darwin, the question is: How can you maximize long-term reproductive success this way?"

Anderson found part of the answer in the boobies' poor hatching rates. Masked boobies hatch only about 60 percent of their eggs, even under perfect conditions. Other seabird species hatch about 90 percent of their eggs. An "insurance egg" hypothesis -- that a second egg greatly increases the chance of reproductive success -- turned out to be on target. By laying two eggs, masked boobies increased their chance of hatching one chick to about 84 percent.

But what about the violence and the collaboration between parent and chick to eliminate the second-born? In the NSF-funded study that took place from 1992 to 1995, Anderson compared masked boobies to a related species, blue-footed boobies. These birds engage in siblicide only during severe food shortages when the first-born bird drops below 75 percent of normal body weight.

Mixing Species and Behavior

To determine whether parents or offspring, or both, controlled the siblicide, Anderson and his team put blue-footed booby chicks in masked booby nests and vice versa. When masked booby chicks were raised by blue-footed booby parents, siblicide happened in 80 percent of the nests, compared to the normal 100 percent when masked booby chicks are cared for by masked booby parents. In the other nests, about half of the blue-footed booby chicks killed their siblings when raised by masked booby adults.

"The masked booby parents seem to be collaborating with the first chick to get rid of the second chick," Anderson observes.

Further research suggests that masked booby parents even stand over their chicks to give the attacking chicks more room to maneuver. The masked boobies' nests, flatter than those of the blue-footed species, may also make the job easier. In contrast, blue-footed booby parents tuck their chicks down into bowl-shaped nests.

Despite the evidence of collaboration between masked booby parents and their first-born chicks in conducting siblicide, Anderson has not yet completed analyzing data to answer the ultimate question: What are the long-term effects on masked booby adults forced to raise two chicks?

Preliminary data clearly show a cost. "They show up at the breeding colony in about the same frequency the second year, but are much less likely to breed," says Anderson, who visits the Galapágos each fall to set up his team's research camp, returns in July to close another season of data-gathering, and supervises the team via computer e-mail and satellite telephone in the interim.

"The parents are tuckered out from the year before," he says. "If the rest of the data show the same trend, then we could say that parents seem to maximize their long-term reproductive success by collaborating in the siblicide, because the effort required to raise two chicks at once is so high. The insurance value of the second egg is there, but once the first egg hatches, the second chick is clearly a liability."

Darwin may not have envisioned the siblicide scenario, says Anderson, but he might well have been perfectly comfortable with these results.

Continue to sidebar - Evolutionary Biology: The History of Life.

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