The death of a nine-day-old rare western lowland gorilla infant from head trauma at Chicago's Lincoln Park Zoo highlights the mysterious dynamics of child rearing in gorilla tribes, where only two out of three infants survive.

Zoo authorities say the unnamed baby, who had already become a big hit with visitors, was observed being carried, lifeless, by its 16-year-old first-time mom, Bana, in the zoo's gorilla enclosure on Friday morning. Authorities believe the mother may have accidentally sat on the baby, but admit they don't fully understand what happened.

"There are no bite marks or cuts and no sign of aggression, but it is possible she may have been dropped or even sat on," Zoo spokeswoman Sharon Dewar told the Daily Mail. "It happened at night and it was too dark to be captured by the cameras in the exhibit."

Last year, a baby gorilla lost part of its leg after a family scuffle in an enclosure at the Louisville Zoo, in Kentucky. Several years ago, a gorilla infant at Zoo Atlanta, which has the largest population of lowland gorillas in the US, was hurt, perhaps accidentally, by the dominant male silverback in the group. Earlier this year, a gorilla infant named Tiny at the London Zoo was killed during a group squabble after a new male was introduced to the group.

Most infanticides in the wild occur when the dominant silverback dies, which often causes females with babies to seek protection inside other family groups rather than face the potential of a rival male murdering the deceased male's offspring.

“Squabbles within a gorilla group do happen,” Louisville Zoo Animal Curator Steve Wing said in a statement. “Gorillas exhibit complex and dynamic relationship behaviors. It is challenging to identify the reason for this occurrence."

But while relationships can be managed at zoos, it's impossible to anticipate the full dynamics of gorilla behavior. In Chicago, zoo staff had worked with Bana to help her figure out how to rear the baby, and all signs showed that she was a good and willing mother.

The zoo says workers allowed Bana to hold onto her baby for several hours "to make peace with what happened."