Evaluating optimal diet models for an African browsing ruminant, the kudu: How constraining are the assumed constraints?

Abstract

The linear programming model (LPM) of Belovsky (1978, 1986) and modifications of the classical or contingency model incorporating a digestive constraint (CM) were tested using foraging data recorded for kudus (Tragelaphus strepsiceros) browsing savanna vegetation over the late wet season. Food choice was between the herbaceous and woody plant components for LPM and among plant species or categories for CM. The constraints considered were consumption (cropping) rate, foraging time and digestive capacity. Woody communities dominated byBurkea andAcacia represented alternative habitat types. Following a minor adjustment, LPM represented the overall average diet and predicted the dietary differences between habitat types. However, the kudus failed to respond dietarily to variations among days and foraging sessions (meals) in the parameters constraining intake. The kudus accepted a wider dietary range than predicted to be optimal by CM. Evidence suggested that neither foraging time, nor digestive capacity, formed an effective constraint under the study conditions. Thermal tolerance and gut space may become limiting only towards the extremes of environmental variability that animals experience. LPM is vulnerable to circularity if average parameter values are used to estimate constraint settings. The energy maximizer—time minimizer dichotomy fails to take into account the fitness consequences of alternative foraging responses. CM is less cryptic in its application than LPM and so has greater heuristic value, despite its predictive failures. However, there may be no consistent ranking of food types where multiple constraints that are variable in their effectiveness apply. Dynamic programming models offer a solution, but pose a formidable challenge in complex natural environments.