NUTRITIONAL ECOLOGY OF DIMORPHIC HERBIVORES - DIGESTION AND PASSAGE RATES IN NUBIAN IBEX

We compared forage digestion and passage rates among three groups of N ubian ibex (Capra ibex nubiana) - mature males, non-lactating females, and lactating females - to test hypotheses relating intraspecific dig estive ability to body mass and reproduction costs, We hypothesized th at large males (60 kg) would exhibit longer forage retention times and more complete digestion of fermentable cell walls than adult females (23 kg). We tested these predictions by measuring digestion and retent ion of a grass hay and an alfalfa hay, forages that exhibited contrast ing rates and extents of cell wall digestion. Consistent with predicti ons, males retained both forages longer than non-lactating females. Ho wever, by substantially increasing gut fill, lactating females increas ed both intake and retention time with respect to non-lactating female s, Contrary to predictions, all three groups digested the grass (66% d igestible) and alfalfa hay (63%) equally well. Alfalfa cell wall was l ess digestible than that of grass hay (60% vs 69% digestible), and ret ention time of alfalfa was consistently, but not statistically signifi cantly, shorter. Fiber digestion was not correlated with retention tim e, emphasizing the ability of behavioral processes to modify digestion rate. We postulate that females achieved their greater digestion rate by masticating forages much more thoroughly than males.