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Parent–offspring and sibling conflict in Galápagos fur seals and sea lions

Behavioral Ecology and Sociobiology volume 62, pages 363–375 (2008)Cite this article

Abstract

Parent–offspring conflict theory is well supported by theoretical arguments. However, empirical observations are often difficult to interpret and have contradicted one of its most appealing predictions that parent and offspring should disagree over killing of nest or littermates. We present the first examples of deadly conflict between siblings of different cohorts. In Galápagos fur seals (Arctocephalus galapagoensis) and sea lions (Zalophus wollebaeki), mothers often wean their single offspring at 2 years. This leads to a situation where up to 23% of all pups are born while the older sibling is still being nursed. Younger siblings are disadvantaged by being born lighter than neonates without older still dependent siblings. Pups born while an older sib is still dependent grow less in early life (fur seal) and suffer increased early mortality (both species) through direct aggression or scramble competition with the older sibling. This effect is much stronger in years of high sea surface temperature (El Niño) indicating low marine productivity and if the older offspring is a male. In both species, mothers interfere aggressively in this conflict by defending the younger offspring. In years of El Niño, intense resistance to maternal aggression by the older offspring happens frequently in the fur seal. Such resistance against weaning can induce maternal neglect of the newborn. Given substantial year to year variation in offspring growth, maternal aggression forces weaning in the older sibling only if it has reached sufficient size to support itself by foraging. In Galápagos fur seals, pups with older siblings can either represent insurance against loss of older offspring or extra reproductive value.

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Acknowledgements

This study was supported by the Max-Planck Society, the Deutsche Forschungs-gemeinschaft, the VW-Stiftung and the Friends of the Galápagos, Switzerland. The Galápagos National Park permitted the study of fur seals and sea lions, and the Charles Darwin Station logistically supported the study through its directors Günther Reck, Hendrik Hoeck, Friedemann Köster and Graham Watkins. FT wishes to thank W. Wickler for his steady support and G.L. Kooyman and many field assistants for their wonderful companionship in the field. C. Kraus, J. Krause, D.W. Mock and M. Wikelski gave much appreciated criticism to various drafts of the manuscript. FT wants to thank D.W. Mock for his encouragement to publish the old fur seal data and especially for sharing his many insights and doubts about the state of parent–offspring conflict research during a one month stay in his lab in Norman.

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Authors and Affiliations

  1. Animal Behaviour, University of Bielefeld, P.O. Box 10 01 31, 33501, Bielefeld, Germany

    Fritz Trillmich

  2. Institute for Genetics, Department of Evolutionary Genetics, University of Cologne, Zülpicher Str. 47, 50674, Koln, Germany

    Jochen B. W. Wolf

Authors
  1. Fritz Trillmich
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  2. Jochen B. W. Wolf
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Correspondence to Fritz Trillmich.

Additional information

This contribution is part of the special issue “Sibling competition and cooperation in mammals” (guest editors: Robyn Hudson and Fritz Trillmich).

Communicated by A. Schulte-Hostedde

Appendix

Appendix

Summary of model selection statistics. The number of estimated parameters (K), the residual sum of squares (RSS), Akaike’s information criterion for small sample sizes (AICc), the difference between the minimum AICc of the best candidate model (bold font) and the AICc of the model under consideration (Δ i), Akaike weights (w i) and the Bayesian information criterion (BIC) are given for each model of the candidate set.

Table 4 Influence of presence or absence of older sibling and pup sex on postnatal pup weight in fur seals
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Table 5 Influence of presence or absence of older sibling and pup sex on postnatal pup weight in sea lions 
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Table 6 Influence of SST anomaly and presence or absence of a sib on pup growth for those pups that survived to 30 days of age 
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Table 7 Influence of sea surface temperature anomaly, older sibling’s sex and older sibling’s mass on survival of pups in fur seals
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Trillmich, F., Wolf, J.B.W. Parent–offspring and sibling conflict in Galápagos fur seals and sea lions. Behav Ecol Sociobiol 62, 363–375 (2008). https://doi.org/10.1007/s00265-007-0423-1

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  • DOI: https://doi.org/10.1007/s00265-007-0423-1

Keywords

  • Parent–offspring conflict
  • Sibling conflict
  • Siblicide
  • Parental investment
  • Sexual selection
  • Arctocephalus galapagoensis
  • Zalophus wollebaeki