Effects of dehydration and heat stress on food intake and dry matter digestibility in East African ruminants.

Comparative investigations were made between wild and domestic ruminants from arid and semi-arid regions and those species from non-arid areas in an attempt to evaluate the adaptations of these ruminants in terms of the effects of heat stress and dehydration on food intake and digestibility. The effect of (a) an intermittent heat load (a daily light cycle of 12 h at 22 degrees C and 12 h at 40 degrees C) compared to 22 degrees C throughout the day and (b) dehydration level of 15% weight loss, with and without the heat load, on the intake and digestibility of a poor quality hay was investigated in the Grant's gazelle, Oryx, the domestic Turkana goats, fat-tailed sheep, zebu cattle, Thomson's gazelle and wildebeest. The intermittent heat load with water available ad libitum depressed the food intake of zebu cattle and Turkana goats by more than 40%. It had no significant effect on the food intake of the other species. The Thomson's and Grants gazelle, oryx, wildebeest and fat-tailed sheep appear well adapted to withstanding a periodic heat load. Dehydration at 22 degrees C caused a marked depression on food intake of all the species investigated. Dehydration together with a heat load caused no further reduction in the food intake by the Grants's gazelle, oryx, and goats but it did cause a further reduction in the intake in the other species. The small non-domestic ruminants (i.e. Grant's and Thomson's gazelle) appear much more digestive efficient than any of their domestic counterpart.

Digestive efficiency in two small, wild ruminants: the dik-dik and suni antelopes.

A comparative study, using six dik-dik and eight suni antelope, was undertaken to identify similarities and differences that may exist in the digestive process of these two small, East African ruminant browsers. The suni antelope was the more select feeder, preferring the native, Grewia sinilis leaves over that of lucerne hay. Daily forage consumption rate, per unity body weight, was greater in the dik-dik (40.4 g/kg) than for the suni (30.6 g/kg), while daily fluid intake was considerably less (i.e. dik-dik, 68 ml/kg versus suni, 106 ml/kg body weight). Rumen fermentation studies suggested that the suni antelope attained the more rapid rumen and caecal fermentation activities, when compared to the dik-dik antelope. The difference in ruminal and caecal digestive process of the two antelope is suggested to be partly the result of the dik-dik's arid-adaptation strategies of less fluid intake and a more diverse (less selective) browse consumption, relative to the suni antelope.

Moving cheaply: energetics of walking in the African elephant.

Large animals have a much better fuel economy than small ones, both when they rest and when they run. At rest, each gram of tissue of the largest land animal, the African elephant, consumes metabolic energy at 1/20 the rate of a mouse; using existing allometric relationships, we calculate that it should be able to carry 1 g of its tissue (or a load) for 1 km at 1/40 the cost for a mouse. These relationships between energetics and size are so consistent that they have been characterized as biological laws. The elephant has massive legs and lumbers along awkwardly, suggesting that it might expend more energy to move about than other animals. We find, however, that its energetic cost of locomotion is predicted remarkably well by the allometric relationships and is the lowest recorded for any living land animal.

Scanning electron microscope study of the morphology of the reptilian lung: the Savanna monitor lizard Varanus exanthematicus and the pancake tortoise Malacochersus tornieri.

The morphology of the lungs of two reptilian species, Varanus exanthematicus and Malacochersus tornieri, have been studied on gross preparations, latex casts, and critical-point-dried tissues. The shape of these lungs was observed to conform with that of the body, the lung of the monitor lizard (Varanus) being long and ovoid while that of the pancake tortoise (Malacochersus) was rounded and laterally indented. With respect to the size distribution of the gas exchange compartments, the lungs were observed to be notably heterogenous. In both species these units were generally smaller in diameter in the cranial region of the lung while those in the caudal region were larger. The gas exchange compartments in the tortoise were more profusely compartmented with the primary, secondary, and tertiary septa being well developed while in the lizard only the primary and secondary septa were observed. The tertiary septa in the tortoise lung and the secondary septa in that of the monitor lizard defined the terminal gas exchange units, the faveoli. The cast impressions closely resembled the actual lung tissue and convincingly revealed the hierarchical design of the gas exchange compartments as they radiate from the air chambers and ducts, terminally giving rise to the faveoli. This stratification clearly increases the surface area available for gas exchange in these lungs. Disparate refinements of the basic reptilian lung design, as noted here, may lead to differing anatomic pulmonary diffusing capacities for oxygen to which characteristics like energetics and mode of respiration in this taxon may be attributed.

