ENVIRONMENTAL-EFFECTS ON PRENATAL GROWTH-RATE IN PRONGHORN AND BIGHORN - FURTHER EVIDENCE FOR ENERGY CONSTRAINT ON SEX-BIASED MATERNAL EXPENDITURE

Byers and Moodie (1990) proposed that high levels of maternal expendit ure in polygynous ungulates limit the ability of mothers to support el evated male fetal and neonatal growth rates. This hypothesis assumes t hat females in high-expenditure species are at or near the maximum lev el possible and that females in lower-expenditure species are not To t est this assumption, we examined our long-term data on reproduction of pronghorn (Antilocapra americana) and bighorn (Ovis canadensis) at th e National Bison Range (western Montana, USA) and compared gestation l engths, birth weights, and prenatal growth rates of offspring born fol lowing summers of low versus average or above average precipitation. I n bighorn, these variables were unaffected by the previous summer's ra infall, but in pronghorn, gestation length and prenatal growth rate we re significantly lower following dry summers. Extended samples for bot h species confirmed earlier reports of sex-biased expenditure favoring males in bighorn and the absence of sex-bias in pronghorn. Bighorn pr enatal litter growth rates and birth weights, corrected for maternal m ass, are 63.5% and 38%, respectively, of pronghorn values. These data support the Byers and Moodie (1990) contention that females of high-ex penditure species do not show differential expenditure by offspring se x because they are at a reproductive expenditure maximum, whereas fema les of lower-expenditure species are able to support excess expenditur e in male offspring because optimal allocation to female offspring is farther from such a maximum.