AJP - Gastrointestinal and Liver Physiology, Vol 262, Issue 4 711-G718, Copyright © 1992 by American Physiological Society
Digestive modulation in a seasonal frugivore, the American robin (Turdus migratorius)
D. J. Levey and W. H. Karasov
Department of Zoology, University of Florida, Gainesville 32611.
American robins (Turdus migratorius) switch from eating fruits in the fall
to insects in the spring. Our objective was to determine the physiological
and morphological changes associated with such a switch. Three nonexclusive
hypotheses addressed possible mechanisms operating on different levels.
First, we hypothesized that birds on the two diets would differ in
intestinal absorption rates of sugars and amino acids. We predicted that
individuals on a high-protein low-carbohydrate insect diet would exhibit
higher proline and lower glucose absorption than individuals on a
low-protein high-carbohydrate fruit-based diet. Uptake rates of glucose and
proline were measured in vitro, using an everted sleeve technique. We found
no significant differences in uptake between the two groups and so rejected
the hypothesis. The second hypothesis, that retention time of digesta in
the gut changes with diet, was tested using an inert marker (polyethylene
glycol) injected into insects or fruit and fed to birds. We predicted and
found that the marker was excreted significantly faster in birds eating
fruit rather than insects. Thus this hypothesis is supported. The third
hypothesis focused on changes in gut morphology as a basis for shifts in
digestive processing. We found no differences in gut length, nominal
surface area, or volume and so rejected this hypothesis. Note that
hypotheses two and three are closely related, because retention time is
proportional to gut volume/digesta flow. Because gut volume did not differ
between the two groups, the difference in retention time must have been due
to a change in flow. Taken together, these results suggest that short
retention times are likely an important adaptation to frugivory.
