Gut size flexibility in rodents: what we know, and don't know, after a century of research
Phenotypic plasticity comprises a central concept in the understanding of how organisms interact with their environment, and thus, is a central topic in ecology and evolution. A particular case of phenotypic plasticity is phenotypic flexibility, which refers to reversible change in organism traits due to changes in internal or external environmental conditions. Flexibility of digestive features has been analyzed for more than a century in a myriad of different species and contexts. Studies in rodents on gut size flexibility have been developed mainly from two different áreas of the biological sciences, physiology and ecology. However, as for several other topics related with physiological ecology, both kinds of studies largely developed along sepárate paths. Herein, I evaluate altogether the information belonging to both áreas. The major conclusions reached are: (1) there is a clear match between digestive morphology adjustments and change in environmental conditions, and gut size flexibility could be considered a widespread physiological mechanism oceurring in laboratory and wild species, and under laboratory, semi-natural and natural conditions. (2) For laboratory species, the experimental factors that have been more investigated are diet quality, reproductive status, environmental temperature and fasting, while for wild species the more analyzed factors are diet quality and temperature. (3) For wild rodent species, no differences in small intestine length flexibility between methodological approaches ñor species feeding categories has been identified. (4) It appears that high energetic demands are mainly coped with by changes at the small intestine level, while changes in the amount of undigestible material in the diet are mainly coped with by changes in the hindgut. (5) Change in gut length may be related to a decrease in food retention time (e.g., during diet dilution), while change in gut mass appears to be related to a need of higher specific absorption (e.g., during highly demanding periods). (6) The qualities of an energetic demand (e.g., its relative intensity) rather than simply its presence or absence can affect the amount of digestive flexibility. (7) Quantitative comparisons of the existing data are difficult due to several factors, such as the disparity of experimental treatments and differences in the types of data collected. At the end of this review, further directions for the study of digestive flexibility in rodents are presented.