Tenny, one of the five American bison living in the park, had recently become ill. After separating herself from the other bison, Tenny "laid down and basically never got up again," San Francisco Zoo spokeswoman Lora LaMarca said.
"She really went downhill very quickly. The first sign was when she separated herself from the herd," LaMarca said.
A grazing animal, bison usually feed standing up, and after treating Tenny for two days, zoo staff concluded that Tenny's quality of life had deteriorated. She was euthanized before noon Tuesday.
LaMarca said the precise cause of Tenny's illness could not be known until after a necropsy had been performed.
LaMarca said that the zoo is working with the parks department to introduce more bison in Golden Gate Park. "Hopefully by year's end there will be more because the bison here are old," LaMarca said.
Bison typically live anywhere from 20 to 30 years. Of the remaining four bison, two are 20 years old and two are 28 years old, so they are not expected to live much longer. Tenny was 20 years old.
Tenny and the other four bison are descendents of two bison introduced in 1984 as a gift to then-San Francisco Mayor Dianne Feinstein from her husband, Richard Blum.
Bison were first introduced to Golden Gate Park in February 1891 by the Park Commission as the native American animals were on the brink of extinction.
Today, Golden Gate Park is one of several protected habitats for bison, and the zoo is collaborating with the parks department to maintain them in San Francisco.