Foraging Responses of Kudus to Seasonal Changes in Food Resources: Elasticity in Constraints

Foraging behavior provides the mechanistic link between species perfor- mance and food resources in the environment. I document the foraging responses of a browsing ruminant, the kudu (Tragelaphus strepsiceros), to changes in food abundance and quality over the seasonal cycle and interpret the results in the context of optimal foraging theory. The study was carried out on hand-reared but free-ranging animals in a savanna region with summer wet season and winter dry season in Transvaal, South Africa. During the dry season, kudus expanded their diet to include evergreen and unpalatable deciduous woody species neglected during the wet season. The acceptability rating of woody plants was correlated with food value expressed in terms of a protein-condensed tannin index, relative to handling time during ingestion or digestion. However, dietary expansion alone was inadequate to satisfy the daily energy requirement during this period. The animals also increased the fraction of trees of palatable species that was accepted for feeding, extended feeding duration at feeding stations, and increased encounter rate with evergreen trees still retaining leaves. Both total time active and proportion of active time spent foraging increased over the dry season. Digestive capacity was apparently increased to accommodate a higher daily food intake, which compensated for reduced diet quality. Only in September at the end of the dry season, when little foliage remained, were compensatory adjustments inadequate to meet energy requirements. The functional response relating consumption rate to seasonal changes in food abundance showed little variation, despite a decline in foliage biomass by more than an order of magnitude. The kudus were neither energy maximizers, nor time minimizers, but rather targeted on their energy requirements with least overall cost. Findings demonstrate that the foraging time and digestive capacity constraints assumed in optimal diet models are somewhat elastic. Dietary predictions obtained using average parameter settings may be misleading, because constraints may become effective only under extreme conditions. Further research is needed to establish the costs associated with stretching physiological constraints towards their upper tolerance limits.