Survival costs of reproduction are mediated by parasite infection in wild Soay sheep

Reproduction in wild Soay sheep carries significant survival costs, mediated by increased parasite infection and reduced body weight. A 20-year study on St. Kilda, Scotland, found that females that gave birth—especially those that successfully weaned lambs—experienced higher faecal egg counts of strongyle nematodes during the lambing season. Structural equation modelling further revealed that these reproductive costs negatively impacted overwinter survival by reducing parasite resistance and body weight. This suggests a trade-off between current reproduction and future fitness, where resource allocation to offspring weakens immune defences against parasites.

Leivesley, J. A., Bussière, L. F., Pemberton, J. M., Pilkington, J. G., Wilson, K., & Hayward, A. D. (2019). Survival costs of reproduction are mediated by parasite infection in wild Soay sheep. Ecology Letters, 22(8), 1203–1213. Portico. https://doi.org/10.1111/ele.13275