David S. Hinds

WATER REGULATORY EFFICIENCY IN HETEROMYID RODENTS: A MODEL AND ITS APPLICATION'

We have developed and tested a model of water regulatory efficiency for the rodent family Heteromyidae. The model is based on the premise that granivory coupled with increasing aridity and seasonality of rainfall has been the major theme throughout the evolutionary history of the family; it states further that water regulatory efficiency is negatively correlated with body mass in the family, and that member species are largely dependent upon a common resource packet (seeds) to meet both energy and water needs. A test of the model with five genera and 13 species revealed positive results across the entire family, but the two most speciose genera, Perognathus and Dipod- omnys, showed seemingly conflicting patterns: the smaller (in mass) genus, Perognathus, conformed strongly to the model, while in Dipodomys water regulatory efficiency was fixed at a level equivalent to that of the largest Perognathus spp. (-35-40 g). At this mass in these genera, in addition, occurs the demarcation between strict quadrupedality (Perognathus) and bipedality (Dipodomys), a strict scaling of cheek pouch volume with mass in Perognathus but independence of mass in Dipodomys, and the common use of torpor in Perognatlhus, with its near absence in Dipodomys. We interpret our findings to indicate that initially selection favored a decrease in mass with a concomitant increase in water regulatory efficiency and reduction in absolute energy need in heteromyids, tracking pro- gressive aridity during the Tertiary. In Dipodomys, the option of bipedality was adopted, apparently freeing them from energetic constraints imposed strictly by mass, coupled with an intermediate and fixed level of water regulatory efficiency that dictates use of seeds with high metabolic water yields. The quadrupedal Perognathus have retained a mass-specific water regulatory efficiency, ensuring maintenance of both water and energy balance on a broad array (with respect to protein: lipid: carbohydrate and metabolic water yield) of seeds, when their more-limited locomotor powers are consistent with seed availability; torpor is the tradeoff, enhancing survival during energetically de- manding periods.

Scaling of Respiratory Variables and the Breathing Pattern in Birds: An Allometric and Phylogenetic Approach

Allometric equations can be useful in comparative physiology in a number of ways, not the least of which include assessing whether a particular species deviates from the norm for its size and phylogenetic group with respect to some specific physiological process or determining how differences in design among groups may be reflected in differences in function. The allometric equations for respiratory variables in birds were developed 30 yr ago by Lasiewski and Calder and presented as “preliminary” because they were based on a small number of species. With the expanded data base now available to reconstruct these allometries and the call for taking account of the nonindependence of species in this process through a phylogenetically independent contrasts (PIC) approach, we have developed new allometric equations for respiratory variables in birds using both the traditional and PIC approaches. On the whole, the new equations agree with the old ones with only minor changes in the coefficients, and the primary difference between the traditional and PIC approaches is in the broader confidence intervals given by the latter. We confirm the lower V̇e/V̇o2 ratio for birds compared to mammals and observe a common scaling of inspiratory flow and oxygen consumption for birds as has been reported for mammals. Use of allometrics and comparisons among avian groups are also discussed.

Scaling of Energy Metabolism and Evaporative Water Loss in Heteromyid Rodents

Oxygen consumption (V̇o2), evaporative water loss (ṁwe), and body temperature (Tb) of 117 individuals of 13 species of heteromyid rodents were determined at ambient air temperatures (Ta) of 5, 15, 25, and 35 C; the 13 species encompass all five genera in the family Heteromyidae, cover the range in habitat distribution, and vary in body mass from 8 to 130 g. The relationship between V̇o2 and Ta generally follows the Scholander model in the 13 species, although the three nonxeric species (Heteromys desmarestianus, Liomys irroratus, L. salvini) exhibit a thermoneutral point, not a zone, within the Tas measured. Thermal conductances, ṁwe, and Tb do not vary between Tas of 5-25 C but do increase at 35 C, except for ṁwe in three Perognathus spp. The allometric relationships of mass-specific V̇o2, ṁwe, and thermal conductances to body mass have similar slopes relative to other eutherian mammals; however, the relationships in heteromyids are depressed below those of other eutherians at all temperatures. In general, these physiological variables are depressed in the semixeric-and xeric-dwelling heteromyid species but are similar to those predicted for other eutherians of the same body mass in the three nonxeric heteromyid species. The reduced levels of metabolism and evaporative water loss are considered adaptive for energy and water conservation linked to the familys dietary specialism, granivory.

Maximum Metabolism and Aerobic Capacity in Heteromyid and Other Rodents

Minimum and cold-induced maximum metabolisms were measured in six heteromyid and one cricetine species of rodents. Minimum and cold-induced maximum metabolisms have similar slopes, and maximum metabolism is significantly elevated over minimum metabolism by approximately 65 times at 45 g. Minimum metabolism adjusted for body mass is statistically lower for heteromyids than for other rodents. The aerobic capacity model for the evolution of endothermy would predict that since heteromyids have a low minimum metabolism they should also have a low maximum metabolism. Such is not the case in the relatively few species examined. The aerobic capacity model also predicts that mass-independent minimum and maximum metabolic rates will be correlated. This is true for the species we measured here when maximum was induced using cold. The correlation is also significant for rodents in general regardless of whether maximum metabolism was elicited with cold or exercise. Cold-induced maximum metabolism is significantly lower than exercise-induced maximum metabolism in rodents.

Temperature Regulation of the Pyrrhuloxia and the Arizona Cardinal

Both the pyrrhuloxia (Cardinalis sinuata) and the cardinal (C. cardinalis) occur sympatrically in the Sonoran Desert (nomenclature of Mayr and Amadon 1951; Mayr and Short 1970). These are not true desert species as defined by Lowe (1968), but they are abundant in riparian and semiriparian communities of the lower Sonoran life zone. In Arizona the cardinal occurs farther to the north while the pyrrhuloxia ranges farther into the hotter, more arid western portion of the state. In southeastern Arizona where the two species overlap, they use the same environment in much the same way, but the cardinal is more typically a bird of the riparian thickets while the pyrrhuloxia occupies the mesquite edge and apparently prefers a more arid habitat (Peterson 1941, p. 240; Gould 1961; Bent et al. 1968). Thus, both are subject to high air temperatures and intense solar radiation, but the pyrrhuloxia tolerates the hotter and more arid portion of the environment continuum. These biological considerations suggested a study of the physiological responses to high temperatures of these two closely related species, to see if any

Scaling of Evaporative Water Loss in Marsupials

Evaporative water loss was determined at ambient air temperatures of 10-35 C in 10 species of dasyurid marsupials ranging in body mass from 9 to 900 g. Mass-specific evaporative water loss (ṁwe, mg H₂O/g·h) of these marsupials is unrelated to ambient temperatures (Ta) at and below 30 C and is inversely related to body mass (g) by the relationship

Acclimatization of Thermoregulation in the Desert Cottontail, Sylvilagus audubonii

\dot{m}_{we} = 13.157 M_{b}^{-0.423}

Energy Scaling in Marsupials and Eutherians

. At 32.5 and 35 C, the elevations of the allometric relationships of ṁwe to Mb are 24% and 51% higher than at 10-30 C. Data taken from the literature at Tas ≤ 30 C show that nondasyurid marsupials have a similar allometric relationship, and the relationship for evaporative water loss in all marsupials is