TRACY, C. R.*; DIAMOND, J.: Altitudinal variation in effects of hibernation and feeding on nutrient uptake in chuckwallas (Sauromalus obesus: Iguanidae)

We examined the effects of hibernation and fasting on intestinal glucose and proline uptake rates of chuckwallas. These lizards show dramatic geographic variation in body size and growth rate that parallels an altitudinal gradient in our study area. At low altitude, food is available only for a short time during the spring; at high altitude, food may also be available during summer and autumn, depending on rainfall conditions in a given year. We hypothesized that low-altitude lizards with a short season of food availability would show more pronounced regulation of gut size and function than high-altitude lizards with prolonged or bimodal food availability. Hibernating lizards from both areas had significantly lower uptake rates per milligram intestine for both nutrients, and they had lower small intestine mass than active lizards. This results in significantly lower total nutrient uptake in hibernating animals compared to active ones. Lizards from low altitudes with a short growing season also showed a greater increase in both uptake rates and small intestine mass when active versus when hibernating, compared to those from high altitudes with longer growing seasons. Thus, compared to those from long season areas, lizards from short season areas have equal uptake capacity during hibernation, but much higher uptake capacity while active and feeding. This pattern of regulation of gut function may be an adaptive response. In areas with a short season, those lizards which can extract nutrients quickly and then reduce the expensive gut will be favored; in areas where food may be available later in the year, those lizards that maintain a relatively active gut would be favored.