Differences in muscle fiber size and associated energetic costs in phylogenetically paired tropical and temperate birds.

Tropical and temperate birds provide a unique system to examine mechanistic consequences of life-history trade-offs at opposing ends of the pace-of-life spectrum; tropical birds tend to have a slow pace of life whereas temperate birds the opposite. Birds in the tropics have a lower whole-animal basal metabolic rate and peak metabolic rate, lower rates of reproduction, and longer survival than birds in temperate regions. Although skeletal muscle has a relatively low tissue-specific metabolism at rest, it makes up the largest fraction of body mass and therefore contributes more to basal metabolism than any other tissue. A principal property of muscle cells that influences their rate of metabolism is fiber size. The optimal fiber size hypothesis attempts to link whole-animal basal metabolic rate to the cost of maintaining muscle mass by stating that larger fibers may be metabolically cheaper to maintain since the surface area∶volume ratio (SA∶V) is reduced compared with smaller fibers and thus the amount of area to transport ions is also reduced. Because tropical birds have a reduced whole-organism metabolism, we hypothesized that they would have larger muscle fibers than temperate birds, given that larger muscle fibers have reduced energy demand from membrane Na(+)-K(+) pumps. Alternatively, smaller muscle fibers could result in a lower capacity for shivering and exercise. To test this idea, we examined muscle fiber size and Na(+)-K(+)-ATPase activity in 16 phylogenetically paired species of tropical and temperate birds. We found that 3 of the 16 paired comparisons indicated that tropical birds had significantly larger fibers, contrary to our hypothesis. Our data show that SA∶V is proportional to Na(+)-K(+)-ATPase activity in muscles of birds.

Energy expenditure for egg production in arthropod ectoparasites: the effect of host species.

We studied the energy cost of egg production in two flea species (Parapulex chephrenis and Xenopsylla ramesis) feeding on principal (Acomys cahirinus and Meriones crassus, respectively) and auxiliary (M. crassus and A. cahirinus, respectively) rodent hosts. We predicted that fleas feeding on principal as compared with auxiliary hosts will (a) expend less energy for egg production; (b) produce larger eggs and (c) live longer after oviposition. Both fleas produced more eggs and spent less energy per egg when exploiting principal hosts. Parapulex chephrenis produced larger eggs after exploiting auxiliary hosts, while the opposite was true for X. ramesis. After oviposition, P. chephrenis fed on the auxiliary hosts survived for a shorter time than those fed on the principal hosts, while in X. ramesis the survival time did not differ among hosts. Our results suggested that one of the proximate causes for lower reproductive performance and subsequent lower abundance of fleas on auxiliary hosts is the higher energy cost of egg production. However, in some species, lower offspring number may be compensated to some extent by their size, although this compensation may also compromise their future reproduction via decreased survival. In addition, the reproductive strategy of exploitation of low profitable (i.e. auxiliary) hosts may differ between flea species.

Physiological adjustments of sand gazelles (Gazella subgutturosa) to a boom-or-bust economy: standard fasting metabolic rate, total evaporative water loss, and changes in the sizes of organs during food and water restriction.

To test the hypothesis that desert ungulates adjust their physiology in response to long-term food and water restriction, we established three groups of sand gazelles (Gazella subgutturosa): one that was provided food and water (n = 6; CTRL) ad lib. for 4 mo, one that received ad lib. food and water for the same period but was deprived of food and water for the last 4.5 d (n = 6; EXPT(1)), and one that was exposed to 4 mo of progressive food and water restriction, an experimental regime designed to mimic conditions in a natural desert setting (n = 6; EXPT(2)). At the end of the 4-mo experiment, we measured standard fasting metabolic rate (SFMR) and total evaporative water loss (TEWL) of all sand gazelles and determined lean dry mass of organs of gazelles in CTRL and EXPT(2). Gazelles in CTRL had a mean SFMR of 2,524 +/- 194 kJ d(-1), whereas gazelles in EXPT(1) and EXPT(2) had SFMRs of 2,101+/- 232 and 1,365 +/- 182 kJ d(-1), respectively, values that differed significantly when we controlled for differences in body mass. Gazelles had TEWLs of 151.1 +/- 18.2, 138.5 +/- 17.53, and 98.4 +/- 27.2 g H(2)O d(-1) in CTRL, EXPT(1), and EXPT(2), respectively. For the latter group, mass-independent TEWL was 27.1% of the value for CTRL. We found that normally hydrated sand gazelles had a low mass-adjusted TEWL compared with other arid-zone ungulates: 13.6 g H(2)O kg(-0.898) d(-1), only 17.1% of allometric predictions, the lowest ever measured in an arid-zone ungulate. After 4 mo of progressive food and water restriction, dry lean mass of liver, heart, and muscle of gazelles in EXPT(2) was significantly less than that of these same organs in CTRL, even when we controlled for body mass decrease. Decreases in the dry lean mass of liver explained 70.4% of the variance of SFMR in food- and water-restricted gazelles. As oxygen demands decreased because of reduced organ sizes, gazelles lost less evaporative water, probably because of a decreased respiratory water loss.

