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Oecologia (2013) 171:893903 DOI 10.1007/s00442-012-2469-7
POPULATION ECOLOGY - ORIGINAL RESEARCH
Mamas boy: sex differences in juvenile survival in a highly dimorphic large mammal, the Galapagos sea lion
C. Kraus B. Mueller K. Meise P. Piedrahita
U. Prschmann F. Trillmich
Received: 12 October 2011 / Accepted: 4 September 2012 / Published online: 2 October 2012 Springer-Verlag 2012
Abstract In many mammals, early survival differs between the sexes, with males proving the more fragile sex [Fragile male (FM) hypothesis], especially in sexually dimorphic species where males are the larger sex. Male-biased allocation (MBA) by females may offset this difference. Here, we evaluate support for the FM and MBA hypotheses using a dataset on Galapagos sea lions (Zalophus wollebaeki). We statistically model sex-specic survival as it depends on body mass and environmental conditions (sea surface temperature, SST, a correlate of marine productivity) at three developmental stages, the perinatal phase (1st month), the main lactation period (1st year), and the weaning period (2nd year). Supporting the FM hypothesis, we found that, early in life (1st month), at equal birth mass, males survived less well than females. During the remainder of the rst year of life, male survival was actually less sensitive to harsh environmental conditions than that of females, contradicting the FM hypothesis and supporting the MBA hypothesis. During the second year of life, only male survival suffered with high SSTs as predicted by the FM hypothesis. At each developmental stage, observed survival rates were almost equal for both sexes, suggesting that mothers buffer against the inherent fragility of male offspring through increased allocation,
thereby masking the differences in survival prospects between the sexes.
Keywords Environmental variability Fragile males
Male-biased allocation Sex differential mortality
Zalophus wollebaeki
Introduction
In humans and other mammals, males are often the more fragile sex, leading to reduced survival early in life (Clutton-Brock et al. 1985; Kraemer 2000). During the preand early perinatal period, this might in part be due to males being the heterogametic sex (XY). Hence, deleterious alleles on their sex chromosomes cannot be compensated (reviewed in Austad 2006; Smith 1989; Smith and Warner 1989). Indeed, heterozygosity has been shown to inuence juvenile and in particular perinatal survival (Bean et al. 2004; but see Hoffman et al. 2006). Additionally, gonadal steroid hormones might...