It's a scene that occurs daily among nesting colonies of Nazca boobies: A young adult bird struts over to a neighbor's chick and begins biting and pecking it, sometimes causing injuries that lead to the nestling's death. But if the chick survives, it's likely to become just like its tormentor, attacking other nestlings when it reaches maturity and perpetuating this "cycle of violence," researchers report. It's one of the first times that the cycle, which is normally used to explain child abuse in humans, has been discovered in a population of wild animals. The study appears in the October issue of The Auk.
It's well known that children who suffer attacks by adults often grow up to abuse their own kids. But it's been difficult to study this cycle outside of humans or captive species, such as rhesus monkeys, that may exhibit some similar behaviors—because it is apparently rare, or at least seldom witnessed. On Española Island in the Galápagos, however, adult Nazca boobies attack chicks at an alarming rate, and the researchers say the birds' behavior offers a somewhat parallel model to that of humans. The birds are indifferent to human observers, so it's easy to spot and record the entire sequence of events, the researchers say.
The sea birds' chick abuse is "one of the first things you notice; it's that obvious and disturbing," says David Anderson, an evolutionary ecologist at Wake Forest University in Winston-Salem, North Carolina, and one of the study's co-authors, who has been observing the boobies since 1984. The behavior is also surprising. "You don't expect to see animals wasting time, bothering with a neighbor's chick, when it could be doing something that benefits its fitness, like finding a mate."
To understand more about how this behavior arises in the boobies, Anderson and his graduate students, including first author Martina Muller, collected data over three breeding seasons. They identified 24 individuals, some of which were attacked as nestlings and some of which weren't. They tagged these birds with bright blue leg bands and tracked them again when they were adults. The scientists discovered that those adults that had suffered the most abuse as nestlings were themselves the worst chick abusers as adults. "A bird's history as a target of abuse proved to be a strong predictor of its adult behavior," Anderson says. The bird's behavior is thus similar to that of humans—but with a key difference, he adds. In humans, the abuser is most often related to the child. The boobies, however, are attacking unrelated chicks.
The boobies' behavior seems to be at least partly linked to the birds' natural history. Boobies often lay two eggs, even though the parents can care for only one chick. If both eggs hatch, the nestlings fight each other to the death—a behavior that is governed by hormones.
The latest find—that these survivors often become abusers as adults—may be due to this previous behavior or to lingering hormonal effects. Because the scientists have not yet found an evolutionary explanation for the birds' behavior, they suggest there may be "some psychological element at play," given the abusers' "intense interest in the chicks," Anderson says.
Because the attacks by unrelated adults are similar to the sibling attacks, there may be a "maladaptive side effect" of something that makes evolutionary sense, says Scott Forbes, a biologist at the University of Winnipeg in Canada. Forbes agrees with Anderson that the boobies can provide a "useful model system" to study the causes of human child abuse. Others, however, are not yet convinced that the chick-abusing boobies and child-abusing humans are sufficiently similar to warrant using the birds as a model. "It is very interesting to see these intergenerational effects of early experience and hormones in an animal," says Dario Maestripieri, a behavioral biologist at the University of Chicago in Illinois. "But what happens in humans is pathological," he continues, "whereas the birds are programmed to attack each other as siblings," because the parents can handle only one.