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Litter reductions reveal a trade-off between offspring size and number in brown bears

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Abstract

Experimental manipulations have revealed positive effects of litter reduction on offspring mass in small mammals, but little is known about this trade-off in large mammals. We examined the determinants of natural litter size variation and quantified the effects of litter size, maternal characteristics, and litter composition on yearling mass using 24 years of data on marked brown bears (Ursus arctos) in Sweden. Infanticide by adult males is a major cause of cub-of-the-year mortality, leading to litter size reductions. Litter size (n = 265) at den emergence ranged from one to four cubs (average, 2.7) and increased with maternal age. Litter size, however, appeared independent of maternal size, population density, interlitter interval, study area, or previous litter sex ratio. Yearling body mass increased with maternal body size but was independent of litter sex ratio. Litter size and yearling mass were negatively correlated, mostly because singletons were about 30 % heavier than yearlings from litters of two to four cubs. In reduced litters, survivors were on average 8 % heavier as yearlings than individuals from intact litters, suggesting that sibling competition reduces growth. Trade-offs between litter size and yearling mass in bears appear similar in magnitude to those found in small mammals.

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Acknowledgments

We thank the field personnel of the Scandinavian Brown Bear Research Project (SBBRP) for collecting data over the years. The SBBRP was funded by the Swedish Environmental Protection Agency, the Norwegian Directorate for Nature Management, the Swedish Association for Hunting and Wildlife Management, WWF Sweden, and the Research Council of Norway. M.F.B. and F.P. acknowledge the financial support of NSERC of Canada. F.P. is supported by the Canada Research Chair in Evolutionary Demography and Conservation. A.Z. was supported financially by the Norwegian Research Council. This is paper No. 129 from the Scandinavian Brown Bear Research Project.

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Authors and Affiliations

  1. Département de biologie, Université de Sherbrooke, 2500 boul. de l’Université, Sherbrooke, QC, Canada, J1K 2R1

    Ofelia Gonzalez, Fanie Pelletier & Marco Festa-Bianchet

  2. Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, Post Box 5003, 1432, Ås, Norway

    Andreas Zedrosser & Jon E. Swenson

  3. Department of Integrative Biology and Biodiversity Research, Institute for Wildlife Biology and Game Management, University of Natural Resources and Applied Life Sciences, Vienna, Gregor Mendel Str. 33, 1180, Vienna, Austria

    Andreas Zedrosser

  4. Norwegian Institute for Nature Research, 7485, Trondheim, Norway

    Jon E. Swenson

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  1. Ofelia Gonzalez
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  2. Andreas Zedrosser
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  5. Marco Festa-Bianchet
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Correspondence to Ofelia Gonzalez.

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Communicated by K. Ruckstuhl

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Gonzalez, O., Zedrosser, A., Pelletier, F. et al. Litter reductions reveal a trade-off between offspring size and number in brown bears. Behav Ecol Sociobiol 66, 1025–1032 (2012). https://doi.org/10.1007/s00265-012-1350-3

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