Morphological plasticity of vertebrate aestivation

Aestivation or daily torpor is an adaptive tactic to survive hot and dry periods of low food availability, and has been documented for species of lungfishes, teleost fishes, amphibians, reptiles, birds, and mammals. Among these species, aestivation is characterized by inactivity and fasting, and for lungfishes and amphibians the formation of a cocoon around the body to retard water loss. While metabolic and physiological changes to aestivation have been well examined, few studies have explored the morphological responses of organs and tissues to aestivation. Predictably, inactive tissues such as skeletal muscles and those of the gastrointestinal tract would regress during aestivation, and thus aid in the reduction of metabolic rate. African lungfishes experience changes in the structure of their skin, gills, lungs, and heart during aestivation. For anurans, the group most thoroughly examined for morphological responses, aestivation generates significant decreases in gut mass and modification of the intestinal epithelium. Intestinal mucosal thickness, enterocyte size, and microvillus length of anurans are characteristically reduced during aestivation. We can surmise from laboratory studies on fasting reptiles, birds, and mammals that they likewise experience atrophy of their digestive tissues during torpor or aestivation. Aestivation-induced loss of tissue structure may be matched with a loss of cellular function generating an integrative decrease in tissue performance and metabolism. Ample opportunity exists to remedy the paucity of studies on the morphological plasticity of organs and tissues to aestivation and examine how such responses dictate tissue function during and immediately following aestivation.