Tina saw the bison in Golden Gate Park. She says that her friend mentioned that bison only have a single lung cavity, and that's why you can fell one with a single bullet (or arrow, as the case may be).
http://www.medterms.com/script/main/art.asp?articlekey=25357
Definition of Buffalo chest
Buffalo chest: A single chest cavity with no anatomic separation of the two hemithoraxes due, for example, to tension pneumothorax (air under pressure collapsing a lung). It is called buffalo chest because this anatomical peculiarity of the North American buffalo, or bison, helped the Indians of the Great Plains to kill them. A single Indian arrow to the chest frequently was enough to let air in to collapse both lungs and fell the breathless bison.
The use of the term "buffalo chest" came to medical attention through a 2003 report in The New England Journal of Medicine. However, the term dates back at least to 1984, as indicated by the report by GR Schorlemmer and others entitled "Bilateral pneumothoraces secondary to latrogenic buffalo chest" (Ann Surg. 1984;199:372-4). When the term was originally introduced into medicine is not known.