Heterothermy and the water economy of free-living Arabian oryx (Oryx leucoryx).

To test the idea that large, free-living, desert ungulates use heterothermy to reduce water loss, we measured core body temperature (T(b)) of six free-ranging, adult Arabian oryx (Oryx leucoryx) during 2 years in the arid desert of west-central Saudi Arabia. We report the first case of heterothermy in a free-living ruminant in a desert environment: T(b) varied by 4.1+/-1.7 degrees C day(-1) during summer (June to September) and by 1.5+/-0.6 degrees C day (-1) during winter (November to March). Over both seasons, mean T(b) was 38.4+/-1.3 degrees C. During the day in both summer and winter, T(b) increased continually, suggesting that oryx store heat instead of dissipating it by evaporation, whereas at night T(b) decreased. The minimum T(b) was lower in summer (36.5+/-1.16 degrees C) than in winter (37.5+/-0.51 degrees C) despite the fact that the temperature gradient between T(b) and air temperature (T(a)) was larger and solar radiation was lower in winter. Throughout the year, daily variation in T(b) appeared to reflect thermal load (T(a,max)-T(a,min)) rather than an endogenous rhythm. Behavioural thermoregulation was used by oryx to cope with thermal stress during summer: animals lay down in shade in the morning shortly before T(a) exceeded T(b) and remained there until evening when T(b)-T(a) became positive. The use of heterothermy by oryx resulted in storage of 672.4 kJ day(-1) animal(-1) in summer and 258.6 kJ day(-1) animal(-1) in winter, if heat storage is based on calculations involving mean T(b). To dissipate this heat by evaporation would require 0.28 litres H(2)O day(-1) animal(-1) and 0.11 litres H(2)O day(-1) animal(-1) in summer and winter, respectively. Without heat storage in summer, we estimated that oryx would have to increase their water intake by 19%, a requirement that would be difficult to meet in their desert environment. If heat storage was calculated based on the daily change in T(b) rather than on heat storage above mean T(b) then we estimated that oryx saved 0.538 litres H(2)O day(-1) animal(-1) during summer.

Energy expenditure and water flux of Rüppell's foxes in Saudi Arabia.

Scattered populations of Rüppell's foxes (Vulpes rueppelli) occur across the deserts of northern Africa and Arabia. Little is known about the biology of these canids, especially the physiological mechanisms that contribute to their ability to live in such harsh environments. For individuals from Saudi Arabia, we tested the hypotheses that Rüppell's foxes have a reduced basal metabolic rate and total evaporative water loss (TEWL), parameters measured in the laboratory, and a reduced field metabolic rate (FMR) and water flux when free-living. Under basal conditions in the laboratory, males, which averaged 1,858 g in body mass, had an oxygen consumption of 914.9 mL O(2)/h, whereas females, which weighed on average 1,233 g, consumed 682.9 mL O(2)/h; rates of oxygen consumption translated to 441.4 kJ/d and 329.4 kJ/d, respectively. TEWL averaged 52.6 g H(2)O/d for males and 47.5 g H(2)O/d for females. We found no evidence that basal metabolism is reduced in Rüppell's foxes, but their TEWL was remarkably low: 50.9% of allometric prediction for males and 64.5% for females. In the wild during winter, males expended energy at a rate of 1,306.5 kJ/d, whereas females had an expenditure of 722.8 kJ/d. Analysis of covariance with FMR as the dependent variable, sex as a fixed factor, and body mass as a covariate showed no statistical difference in FMR between sexes. Water flux did not differ significantly between sexes and averaged 123 mL H(2)O/d, a value 30% lower than the kit fox from the deserts of southwestern North America. FMR was positively related to nocturnal activity levels as FMR (kJ/d) = -2,900.1+55.5 (% of time moving). The water content of prey items varied between 1.9 and 4.1 g H(2)O/g dry matter consumed. Based on these values and knowledge of their diet, we calculated that foxes captured about one rodent and a variety of anthropods per night of foraging.