Urea excretion as a strategy for survival in a fish living in a very alkaline environment.

Ammonia is toxic to all vertebrates. It can be converted to the less toxic urea, but this is a metabolically expensive process found only in terrestrial vertebrates that cannot readily excrete ammonia and marine fish that use urea as an osmotic filler. Freshwater fish mostly excrete ammonia with only a small quantity of urea. It seems the ornithine cycle for urea production has been suppressed in all freshwater teleosts except for some airbreathers which, when exposed to air, increase urea synthesis via the cycle. Here we show that the tilapia fish Oreochromis alcalicus grahami, the only fish living in Lake Magadi, an alkaline soda lake (pH = 9.6-10) in the Kenyan Rift Valley, excretes exclusively urea and has ornithine-urea cycle enzymes in its liver. A closely related species that lives in water at pH 7.1 lacks these enzymes and excretes mainly ammonia with small amounts of urea produced via uricolysis. It dies within 60 min when placed in water from Lake Magadi. We suggest that urea production via the ornithine-urea cycle permits O. a. grahami to survive the very alkaline conditions in Lake Magadi.

Studies on renal excretion of potassium in the dik-dik antelope.

1. In a study on the renal handling of potassium by the dik-dik antelope, plasma and urine samples were analysed for potassium, sodium and creatinine concentrations and osmolality during dehydration and intra-ruminal loading of potassium solutions. 2. The fractional excretion of potassium was 0.64 during the control period and rose up to as high as 2.3 during potassium loading. Urinary osmolality and potassium concentration decreased as the urine volume increased but the total amounts of potassium excreted were independent of urine volume. 3. Potassium loading led to a steady increase in its urinary excretion but a decrease in plasma potassium concentration was observed. This observation casts doubt on the hypothesis that alterations in potassium intake produce parallel alterations in plasma potassium concentration (which supposedly stimulates or depresses potassium excretion) and thereby maintain potassium homeostasis. 4. A possible alternative signal for increased potassium excretion following increased intake is discussed.

Physiology of the dik-dik antelope.

1. The East African dik-dik antelope represents a miniature model ruminant for comparative studies. 2. Dik-diks are browsers, consuming a diet consistently high in fermentable and digestable plant material. 3. Their foregut structure is designed for a relatively rapid passage of food and effective absorption of fermentative products. 4. Dik-dik antelopes are very economical in their use of water, having a low daily water exchange and excreting a highly concentrated urine. 5. Dik-diks have been observed to employ three thermoregulatory mechanisms; thermopanting, active sweating and a labile body temperature. 6. Their reproductive cycle is polyestrous with peak breeding in June and December, having a gestation period of between 170 and 174 days.

Renal excretion of urea in a small east African antelope, the dik-dik (Rhynchotragus kirkii).

1. A study on the renal handling of urea by the dik-dik antelope (Rhynchotragus kirkii) was conducted. 2. Plasma and urine samples were analysed for osmolality, urea and creatinine concentrations during dehydration and intra-ruminal loading of potassium and sodium solutions. 3. The glomerular filtration rate (GFR) of the dik-dik was found to be 182.6 +/- 11.7 ml/min/100 kg body mass. 4. Dehydration was observed to increase tubular urea reabsorption and increase plasma and urine osmolalities, but had no effect on the amount of urea filtered at the glomerulus. 5. Potassium loading increased both GFR and urine flow rate.

Energetic cost of carrying loads: have African women discovered an economic way?

