Selection for foraging efficiency during a population crash in Soay sheep\ud

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.\ud \ud 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.\ud \ud 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.\ud \ud 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

Feedback effects of chronic browsing on life-history traits of a large herbivore

1 Increasing ungulate populations are affecting vegetation negatively in many areas, but few studies have assessed the long-term effects of overbrowsing on individual life-history traits of ungulates. 2 Using an insular population of white-tailed deer (Odocoileus virginianus Zimmermann; Anticosti, Québec, Canada) introduced in 1896, and whose density has remained high since the first evidence of severe browsing in the 1930s, we investigated potential feedbacks of long-term and heavy browsing on deer life-history traits. 3 We assessed whether chronic browsing contributed to a decline of the quality of deer diet in early autumn during the last 25 years, and evaluated the impacts of reduced diet quality on deer body condition and reproduction. 4 Rumen nitrogen content declined 22% between two time periods, 1977–79 and 2002–04, indicating a reduction in diet quality. 5 After accounting for the effects of year within the time period, age and date of harvest in autumn, peak body mass of both sexes declined between the two time periods. At the end of November, males were on average 12% heavier and adult does 6% heavier in 1977–79 than in 2002–04. Hind foot length did not vary between time periods. 6 The probability of conception increased 15% between the two time periods, but litter size at ovulation declined 7%, resulting in a similar total number of ovulations in 2002–04 and in 1977–79. 7 Our results suggest that following a decline in diet quality, white-tailed deer females modified their life-history strategies to maintain reproduction at the expense of growth. 8 Deer appear to tolerate drastic reductions in diet quality by modifying their life history traits, such as body mass and reproduction, before a reduction in density is observed. Such modifications may contribute to maintain high population density of large herbivores following population irruption