Differential investment in twin offspring by female pronghorns (Antilocapra americana)

Abstract

Differential investment in offspring has been reported for many mammals, often in the context of the Trivers– Willard model of male-biased investment, but evidence of differential investment in pronghorns (Antilocapra americana) is largely lacking. We assessed the causes and consequences of different birth masses of littermate fawns in a pronghorn population in Oregon. The mass differential for co-twins ranged from 0% to 89% (median ¼ 8.35%). Male-biased investment explained the mass differential in opposite-sex litters but not same-sex litters. The mass differential did not result from mothers producing 1 normal-size fawn and 1 runt fawn, and the smaller fawn was not deficient in physiological condition. Only 29% of fawns survived to 8 weeks and both fawns died in 56% of litters, but co-twin mortalities were largely separate events. Mass did not confer a survival advantage when considering all fawns through age 8 weeks, but there was evidence of such an advantage when comparing fawns within litters before age 18 days. Differential investment in fawns might be a bet-hedging strategy in which the mother accepts a lower expected reproductive success in exchange for a lower variance, but neither the mean nor the variance differed between mothers of different-size (.8.35% mass differential) and similar-size (,8.35%) litters. In fact, there was evidence of increased reproductive success for mothers of different-size litters, much of which stemmed from higher survival 4–6 days after birth. Having different-size fawns reduced the chances of sequential mortality, in which a predator killed one fawn then returned to kill the other.