When travelling in East Africa one is often surprised at the prodigious loads carried by the women of the area. It is not uncommon to see women of the Luo tribe carrying loads equivalent to 70% of their body mass balanced on the top of their heads (Fig. 1). Women of the Kikuyu tribe carry equally large loads supported by a strap across their foreheads; this frequently results in a permanently grooved skull. Recent experiments on running horses, humans, dogs and rats showed that the energy expended in carrying a load increased in direct proportion to the weight of the load for each animal at each speed, that is, carrying a load equal to 20% of body weight increased the rate of energy consumption by 20% (ref. 1). The purpose of the present study was to determine whether these African women use specialized mechanisms for carrying very large loads cheaply. We found that both the Luo and Kikuyu women could carry loads of up to 20% of their body weight without increasing their rate of energy consumption. For heavier loads there was a proportional increase in energy consumption, that is, a 30% load increased energy consumption by 10%, a 40% load by 20% and so on. We suggest that some element of training and/or anatomical change since childhood may allow these women to carry heavy loads economically.

Thermoregulation and heat balance in the dik-dik antelope (Rhynchotragus kirki): a field and laboratory study.

Experiments were conducted in the field to study the physiological responses of dik-dik antelope to direct solar radiation and shade. The results were compared to those obtained in the laboratory. The rates of metabolic heat production when the animals were exposed either to the sun or the shade were identical. Dik-dik antelopes lost about 50% more heat evaporatively when exposed to the sun compared to the shade at an ambient temperature (Ta) of 28 degrees C or a Ta of 40 degrees C in a climatic chamber. Heat storage in the laboratory at Ta 40 degrees C or at Ta 28 degrees C in the shade accounted for between 30 and 35% of the total heat production. The corresponding value in the sun was 55%. The net rate of heat gain under the sun was four times greater than under shade at 28 degrees C or in the laboratory at 40 degrees C. Behavioural mechanisms for avoidance of high insolation must constitute important adaptations that the dik-dik uses to avoid dehydration and dependence on drinking water in their natural environment.

Function of the lower intestine and osmoregulation in the ostrich: preliminary anatomical and physiological observations.

The gross anatomy of the lower intestine of the African ostrich (Struthio camelus) was investigated in four adult birds shot in the wild. The presence of 80 cm long paired caeca, and approximately 10 m of colon between ileum and the cloaca was confirmed. Urine, but not faeces, was found in the coprodeum. Retrograde flow into the colon was not observed. Samples of contents from the lower intestine were secured from these birds and the water content, osmolality and concentrations in the supernatant of short-chain fatty acids (SCFA), acetate, Na, K and Cl and pH were measured. In the caeca and the orad wide part (2-3 m) of the colon an avid production of SCFA takes place since the concentration of SCFA reached around 200 mM in these segments. As judged from a pronounced fall in the concentrations of Na and SCFA along the length of the colon, these ions are absorbed together with water. The water content falls from 92 to 67% (i.e. from 11.5 to 2.0 ml H2O/g dry matter). The mid-gut fermentation of carbohydrate is in agreement with the observation that the birds were feeding exclusively on Euphorbia heterochroma. This succulent plant (water content 87%) apparently makes the birds independent of surface water. Laboratory studies were performed on two captive male chicks. Hyperosmotic NaCl loading was unable to activate the nasal glands to secretion. Their ducts passed directly from the frontal/lacrimal bone to the nasal cavity. Dehydration confirmed a maximal osmolality of the cloacal urine of around 800 mosmol/l, the osmotic urine to plasma ratio being 2.5. Feeding of either a low- or a high-NaCl diet did not affect the transepithelial electrical potential difference of the coprodeal wall. It remained less than 5 mV, lumen negative.

The design and the role of the nasal passages in temperature regulation in the dik-dik antelope (Rhynchotragus kirkii) with observations on the carotid rete.

