Environmental effects on prenatal growth rate in pronghorn and bighorn: further evidence for energy constraint on sex-biased maternal expenditure

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Abstract
Byers and Moodie (1990) proposed that high levels of maternal expenditure in polygynous ungulates limit the ability of mothers to support elevated male fetal and neonatal growth rates. This hypothesis assumes that females in high-expenditure species are at or near the maximum level possible and that females in lower-expenditure species are not. To test this assumption, we examined our long-term data on reproduction of pronghorn (Antilocapra americana) and bighorn (Ovis canadensis) at the National Bison Range (western Montana, USA) and compared gestation lengths, birth weights, and prenatal growth rates of offspring born following summers of low versus average or above average precipitation. In bighorn, these variables were unaffected by the previous summer's rainfall, but in pronghorn, gestation length and prenatal growth rate were significantly lower following dry summers. Extended samples for both species confirmed earlier reports of sex-biased expenditure favoring males in bighorn and the absence of sex-bias in pronghorn. Bighorn prenatal litter growth rates and birth weights, corrected for maternal mass, are 63.5% and 38%, respectively, of pronghorn values. These data support the Byers and Moodie (1990) contention that females of high-expenditure species do not show differential expenditure by offspring sex because they are at a reproductive expenditure maximum, whereas females of lower-expenditure species are able to support excess expenditure in male offspring because optimal allocation to female offspring is farther from such a maximum.

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