Even among vertebrate species of the same body mass and higherlevel taxonomic group, metabolic rates exhibit substantial differences, for which diverse explanatory factors-such as dietary energy content, latitude, altitude, temperature, and rainfall-have been postulated. A unifying underlying factor could be food availability, in turn controlled by net primary productivity (NPP) of the animal's natural environment. We tested this possibility by studying five North American species of Peromyscus mice, all of them similar in diet (generalist omnivores) and in gut morphology but differing by factors of up to 13 in NPP of their habitat of origin. We maintained breeding colonies of all five species in the laboratory under identical conditions and consuming identical diets. Basal metabolic rate (BMR) and daily ad libitum food intake both increased with NPP, which explained 88% and 90% of their variances, respectively. High-metabolism mouse species from high-NPP environments were behaviorally more active than were lowmetabolism species from low-NPP environments. Intestinal glucose uptake capacity also increased with NPP (and with BMR and food intake), because species of high-NPP environments had larger small intestines and higher uptake rates. For metabolic rates of our five species, the driving environmental variable is environmental productivity itself (and hence food availability), rather than temporal variability of productivity. Thus, species that have evolved in the presence of abundant food run their metabolism ''fast,'' both while active and while idling, as compared with species of less productive environments, even when all species are given access to unlimited food.M etabolic rate means the rate at which an animal burns calories to produce energy. Among vertebrate species, there is a 10 7 -fold range of metabolic rates. Much research has aimed at understanding the reasons behind this variation. Why are some animals seemingly extravagant, consuming and expending calories rapidly, whereas others are frugal, consuming very little and metering their expenditure accordingly so as to remain in energy balance?The two factors underlying most of this variation are well known: body mass and higher-level taxonomic affiliation. First, within the same higher-level taxonomic group (e.g., mammals), metabolic rate increases with body mass (the so-called mouseto-elephant curve) according to approximately the 0.75 power of body mass (1). (Metabolic rate per g of body mass decreases with body mass but by a power smaller than 1.0, so the absolute metabolic rate of a whole animal, the subject of this paper, increases with mass). Second, for species of the same mass there are some differences between higher-level taxonomic groups, notably the ca. 8-fold difference between endotherms (''warmblooded'' birds and mammals) and ectotherms (''cold-blooded'' reptiles, amphibia, and fish) (2), and also the ca. 1.3-fold difference between marsupials and placentals (3) and the ca. 2-fold difference between sloths and other placentals (4).Ho...

This review summarizes and contrasts published information on the proximate chemical composition of native fruits and leaves eaten by duikers, and adds new data concerning the soluble carbohydrate, mineral, and tannin content of duiker foods. Information on circulating plasma concentrations of minerals and the fatsoluble vitamins A and E is also summarized in relation to free-ranging vs. zooheld duikers and differences in the nutrient content of their respective diets. In nature, duikers consume diets containing low starch (in general <0.1% of dry matter (DM)) and moderate-to-high fiber (averaging 32% DM) and protein (fruits <10%, leaves <20% DM) levels, and do not avoid foodstuffs containing tannins. Native food samples were considered low to marginal in calcium (Ca) and phosphorus (P) content compared with domestic ruminant dietary requirements, but Ca levels were generally two-to threefold higher than P concentrations. Diets fed to duikers in captivity are often low in fiber and tannin content, and high in starch and protein, as well as low in Ca relative to P (compared with native plants), which may lead to nutrient imbalances and health problems. Plasma alpha-tocopherol values measured in zoo duikers ranged from 0.4 to 8.3 µg/ml, and were less variable in free-ranging animals (1.7-7.0 µg/ml). Retinol values did not differ among duiker species or between sample populations, and were within normal ranges (0.2-0.7 µg/ml) expected for herbivores. Domestic hoofstock appear to be suitable physiological models for evaluating fat-soluble vitamin status, but mineral homeostasis appears to differ in duikers. Free-ranging duikers have somewhat higher Ca, and lower P concentrations compared with captive-held individuals, and inverse Ca:P ratios and copper (Cu) deficiency have been reported in captive duikers-both conditions in which captive diet may

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Summary The metabolic consequences of submaximal exercise following long term nutritional deprivation were investigated in 6 donkeys. Animals were fed all roughage diets, either adequate (Timothy hay: H) or deficient in energy and protein (wheat straw: S) in a crossover design. After 8–10 weeks of dietary adaptation, responses to 60 min of moderate intensity draft loading exercise were compared. Jugular blood was sampled at rest and during exercise for glucose, lactate and free fatty acids (FFA) concentrations, haematocrit, Cortisol and insulin levels. Heart rate (HR) was monitored electronically. Resting lactate concentration was higher in animals fed hay (H) than in animals fed straw (S); only in S animals, however, did post exercise lactate concentration (1.55 ± 0.9 mmol/l) exceed pre‐exercise lactate concentration (0.6 ± 0.1 mmol/l). Hypoglycemia occurred in all animals in early exercise but was more pronounced in S animals. Straw‐fed animals had higher FFA concentrations at rest than H animals (mean ± s.e. 433 ± 71 vs. 102 ± 9 μmol/l). Plasma FFA concentrations declined precipitously in S animals at the onset of exercise, thereafter FFA increased over resting concentrations in both diets. Cortisol concentrations were higher at all times in H animals and increased during exercise regardless of diet. Insulin was lower in S animals and decreased during exercise in both diet groups. Resting HR was lower in S than in H animals (mean ± s.e. 34 ± 2 vs. 45 ± 2 beats/min). Exercising HR was greater in S animals. These data suggest that long term undernutrition in donkeys leads to decreased carbohydrate reserves, diminished maximal aerobic capacity and endocrine alterations. Work difficulty and stress level may increase, with possible negative implications for animal health and endurance.

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