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1 February 2013 Differential investment in twin offspring by female pronghorns ( Antilocapra americana)
Dirk H. Van Vuren, Martin P. Bray, Jeannie M. Heltzel
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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 = 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.

Dirk H. Van Vuren, Martin P. Bray, and Jeannie M. Heltzel "Differential investment in twin offspring by female pronghorns ( Antilocapra americana)," Journal of Mammalogy 94(1), 155-161, (1 February 2013).
Received: 26 April 2012; Accepted: 1 July 2012; Published: 1 February 2013

Antilocapra americana
bet hedging
body mass
fawn survival
maternal investment
sex-biased investment
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