Physiological Capacity for Estivation of the Sonoran Mud Turtle, Kinosternon sonoriense
Sonoran Mud Turtles (Kinosternon sonoriense) are widely believed to be restricted to permanent bodies of water, yet some populations apparently thrive in canyons that dry completely. We investigated the possibility that these turtles could estivate by subjecting individuals collected in the Peloncillo Mountains of New Mexico to 11 weeks without food or water. Turtles apparently used water stored in the urinary bladder to osmoregulate early in the period of deprivation. However, by the end, several blood chemistry parameters (including plasma osmolality, sodium and potassium concentrations, and blood urea nitrogen) had increased anhomeostatically to very high levels; most variables returned to original levels on rehydration. Hematocrit and plasma protein levels were unaffected, suggesting preferential defense of plasma volume by dehydrating turtles. Individuals varied in their burial behavior, and individuals that were surface active became more dehydrated and accumulated more urea than those that remained buried. We were unable to demonstrate metabolic depression or a decrease in evaporative water loss (EWL) during the trial, but rates of resting metabolism and EWL in K. sonoriense were similar to those of Oklahoma-collected K. flavescens (a species well known to estivate) treated identically. Kinosternon flavescens produced less nitrogenous waste than did K. sonoriense and may have been more uricotelic but also incurred more severe hypernatremia. All but one K. sonoriense recovered upon return to water. We conclude that Sonoran mud turtles from this population are physiologically capable of surviving many weeks without food and water. The concept of estivation as a special physiological state (as opposed to a behavior) is critically discussed.Abstract
Changes in body mass of kinosternid turtles subjected to food and water deprivation (starting at day 0), followed by rehydration: (A) individual body mass data; leftmost symbols are masses at first capture for Kinosternon sonoriense; (B) means for female K. sonoriense (inverted triangles), male K. sonoriense (triangles), and female K. flavescens (circles), standardized to the maximum body mass recorded for each individual; (C) mean values of a condition index (mass divided by carapace length raised to the power of 2.6; see Methods for explanation). Some symbols offset by two days for clarity. Error bars are ± 1 SE of the mean
Changes in (A) total osmotic concentration of blood plasma and voided urine, and blood plasma levels of (B) sodium and glucose, (C) calcium and potassium, and (D) cholesterol in kinosternid turtles during food and water deprivation, followed by rehydration. Symbols represent means ± 1 SE, as follows: circles = Kinosternon flavescens females; triangles = K. sonoriense males; inverted triangles = K. sonoriense females; diamonds = K. sonoriense sexes pooled; squares = all sexes and species pooled. Dotted line in (A) is an extrapolation to the pretrial sampling date (when no urine was obtained). Some symbols offset by two days for clarity
Changes in (A) blood urea nitrogen, (B) plasma uric acid, (C) total plasma protein and plasma albumin, and (D) hematocrit (packed cell volume, PCV) in kinosternid turtles during food and water deprivation, followed by rehydration. Symbols as in Figure 2
Rates of evaporative water loss measured in kinosternid turtles (A) before a period of food and water deprivation and (B) after. Symbols are values for individual turtles; diamonds = Kinosternon sonoriense; circles = K. flavescens. In (A), the allometric regression line (with 95% confidence limits) and equation are shown. In (B), filled symbols represent posttrial values, and hollow symbols represent pretrial values with regression line [i.e., the same data as (A)], with corresponding measurements for individuals connected by a dashed line. Note logarithmic coordinates and difference in scale
Metabolic rates (oxygen consumption) measured in putatively resting kinosternid turtles at 28 C, (A) before a period of food and water deprivation and (B) after. Symbols and format as in Figure 6
Total osmotic concentration of blood plasma plotted against total osmotic concentration of simultaneously collected voided urine. Horizontal dashed lines denote the range of plasma osmolalities measured in normally hydrated animals; diagonal dotted line represents isosmoticity. In (B), filled symbols represent three individuals from which urine samples were obtained during and after food and water deprivation (individuals represented by squares and triangles) or during and after deprivation and following rehydration (circles); arrows connect samples in chronological order
Effect of activity during deprivation period (index number increases with decreasing activity; see Results text for explanation) on plasma concentrations of total solutes (osmolality), sodium, potassium, and urea, measured in dehydrated turtles after a period of food and water deprivation. Symbols (for individual measurements) as in Figure 4. Spearman rank correlation coefficients are for Kinosternon sonoriense data only