food-restricted turtles had not only smaller guts than ad libitum turtles in terms of mass and

length but also decreased intestinal surface area (Roark and Bjorndal, unpublished data). Because

I did not evaluate density of epithelial transporters, I cannot rule out the possibility that uptake

rates may have been enhanced in food-restricted turtles despite a significant reduction in

intestinal mass, as has been shown in food-restricted birds (Brzek and Konarzewski 2001).

 Alternatively, the enhanced FCE of R-AL turtles relative to AL turtles, especially in the

first few weeks after the switch to ad libitum feeding, may have resulted from decreased

metabolic expenditure. I did not quantify metabolic rates, but I did find that several major

visceral organs (e.g., liver, stomach, and intestine) in R turtles were smaller than those in AL

turtles. By down-sizing organs that would otherwise require disproportionate metabolic

expenditure to maintain them, R turtles may have been able to allocate less of their assimilated

food into maintenance metabolism and more into growth. Similar results have been obtained in

studies of fasted or food-restricted migratory birds (Lee et al. 2002, Karasov et al. 2004), which

had significantly smaller digestive and assimilatory organs compared to birds fed ad libitum. In

the intestines, this decrease in size was due largely to changes in the mucosal layer of the villi.

The observed decrease in organ mass in these birds was reversed by several days of feeding ad

libitum.

 Because I sampled only at times to, t5, and t12, I was unable to determine the time course

over which the organ sizes of R and R-AL turtles changed. Upon a return to ad libitum

conditions, R-AL turtles may have experienced a delay in up-regulation of visceral organ size.

The switch to ad libitum feeding at a time when maintenance expenditure was minimized would

have allowed for the rapid growth I documented in R-AL turtles during weeks 7 through 9 (Ali et

al. 2003).