A river hippopotamus born at the San Diego Zoo five days ago has died, the zoo reported Thursday.

Animal care staff noticed it had passed away early Thursday morning during regular observations. The cause of death is unknown.

On Wednesday, the baby hippo spent time with its mother, Funani, lounging in their 150,000-gallon pool. Zoo representatives said Funani had kept the calf close and keepers had not been able to determine if the baby was male or female. With so many obstacles in the wild, mother hippos are known to be very protective of their newborns.

Zoo staff consider Funani an experienced mother and very attentive. She last gave birth in 2011.

According to the zoo, the river hippopotamus is a threatened species, facing both natural predators and man-made dangers such as poaching.

City News Service contributed to this report.

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