A baby chimpanzee was killed at the Los Angeles Zoo by an adult male chimpanzee as visitors watched on Tuesday.
The baby chimp — born March 6 — had been gradually introduced to the shared habitat and there had been no sign of problems, the LA Times cited the zoo as saying.
Chimpanzees are the human species' closest living relative, and the LA Zoo troop is one of the largest in a North American zoo.
"Chimpanzee behavior can sometimes be aggressive and violent, and the zoo is sorry that visitors had to be exposed to this," the statement said.
"This is a heartbreaking and tragic loss for the zoo and especially the Great Ape Team who have worked diligently to care for the infant and its mother since its birth."
The baby's mother, named Gracie, "had proved to be a caring mother," the zoo said in a statement.
The Associated Press cited zoo spokesman Jason Jacobs as saying visitors reported seeing the adult attack the 3-month-old infant.
Zoo staff were unable to stop the attack owing to their own safety concerns, Jacobs said.
"Gracie is being allowed to keep the infant overnight to allow her the opportunity to grieve," he said.
A recent report in Scientific American said that while chimpanzees were usually "peaceable creatures," they occasionally "engage[d] in these lethal bouts of aggression" and killed their own.
Some scientists had theorized that this was an evolutionary strategy for reducing competition for resources — and mating partners — while others suggested that human disturbance, including deforestation, had triggered the behavior.
A study of killings in chimp communities across Africa found that found that kills occurred in most chimpanzee communities, and that victims tended to be infants and adult males outside the killer’s social group, with most killings carried out by groups of males.
The higher the number of males in a group, the higher the number of kills, the study found.
Study author Michael Wilson of the University of Minnesota, noting that the research "tells us something about human evolution," told the Scientific American that: "The number of males is important because the more males there are, the more competition there is for mates in the community."
The AP concurred, writing that male chimpanzees were known to attack the offspring of rivals both in the wild and in captivity, especially if a desired female was involved.