How physiology affects the distribution of animals and the patterns and processes by which physiological variation

patterns and processes by which physiological variation evolves are fundamental components of ecological physiology (see Feder and Block, 1991). Historically, water balance has been a major focus of investigations dealing with desert animals. Quantifying geographic variability in physiological variables related to desiccation is essential to understand the time frame through which adjustments act in dealing with a harsh environment and to determine the origins of potential variation in these physiological variables. Adjustments to changing conditions may take place through three nonexclusive modes. The first is acclimational responses that allow individuals to change their physiological characteristics in response to environmental changes. A second mode of response is ontogenetic lability. Developmental plasticity can define individual physiological capacities early during development and can have consequences for the survival of individuals later in their ontogenies. For example, postweaning water restriction causes kidney hypertrophy in domestic (Blount and Blount, 1968) and desert-adapted rodent species (Hewitt, 1981; Buffenstein and Jarvis, 1985) and results in an elevated capacity to concentrate urine. Finally, it is possible that physiological changes relating to desiccation occur at the level of natural selection, whereby populations exhibit adaptation to local conditions. Within the North American deserts, free-standing water is not readily available to rodents such as Merriam’s kangaroo rat (Dipodomys merriami), and water input is restricted to metabolic production from the catabolism of foodstuffs and preformed input from the diet. Consequently, water losses must be minimized. Routes of water loss include urine production, fecal water loss, lactation and evaporation (either ventilatory or insensible cutaneous evaporative water loss, as these species lack sweat glands on all but their feet). One subspecies of Merriam’s kangaroo rat, Dipodomys merriami merriami, ranges from areas of extreme aridity and temperature in southwestern Arizona to milder areas in central and northwestern Arizona (Hoffmeister, 1986; Schmidly et al., 1993; Turner and Brown, 1994). In a recent study of this subspecies, R. L. Tracy and G. E. Walsberg (in preparation) found that, while fecal and urinary loss did not vary within this subspecies, evaporative water loss varied among individuals from locations of differing aridity. Evaporative water loss was significantly lower among individuals from more xeric areas than among those from the more mesic areas. These differences are important because evaporative loss typically accounts for 75 % of the total water loss from this species (Schmidt-Nielsen and Schmidt-Nielsen, 1952). Two significant questions remain unaddressed regarding this dominant mode of water loss. The first is the source of the intraspecific variation observed, i.e. whether it reflects acclimation, developmental plasticity or genetically fixed 773 The Journal of Experimental Biology 203, 773–781 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2322