Exhaled air temperatures (Tex) and rectal temperatures (Trec) were measured in five dik-dik antelopes under controlled environmental temperatures (Ta) between 15 and 40 degrees C. In addition morphometric analysis of the nasal passages, gross and histological identification of the carotid rete were carried out in an attempt to illustrate a possible brain cooling system in this small ungulate. Below Ta of 30 degrees C, Tex decreased with decreasing Ta leading to the establishment of a temperature gradient of about 20 degrees C between Ta and Trec at a Ta of around 15 degrees C. At Ta of 30 degrees C Tex was approximately half a degree lower than the Trec. Gross and histological studies revealed the presence of carotid rete and profuse vascularisation of the nasal turbinates. Morphometric analysis established a mean volume density of the nasal passages (np) in the splanchnocranium (sp) (VV(np,sp], surface density (SV(np,sp] and harmonic mean width of the nasal passages (W) of 9.5%, 2.15 cm2/cm3 and 1.23 mm, respectively. Combined physiological and anatomical measurements suggest that the dik-dik may possess an efficient brain cooling and water conserving system. The design of the upper respiratory system in these antelopes may help these animals to tolerate the extremes of temperatures and insolation encountered in their habitat.

The mechanism of intracardiac shunting in the lizard Varanus exanthematicus.

Intracardiac shunting was studied in unanaesthetized and unrestrained specimens of Varanus exanthematicus by simultaneous injection of radioactively labelled microspheres (15 micron) into the right and left atria. Lung ventilation was monitored by intratracheal pneumotachography. It was found that intracardiac shunting was not significantly affected by the spontaneously occurring periods of ventilation and apnoea: the right-to-left shunt averaged 29 and 31%, respectively, and the left-to-right shunt was 11% in both conditions. The observed shunting, although rather constant with time and independent of the ventilatory state, varied in different individuals. Anatomical studies and intracardiac pressure measurements revealed that, in spite of crocodilian-like systolic pressure separation between pulmonary and systemic circulation (based on the muscular ridge, 'Muskelleiste', between cavum venosum and cavum pulmonale), the cavum venosum is shared by both the pulmonary and the systemic circulation. Intracardiac shunting appears to be mainly due to wash-out of the cavum venosum: blood remaining in this chamber at the end of systole (oxygenated) or at the end of diastole (deoxygenated) is washed into the respective 'inadequate' vascular bed during the next half-cycle of heart action. Thus the extent of intracardiac shunting is expected to depend primarily on the volume and the changes in volume of the cavum venosum during the cardiac cycle.

A comparative study of basal metabolism and thermoregulation in a folivorous (Colobus guereza) and an omnivorous (Cercopithecus mitis) primate species.

1. Abdominal temperatures (Tab) and oxygen consumption (VO2) were measured in two males each of colobus and Sykes monkeys. 2. Tab in both species had the same range (36 38.5 C), but there were marked differences in the daily rhythms. 3. Low ambient temperatures (Ta) had little effect on Tab; at Ta = 33.5 35.5 C. however, Tab rose quickly to above 40 C. 4. The thermoneutral zone (TNZ) extended from about 5 to 28 C in both species. 5. In the colobus monkeys the basal metabolic rate (BMR) was considerably lower than in the Sykes monkeys: 85% vs 113% of the value predicted from body mass.

Molar proportions of volatile fatty acids in the gastrointestinal tract of East African wild ruminants.

The molar proportions of seven individual VFA's were determined at select sites along the gastrointestinal tract of sixteen species of East African wild ruminants. The resulting data were statistically analyzed for species effect, and for effects due to major feeding groups (browsers, grazers, fresh grass grazers, etc.) and for body weight groups (5-750 kg animals). Present data suggest that body weight, rather than diet, is the more influential factor in reticulo-rumen fermentation rate, and in the molar proportion of fatty acids present. The molar proportions of VFA's observed in the mid and hindgut of these wild ruminants appeared more responsive to diet and body weight of the animal than did foregut VFA values.

Digestive physiology of East African wild ruminants.

