THE BISON AND THE BLOW FLY: USING BLOW FLY PUPARIA TO CONSTRAIN THE SEASON OF DEATH AND TAPHONOMIC HISTORY OF AN EARLY-HISTORIC-AGE BISON, CARSON VALLEY, NORTHERN NEVADA

While excavating a bison skeleton buried in flood-plain deposits near Carson City, Nevada we encountered thousands of puparia ("cocoons") of the black blow fly, Phormia regina, which lays its eggs on the bodies of dead animals. Some of the bison bones exhibit cut marks that are too deep and narrow to have been made with stone tools, so we infer that metal tools were involved. Radiocarbon dating has provided ambiguous results, but they are compatible with an early historic age, which is also indicated by the cut marks. Because Phormia regina is important to forensic investigators for determining "time since death" of human accident and crime victims, the life cycle and temperature constraints of this species of fly have been well studied. In this study we use this information to constrain the season of death and taphonomic history of the bison.

The life cycle of P. regina requires a minimum of 8.8 days within a temperature range of 14ºC to 35ºC, so the bison carcass must have been exposed to the air for at least that long, within that temperature range. However, of the thousands of recovered puparia, 35% remained closed and had not produced an adult fly. Of this cohort, only a tiny percentage exhibit small exit holes attributable to parasitoid wasps. We infer that cold night-time temperatures, rather than parasitoid wasps or other causes, were responsible for the high pupal mortality. Based on climate data for the Reno/Carson City region (prior to the recent warming trend), along with P. regina temperature constraints, the high percentage of closed puparia, and seasonal flooding patterns of the Carson River drainage, we conclude that the bison died in the spring.