Illius, A.W., Albon, S.D., Pemberton, J.M., Gordon, I.J., and Clutton-Brock, T.H. (1995) Selection for foraging efficiency during a population crash in Soay sheep. Journal of Animal Ecology, 64 (4). pp. 481-492.

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Abstract

1. There are surprisingly few examples of the fitness consequences of variation in foraging traits, despite their importance for survival and reproduction. We studied the relationship between traits affecting foraging efficiency and individual fitness in Soay sheep on the island of St Kilda, Scotland.

2. Survival during an over-winter population die-off was related to variation in incisor arcade breadth, a morphometric trait that is known to influence food intake. Other morphometric measures, such as body dimensions and weight, were less well related to survival.

3. Individual survival over winter was also related to gastrointestinal parasite burden, estimated in faecal samples in the previous August. Incisor arcade breadth and parasite burden affect nutrient assimilation rate, energy balance and susceptibility to mortality through starvation. An energetic model developed to predict energy intake and expenditure from incisor arcade structure, body reserves and thermal balance was no better at predicting individual survival than incisor breadth alone.

4. Our results raise questions about stabilizing selection on incisor breadth and the effect of spatial heterogeneity in the vegetation on selection pressure in different home ranges. We discuss the way pleiotropic effects may operate through increased fitness of animals with narrow incisor arcades feeding more selectively when food is plentiful at low population density.

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