The cost of reproduction: differential resource specialization in female and male California sea otters

Abstract

Intraspecific variation in behavior and diet can have important consequences for population and ecosystem dynamics. Here, we examine how differences in reproductive investment and spatial ecology influence individual diet specialization in male and female southern sea otters (Enhydra lutris nereis). We hypothesize that greater reproductive constraints and smaller home ranges of females lead to more pronounced intraspecific competition and increased specialization. We integrate stable carbon (δ13C) and nitrogen (δ15N) isotope analysis of sea otter vibrissae with long-term observational studies of five subpopulations in California. We define individual diet specialization as low ratios of within-individual variation (WIC) to total population niche width (TNW). We compare isotopic and observational based metrics of WIC/TNW for males and females to data on population densities, and movement patterns using both general linear and linear mixed-effects models. Consistent with our hypothesis, increasing population density is associated with increased individual diet specialization by females but not by males. Additionally, we find the amount of coastline in a sea otter’s home range positively related with individual dietary variability, with increased range span resulting in weaker specialization for both males and females. We attribute our results to sex-based differences in movement, with females needing to specialize in their small ranges to maximize energy gain, and posit that the paradigm of individual prey specialization in sea otters with increased intraspecific competition may be a pattern driven largely by females. Our work highlights a potentially broader role of sex in the mechanistic pressures promoting and maintaining diet specialization.

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Acknowledgments

We thank all members of the Tinker, Estes and Newsome labs for data collection, support and comments. Additionally, we thank F. A. Smith for invaluable assistance with the manuscript. We thank Luke Tyrrell, Kelli Blomberg, Ryan Jones, and Deborah Boro for laboratory assistance. We also thank the USGS Pacific Nearshore Project team members, especially Keith Miles and Liz Bowen (USGS), Heather Coletti (National Park Service), Tom Dean (Coastal Resource Associates), and USGS personnel Brenda Ballachey, Dan Monson, George Esslinger, Kim Kloecker, and Ben Weitzman for logistical and intellectual support. The manuscript also benefitted greatly from constructive criticism by C. A. Layman and two anonymous reviewers. Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the US government.

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Correspondence to Emma A. Elliott Smith.

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Communicated by Craig A. Layman.

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Elliott Smith, E.A., Newsome, S.D., Estes, J.A. et al. The cost of reproduction: differential resource specialization in female and male California sea otters. Oecologia 178, 17–29 (2015). https://doi.org/10.1007/s00442-014-3206-1

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Keywords

  • Individual diet specialization
  • Stable isotopes
  • Reproductive investment
  • Spatial ecology
  • Enhydra lutris