Mama’s boy: sex differences in juvenile survival in a highly dimorphic large mammal, the Galapagos sea lion

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-specific 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 first 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.

Divergent allele advantage at MHC-DRB through direct and maternal genotypic effects and its consequences for allele pool composition and mating

“…The genetic parameters are MHC-DRB divergence (MHC div and its square, MHC div number of functionally important loci or genome-wide levels of heterozygosity. Surveys on marine mammals in general, and pinnipeds in particular, indicate that pathogens (especially nematodes and viruses) are one of the major causes for mortality in these species [22,40], with mortality being especially high during early life [38,41]. The immunologically important MHC genes therefore provide a clear candidate locus for fitness.…”

“…To provide a maximal contrast in fitness to early deceased offspring, we classified individuals as survivors if they had reached reproductive maturity. Early mortality in this population is approximately 10 -20% in normal years [37,38]; however, for more robust statistical inference we balanced the sample of early deceased individuals with an equally sized random subsample of survivors (selected before genotyping).…”

Lazy sons, self-sufficient daughters: are sons more demanding?

“…Although males are larger than females at all ages studied (see also Kraus et al, 2013;Mueller et al, 2011), Kraus et al (2013) found no overall differences in survival. These findings argue for higher maternal investment in sons than in daughters.…”

“…To avoid disturbance of mothereoffspring interactions, pups as well as older offspring were captured when mothers were away or well separated (out of sight) from the offspring. Yearlings with TDRs survived as expected (77% survival over the second year of life is expected for yearlings without instruments according to Kraus et al, 2013). We resighted 85% of the 40 animals equipped with TDRs in 2010 and 2011 after !1 year and 70.8% of the animals equipped in 2012 after !6 months.…”

“…Except for providing milk, mothers show no specific maternal care behaviours towards their offspring after the perinatal period. Since male offspring are larger than female offspring and mortality to weaning is nearly identical in both sexes (Kraus et al, 2013), maternal investment is likely to be higher in males. However, juveniles begin to dive and forage at the age of 6 months to 1 year, but still largely depend on maternal milk to supplement energy demands (Jeglinski, Werner, Robinson, Costa, & Trillmich, 2012).…”