Water Regulatory Efficiency in Heteromyid Rodents: A Model and Its Application

Abstract

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 Dipodomys, 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 Perognathus, 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 progressive 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 demanding periods.

Kidney Mass and Relative Medullary Thickness of Rodents in Relation to Habitat, Body Size, and Phylogeny

“…This result is consistent with the finding of convergent trends in kidney structure, including a thick medulla relative to kidney size, that have been observed in a variety of lineages of small mammals from diverse desert habitats (e.g., rabbits [Heisinger and Breitenbach 1969], hedgehogs [Yaakobi and Shkolnik 1974], insectivorous bats [Geluso 1978], and heteromyid rodents [MacMillen and Hinds 1983]). However, as noted in the previous paragraph, maximum loop length alone does not determine variation in urine concentrating ability.…”

Seasonal acclimatization in water flux rate, urine osmolality and kidney water channels in free-living degus: molecular mechanisms, physiological processes and ecological implications

“…According to MacMillen and Hinds (1983), this view is probably correct for rodents that lose body mass when deprived of water and are kept on a dry diet. Under such conditions, rodents should exercise maximal capabilities of water conservation, reflected in maximal urine osmolalities.…”

Allometry of evaporative water loss in marsupials: implications of the effect of ambient relative humidity on the physiology of brushtail possums(Trichosurus vulpecula)

“…counter-current heat and water exchange structures in the nasal passages, and presence or absence of cutaneous sweat glands (e.g. Hulbert and Rose, 1972;Hulbert and Dawson, 1974;Dawson, 1969;Dawson et al, 1969;Dawson, 1973;MacMillen and Hinds, 1983;Bell et al, 1983;Hinds and MacMillen, 1986;Schmidt-Nielsen et al, 1970).…”