An effective immune response is expected to confer fitness benefits through improved resistance to parasites but also energetic costs which negatively impact fitness-related traits such as reproduction. These fitness costs and benefits of an immune response are likely to depend on host age, sex, and levels of parasite exposure. Few studies have examined the full extent to which patterns of natural selection on immune phenotypes vary across demographic groups and environments in the wild. Here, we assessed natural selection on plasma levels of three functionally distinct isotypes (IgA, IgE and IgG) of antibodies against a prevalent nematode parasite measured in a wild Soay sheep population over 25 years. We found little support for environment-dependent selection or reproductive costs. However, antibody levels were negatively associated with parasite egg counts and positively associated with subsequent survival, albeit in a highly age- and isotype-dependent manner. Raised levels of anti-parasite IgA best predicted reduced egg counts but this did not predict survival in lambs, whilst in adult females increased anti-parasite IgG predicted reduced egg counts and improved survival. Our results highlight the potential importance of age-dependent selection on immune phenotypes in nature, and that patterns of selection can vary even amongst functionally-related immune markers.

This data was collected as part of the long term individual based study on the Soay sheep of St Kilda in Scotland. The full methods on how data was collected and processed is included in the main article associated with this dataset (https://doi.org/10.1086/700115):

Natural selection on antihelminth antibodies in a wild mammal population

A. M. Sparks, K. Watt, R. Sinclair, J. G. Pilkington, J. M. Pemberton, S. E. Johnston, T. N. McNeilly, D. H. Nussey

Please see the README files for descriptions of the columns in the dataset and data usage notes.