Abstract
As the number of breeding pairs depends on the adult sex ratio in a monogamous species with biparental care, investigating sex-ratio variability in natural populations is essential to understand population dynamics. Using 10 years of data (2000–2009) in a seasonally monogamous seabird, the king penguin (Aptenodytes patagonicus), we investigated the annual sex ratio at fledging, and the potential environmental causes for its variation. Over more than 4000 birds, the annual sex ratio at fledging was highly variable (ranging from 44.4% to 58.3% of males), and on average slightly biased towards males (51.6%). Yearly variation in sex-ratio bias was neither related to density within the colony, nor to global or local oceanographic conditions known to affect both the productivity and accessibility of penguin foraging areas. However, rising sea surface temperature coincided with an increase in fledging sex-ratio variability. Fledging sex ratio was also correlated with difference in body condition between male and female fledglings. When more males were produced in a given year, their body condition was higher (and reciprocally), suggesting that parents might adopt a sex-biased allocation strategy depending on yearly environmental conditions and/or that the effect of environmental parameters on chick condition and survival may be sex-dependent. The initial bias in sex ratio observed at the juvenile stage tended to return to 1∶1 equilibrium upon first breeding attempts, as would be expected from Fisher’s classic theory of offspring sex-ratio variation.
Highlights
In animal populations, determinants of adult sex ratio (ASR, i.e. the proportion of male to female adult individuals [1]) are likely to occur before adulthood, due to variations in parental sexual allocation of resources, and to fluctuations in sex-specific mortality of these offspring before recruitment into the breeding population
Because colony density of king penguins varies to a large extent over the course of a breeding season and may affect the physiological status of breeding parents [32], we considered whether colony density might affect chick sex ratio
Over the 10-year study period, fledging sex ratio was slightly biased towards males (1941 females vs. 2071 males), implying that an overall greater proportion of male offspring successfully fledged from 2000 to 2009, i.e. 51.62% of male (GLM; z52.05, P50.04, N54012; upper-right panel, Fig. 1)
Summary
Determinants of adult sex ratio (ASR, i.e. the proportion of male to female adult individuals [1]) are likely to occur before adulthood, due to variations in parental sexual allocation of resources (before and after birth, reviewed in [2]), and to fluctuations in sex-specific mortality of these offspring before recruitment into the breeding population. Hamilton pointed out that biased sex ratios were likely to occur in natural conditions [6], if the costs of producing male and female offspring were different In such cases, a bias towards the less costly sex should be expected [3, 6,7,8]. Trivers & Willard [7] hypothesised that mothers in good condition may afford to invest more into the sex with greatest variance in reproductive success, when mothers in poor condition gain more by investing into the sex less affected by poor maternal rearing conditions [7, 9] This seems to be the case in the red deer (Cervus elaphus) in which high population densities during female pregnancy lead to adverse nutritional stress causing mothers to produce fewer male than female offspring [10]. Offspring production is biased towards males in those years and this male-biased sex ratio has been suggested as an adaptive strategy to decrease later resource competition between mothers and daughters on similar food patches [13]
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