Session A – ECOLOGY S1. DO BISON FEMALES SYNCHRONIZE PARTURITION BY GESTATION

Gregarious mammals generally synchronize birthing by adjusting the time of estrus. Berger (Ecology 73: 323-329, 1992) proposed that American bison (Bison bison) females in good condition are capable of adjusting gestation length, as well. He hypothesized that females in good condition that breed late in the season can shorten their gestation length so as to give birth during the birthing peak. We studied a herd of bison at Fort Niobrara National Wildlife Refuge, Nebraska to test the gestation adjustment hypothesis. For a subset of females observed in 2004 and 2005, dates of conception and birth were observed in the field and confirmed by genetic parentage analysis, fecal progesterone confirmed hormonal estrus, and calves were weighed during the annual roundup in September. Average (± SD) length of gestation was 268 days (± 5). Gestation length of females that conceived after the median breeding date was an average of 3 days shorter than that of females conceiving earlier in the season. The weight of the dam was not correlated with gestation length. Calf weight was positively correlated with dam weight, but was not related to gestation length. The sex of the calf did not influence either gestation length or calf weight.