Marsha, a 32-year-old black rhino, has died at the Oklahoma City Zoo, zoo officials say. She had been at the zoo for 15 years and was known as an "avid painter," the zoo said in a statement Monday. "She thoroughly enjoyed the enriching exercise of painting with her caretakers and contributed many masterpieces to the Zoo's annual Art Gone Wild animal art show that supports conservation," the statement added. Zoo officials said the rhino had become lethargic and wasn't eating, but veterinarians found last week she was improving, KFOR-TV, Oklahoma City, reported. Marsha relapsed during the weekend and died during a medical exam. Animal care officials at the zoo say they believe she died of old age. Animal experts say black rhinos have a life expectancy of 25 to 40 years.