Facilitation of Reproductive Synchrony by Gestation Adjustment in Gregarious Mammals: A New Hypothesis

Abstract

Among gregrious, plaental mammals, breeding synchrony usually occurs by adjustment of the timing of estrus, not births, and it has been suspected that the former process govern synchronized parturition. Gestation length data gathered on wild plains bison (Bison bison) over a 5—yr period demonstrate:(1) females in good body condition who were mated after the seasonal peak shortened gestation by °6 d, synchronizing births with other females; (2) no similar adjustments occurred among females in poor condition; and (3) those females that shortened gestation incurred a cost because their neonates were, on average, °20 kg lighter when 6 mo old. Reproductive synchrony may also be achieved by a less radical tactic; unmated females use olfactory cues to explore the status of other females prior to their own estrus but not after. These data suggest that there is adaptive variation in gestation length in bison cows in good condition and that assumptions about the constancy of gestation are unwarranted.