A Pacific white-sided dolphin calf born last week at Chicago's Shedd Aqauarium has died, officials said Friday.
"This is an extremely sad day for the Shedd Aquarium family," Shedd President and CEO Ted A. Beattie said in a news release.
Though the calf appeared to be healthy when it was born June 3, officials said it didn't learn the vital nursing skills as quickly as expected. About 12 hours after the birth, Shedd's experts said they took a precautionary measure and began feeding the calf with her mom’s milk.
The calf died Thursday.
A necropsy has already been performed, but experts said they weren't able to find a conclusive cause of death.
"Additional evaluations will be conducted; however, as is often the case, a cause of death may never fully be known," the release said.
The calf was born to 26-year-old Tique, who was impregnated by a male named Lii at the Miami Seaquarium through a strategic partnership. Tique continues to be monitored, officials said.
There are fewer than 20 Pacific white-sided dolphins in North American facilities.