Variation in offspring sex ratio among individual Weddell seal (Leptonychotes weddellii) females of different quality

Abstract

The Trivers–Willard model predicts that in polygynous species, superior-quality females will maximize their fitness by producing male offspring. Using a sample of 1,780 Weddell seal (Leptonychotes weddellii) pups recorded over 31 years, we investigated relationships between offspring sex ratio and maternal age, reproductive experience, an index of maternal lifetime reproductive output, and annual environmental variations. We found evidence that females with higher index of lifetime reproductive output were more likely to produce male than female offspring but found only weak evidence that large-scale environmental variations influenced sex ratios. Our results suggest that mothers manipulate offspring sex to maximize their own fitness, and inherent maternal quality may influence offspring sex. These findings support the Trivers–Willard sex-allocation model.

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Acknowledgments

We thank all the personnel who participated in the long-term Weddell seal demography study since 1969. We also wish to thank Don B. Siniff for his insightful comments on an earlier version of this manuscript and three anonymous reviewers for constructive comments on this manuscript. Funding for this project was provided by a National Science Foundation grant, OPP-0225110, to R. A. Garrott, J. R. Rotella, and D. B. Siniff. The data were collected under various National Science Foundation grants to R. A. Garrott and J. R. Rotella at Montana State University, D. B. Siniff at the University of Minnesota, and J. W. Testa at the University of Alaska, Fairbanks.

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Correspondence to Kelly M. Proffitt.

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Communicated by F. Trillmich

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Table S1

Candidate a priori models explaining the effects of maternal and environmental characteristics on the odds of producing a male offspring. Covariates evaluated included an index of lifetime reproductive output (iLRO), maternal age (AGE), parity (PARITY), maternal identity (ID), summer sea-ice extent (SumExtent), winter sea-ice extent (WinExtent), southern oscillation (SOI), and year (YEAR) (DOC 51 kb).

Table S2

Histogram of index of maternal lifetime reproductive output for the 332 mothers included in analysis (DOC 24 kb

Table S3

Complete model-selection results for a priori models examining variation in Weddell seal offspring sex. Models are presented along with the AICc value, the ΔAICc value, and the Akaike model weight (w i ). Covariates evaluated include index of maternal lifetime reproductive output (iLRO), maternal age (AGE), sea-ice extent in the winter prior to conception (WinExtent), sea-ice extent in the summer of breeding and pregnancy (SumExtent), and the southern oscillation index (SOI) in the 3 months prior to conception. The random effects of maternal identity are represented by ID, and the random effects of year are represented by YEAR (DOC 61 kb).

Table S4

S4 Model-averaged coefficient estimates\(\left( {\widehat{\overline \beta }} \right)\), standard error\(\left( {{\text{SE}}\left( {\widehat{\overline \beta }} \right)} \right)\), and 95% confidence interval (CI) of each fixed covariate explaining variation in offspring sex. Predictor weights (w +(j)) are presented from the overall modeling exercise (DOC 30 KB).

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Proffitt, K.M., Garrott, R.A. & Rotella, J.J. Variation in offspring sex ratio among individual Weddell seal (Leptonychotes weddellii) females of different quality. Behav Ecol Sociobiol 62, 1679–1687 (2008). https://doi.org/10.1007/s00265-008-0596-2

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Keywords

  • Sex ratio
  • Trivers–Willard model
  • Lifetime reproductive output
  • Leptonychotes weddellii