Authors

Discussion

Juveniles face strong demands for resources at a crucial time of growth and development. The development of immunity against prevalent parasites is expected to be crucial for survival under natural conditions.  However, since resources are limited, investment in an immune response could come at a cost to growth or survival. Trade-offs between growth and immune function have been shown in experimental studies, but are less well understood in natural conditions. We use a data collected from a long-term study of free-living Soay sheep on St.Kilda to test whether there is evidence of trade-offs between immunity and growth, parasite load and survival in the wild. We measured differential white blood cell counts and  levels of antibodies (IgA, IgE, and IgG) against a prevalent gastrointestinal nematode, Teladorsagia circumcincta (Tc) in samples collected over a 4 year period on St Kilda. Our data suggest that lambs with moderate growth rates have higher levels of anti-Tc IgG & IgE and lower Neutrophil:Lymphocyte ratios. Males investing more in horn growth have lower levels of anti-Tc IgE. Antibody measures were negatively associated with strongyle faecal egg counts, but varied in associations with over-winter survival between sexes. Our data suggest growth costs of immunity may dependent on sex and the immune marker applied under natural conditions.