Knowledge of the physiological mechanisms governing food processing has advanced rapidly, particularly in the growing body of information on the digestive physiology of nondomesticated animals. These studies have revealed physiological adaptations to specialized diets and the ability to adjust or modulate gut function. Alternating between feast and famine can cause gut modulation, but more subtle changes in diet composition also produce gut modulation.

The digestive systems of animals share many basic properties and characteristics, but each species appears to possess unique characteristics related to the resources constituting its own diet. Optimality modeling based on chemical reactor theory provides one approach to understanding the diversity of digestive strategies. This approach models the gut as a chemical reactor and uses reaction kinetics to derive predictions about gut processing, such as efficiency of absorption and changes in retention time.

Application of chemical reactor theory to digestion allows investigators to diagnose the configurations ofdigestive systems and digesta flow within them; to specify how the interplay of processing costs, reactant volumes, and reaction kinetics affects digestive system performance; and to design empirical tests of the predictions ofspecific models. However, improper accounting offoraging costs can lead our analyses to error. Similarly, omission of digestive costs may affect estimates of metabolic costs of foraging in ecological models. The failure to fully account for ecological and physiological costs will limit future progress.

Mathematical models incorporating both ecological and physiological parameters illustrate that both kinds ofprocesses influence harvest rate, diet selection, and the extent of gut filling. Although physiological processing capacity clearly limits foraging, theory suggests that guts are seldom full, suggesting further that spare capacity is an intrinsic feature of gut design. More importantly, this approach demonstrates the coadaptedness of ecological and physiological processes. We encourage and challenge others to further integrate ecological and physiological approaches.

5.6 Suggested Readings

Additional information on digestive structure and function is available for vertebrates (Karasov and Hume 1997), mammals (Chivers and Langer 1994), and invertebrates (Wright and Ahearn 1997). Warner (1981) provides a classic review of techniques for characterizing and measuring the passage of food through the digestive systems of birds and mammals. A discussion of ecological versus digestive constraints, which contrasts somewhat from the view of this chapter, and a comparison of chemical reactor and compartmental models of digestive systems can be found in Penry (1993). Starck (2003) provides a fascinating review of the cellular mechanisms underlying modulation of gut structure and function in mammals, birds, and snakes. All those with an interest in the physiological aspects of foraging will want to consult Starck and Wang (2005).

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