Physiological responses to both natural and anthropogenic perturbations involve changes in glucose use and metabolism to enable animals to respond to challenges. The responses are at least partially mediated via the glucocorticoids, and the proper functioning of these systems are critical to animal health. Prolonged high-sugar diets in humans can compromise physiological functions and lead to a range of diseases including diabetes. Such outcomes are less common in natural, non-human systems, likely because of evolutionarily based balances between sugar intake and the ability to process it. Due to both landscape changes and human feeding of wildlife, the introduction of unnatural diets to wildlife is becoming increasingly common. However, the implications of unnatural dietary changes, and in particular changes in dietary glucose, are not well understood in non-human systems. We tested the effects of a high glucose diet on glucose tolerance in a controlled setting using captive juvenile Green Iguanas (Iguana iguana). In the field, we similarly performed glucose challenges (i.e., glucose tolerance tests) on Northern Bahamian Rock Iguanas (Cyclura cychlura) that are either fed or unfed by tourists, and we evaluated both short (within 8 hrs) and longer-term (within 19 hrs) blood glucose responses as well as plasma corticosterone (CORT) concentrations. Overall, we found significant effects of both experimental glucose supplementation in the laboratory and tourist feeding in the wild on glucose metabolism. In all instances where iguanas received a glucose-rich diet, we found greater acute increases in blood glucose following a glucose challenge. In the wild, tourist-fed iguanas, relative to unfed iguanas, had significantly lower baseline CORT levels, higher baseline blood glucose levels, and slower returns to baseline glucose levels following a glucose challenge. Therefore, glucose intolerance in response to unnatural glucose consumption is manifesting in both laboratory animals with relatively short-term glucose treatment and free-living iguanas exposed to long-term supplemental feeding by tourists. We also found more rapid increases in CORT response to capture and processing in tourist-fed iguanas and that CORT was positively related to blood glucose one-hour post glucose challenge, but only in tourist exposed and fed iguanas. Based on these results and the increasing prevalence of wildlife feeding on unnatural diets, further research is needed to understand how dietary changes affect glucose metabolism broadly across species and whether such changes cause any health ramifications.

Author ORCID Identifier

Susannah French

Document Type




File Format


Publication Date



NSF, Division of Integrative Organismal Systems (IOS)


Utah State University

Award Number

NSF, Division of Integrative Organismal Systems (IOS) 1752908

Award Title

Collaborative Research: The interplay between host diet, immunity, reproduction, and the microbiome across an anthropogenic-disturbed landscape


Physiological and morphological metrics of green and rock iguanas

Scientfic Names

Iguana iguana, Cyclura cychlura

Referenced by

Susannah S. French, Spencer B. Hudson, Alison C. Webb, Charles R. Knapp, Emily E. Virgin, Geoffrey D. Smith, Erin L. Lewis, John B. Iverson, Dale F. DeNardo; Glucose tolerance of iguanas is affected by high-sugar diets in the lab and supplemental feeding by ecotourists in the wild. J Exp Biol 15 April 2022; 225 (8): jeb243932. doi:



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