North Pacific Climate Mediates Offspring Sex Ratio in Northern Elephant Seals

Abstract

Determinants of sex ratios in animal populations have been of general interest to ecologists for decades. We tested the hypothesis that offspring sex ratio in a population of northern elephant seals (Mirounga angustirostris) was related to large-scale environmental indices reflecting ocean climate and productivity that affected the condition of reproductive females over 31 years . More males were produced during years of weaker pressure differences and warmer sea surface temperature anomalies in the northeastern Pacific, conditions that reduce or disperse prey resources for gestating females in the North Pacific Ocean. For this species, sexually distinct forage resources exist, so competition for food resources occurs predominately within sex. These results are consistent with the resource competition model for facultative adjustments of offspring sex ratio. Anthropogenic global warming is predicted to warm the North Pacific. This could result in altered basin-scale ocean productivity, increased nutritional stress, and an overabundance of males that may adversely affect this and other similarly regulated mammalian populations.Key words: climate change, elephant seal, Farallon Islands, Mirounga, Northern Oscillation Index, Pacific Decadal Oscillation, Phocidae, sex ratio Evolutionary theory predicts that parents should invest in offspring in ways that enhance their own lifetime reproductive success (Fisher 1930). Darwin (1871) thought that natural selection controls sex ratio of animals. Selection theoretically should result in an equal offspring sex ratio (probability of a son ¼ 0.5), but vertebrates often deviate from this ideal (Clutton-Brock and Iason 1986; Darwin 1871). Hypotheses concerning adaptive (or facultative) parental adjustment of offspring sex ratio in mammals focus on differences in the expected future reproductive success of the 2 sexes.The 2 major hypotheses explaining offspring sex-ratio adjustment in mammals are the Trivers-Willard (TW) model (Trivers and Willard 1973) and the resource competition (RC) model (Clark 1978;Silk 1983). Trivers and Willard (1973) postulated that when parental investment affects offspring reproductive success, mothers who can afford to invest heavily (i.e., mothers in good condition) should invest more in the sex with greater variance in reproductive success. In polygynous, sexually dimorphic species, females in good condition could produce larger male offspring that would have a disproportionately higher mating success, therefore the TW model predicts that the sex ratio of offspring should be male-biased during years of abundant food resources (Trivers and Willard 1973). Mothers in poor condition would gain more reproductively from producing daughters than sons because the survival and reproductive success of daughters is usually less affected by low levels of preweaning investment (Clutton-Brock et al. 1988). The RC model (Clark 1978), a generalization of the local mate competition model of Hamilton (1967), proposes that competition for resources between mothers a...

Environmental sex reversal, Trojan sex genes, and sex ratio adjustment: conditions and population consequences

“…2002, 2004; Wang 2005; Cotton & Wedekind 2008; Grubler et al. 2008; Lee & Sydeman 2009). Global temperature change has received particular attention in this context, as it is expected to alter the sex ratios of species with TSD (Nelson et al.…”

“…2002; Schwanz & Janzen 2008; Hulin et al. 2009; Lee & Sydeman 2009; Zhang et al. 2009) or with temperature‐induced sex reversal (Wedekind et al.…”

Emerging sex allocation research in mammals: marsupials and the pouch advantage

“…2003, Blanchard et al. 2005, Lee & Sydeman 2009), as well as the potential for good‐condition mothers to value female‐biased sex allocation (Hewison & Gaillard 1999, Hewison et al. 2005).…”

“…For example, better maternal condition, higher maternal rank, greater maternal age and greater competition with daughters have long been examined as potential correlates of male-biased sex ratio (Myers 1978, Perret 1990, Hiraiwa-Hasegawa 1993, Hewison & Gaillard 1999, Saltz 2001, Cockburn et al 2002, Hewison et al 2002, Saltz & Kotler 2003. More recently, the complicating effects of population density and climate on maternal traits have become apparent (Kruuk et al 1999, Bradshaw et al 2003, Blanchard et al 2005, Lee & Sydeman 2009), as well as the potential for good-condition mothers to value female-biased sex allocation (Hewison & Gaillard 1999, Hewison et al 2005). In addition, correlative studies are common in mammals, yet can be limited in the ability to measure appropriate maternal traits at a relevant time during reproduction (Cameron 2004, Sheldon & West 2004.…”

Ontogenetic changes in feeding habits of northern elephant seals revealed by δ15N and δ13C analysis of growth layers in teeth

“…The diving pattern of elephant seals suggests that the sexes consume different prey (Le Boeuf 1994, Le Boeuf et al 2000. To the extent that this is so, competition for prey is predominantly intrasexual (Lee & Sydeman 2009), owing to spatial and temporal separation at sea. The reduced diving capacity of juveniles relative to adults (Le Boeuf et al 1996), as well as the rapid development of differences in foraging strategies (Le Boeuf et al 1993), has been observed during the first 2 yr of life (Le Boeuf et al 1996).…”