Life history theory predicts a trade-off between offspring size and number. However, the role of intra-litter phenotypic variation in shaping this trade-off is often disregarded. We compared the strength of the relationship between litter size and mass from the perspective of the lightest and the heaviest yearling offspring in 110 brown bear litters in Sweden. We showed that the mass of the lightest yearlings decreased with increasing litter size, but that the mass of the heaviest yearling remained similar, regardless of litter size.Consistent with a conservative reproductive strategy, our results suggest that mothers maintained a stable investment in a fraction of the litter, while transferring the costs of a larger litter size to the remainder of the offspring. Ignoring intra-litter phenotypic variation may obscure our ability to detect a trade-off between offspring size and number.

This dataset includes data on brown bear yearling body masses from south-central Sweden, 1990-2016. Included are also litter sizes, maternal size and age, year of capture, and index of local population density. This dataset was used to conduct analyses from the journal article "Trade-off between offspring mass and number: the lightest bear the cost", by Van de Walle, J, Zedrosser, A., Swenson, J.E., and Pelletier, F.

These data were collected as part of the long-term monitoring program of the Scandinavian Brown Bear Research Project.