MCWILLIAMS, S.*; W. KARASOV; J. LEAFLOOR; E. CAVIEDES-VIDAL: Digestive adjustments in geese to reduced forage quality and its ecological implications.

When habitat quality on the breeding grounds is poor, arctic-nesting geese must eat foods of lower quality than their preferred foods. We tested the hypothesis that when geese eat foods of lower quality (i.e., higher fiber), (a) they eat more, (b) their digesta retention time and digestive efficiency does not change, (c) their total capacity for breakdown and absorption of nutrients increases, and (d) the mechanism responsible for the increase in total capacity will be an increase in amount of intestine rather than an increase in intestinal tissue-specific enzyme activity or nutrient transporter activity. Contrary to predictions, Canada geese fed lower quality food (50% fiber) had lower food intake, longer retention times, and larger foreguts compared to geese fed higher quality food (30% fiber). Digestive efficiency and tissue-specific enzyme and nutrient uptake activity did not change with forage quality. Patterns of specific uptake rates and enzyme hydrolysis rates suggest that geese feeding on relatively low quality diets maintain high rates of ileal uptake and thereby increase their integrated uptake capacity without the costs associated with increasing hindgut mass. In the wild, geese compensate for the reduction in food intake associated with reduced diet quality by increasing feeding time; however, there are limits to this strategy, especially when risk of predation or disturbance is high or when daylight is short. The increase in gut size associated with reduced diet quality may represent the primary digestive constraint that determines the lowest quality of food eaten by geese that must fly.