The physiology of digestion of wild ruminants in their natural habitat was determined. The study consisted of fifty-one adult male animals representing sixteen species. While diet and body weight were both related to the gastrointestinal composition, body weight appeared to be the more influential factor. Reticulo-rumen and caecal-colon composition, relative to abomasal and small intestinal composition, showed the greater species, diet and body weight effects. The buffalo, oryx and gerenuk were somewhat more unique in gastrointestinal composition than were other species of wild ruminants, and were deserving of special consideration.

Respiration and metabolism in the giraffe.

Measurements have been made on respiration of three resting unstressed adult giraffe under normal conditions. Tracheal dimensions and body dimensions have also been measured in a large number of giraffe and other mammals. The results indicate that contrary to statements in the literature the giraffe does not have an abnormally large dead space, though the trachea is abnormally long and narrow. The respiratory measurements indicate that the giraffe breathes as predicted by published scaling equations, and at rest shows no abnormalities of rate or depth. The respiratory evaporative water loss is very small. Body temperature is labile with a range of at least 3.3 degrees C, and oxygen consumption, respiratory frequency, minute volume and respiratory evaporative water loss are all strongly correlated with body temperature.

Energetics and mechanics of terrestrial locomotion. I. Metabolic energy consumption as a function of speed and body size in birds and mammals.

This series of four papers investigates the link between the energetics and the mechanics of terrestrial locomotion. Two experimental variables are used throughout the study: speed and body size. Mass-specific metabolic rates of running animals can be varied by about tenfold using either variable. This first paper considers metabolic energy consumed during terrestrial locomotion. New data relating rate of oxygen consumption and speed are reported for: eight species of wild and domestic artiodactyls; seven species of carnivores; four species of primates; and one species of rodent. These are combined with previously published data to formulate a new allometric equation relating mass-specific rates of oxygen consumed (VO2/Mb) during locomotion at a constant speed to speed and body mass (based on data from 62 avian and mammalian species): VO2/Mb = 0.533 Mb-0.316.vg + 0.300 Mb-0.303 where VO2/Mb has the units ml O2 s-1 kg-1; Mb is in kg; and vg is in m s-1. This equation can be expressed in terms of mass-specific rates of energy consumption (Emetab/Mb) using the energetic equivalent of 1 ml O2 = 20.1 J because the contribution of anaerobic glycolysis was negligible: Emetab/Mb = 10.7 Mb-0.316.vg + 6.03 Mb-0.303 where Emetab/Mb has the units watts/kg. This new relationship applies equally well to bipeds and quadrupeds and differs little from the allometric equation reported 12 years ago by Taylor, Schmid-Nielsen & Raab (1970). Ninety per cent of the values calculated from this genera equation for the diverse assortment of avian and mammalian species included in this regression fall within 25% of the observed values at the middle of the speed range where measurements were made. This agreement is impressive when one considers that mass-specific rates of oxygen consumption differed by more than 1400% over this size range of animals.

The relationship between rate of oxygen consumption, heart rate and thermal conductance of the dik-dik antelope (Rhynchotragus kirkii) at various ambient temperatures.

1. The extent of cardiovascular adjustments to heat and cold were investigated between ambient temperatures of 5 and 45 degrees C by measuring conductance and the rates of oxygen consumption and heart beats. 2. Minimum heart rate was observed at 25 degrees C (114 +/- 9 beats/min). In the heat at 45 degrees C heart rate was observed to increase only slightly (127 +/- 12 beats/min) but in the cold -5 degrees C heart rate nearly doubled that at 25 degrees C. 3. Thermal conductance was on average 0.031 mlO2 (g. hr. degrees C)-1 below 25 degrees C but increased by more than 20 times at 40 degrees C. 4. A positive correlation between heart rate and rate of oxygen consumption was demonstrated below 25 degrees C and the relation may be of practical use.

Design of the mammalian respiratory system. II. Measuring maximum aerobic capacity.

We describe a simple and reproducible procedure for measuring maximal O2 consumption (Vo2max) that can be used for mammals spanning a wide range of body mass (Mb). Vo2 of trained animals was measured as a function of speed while they ran on a treadmill. Lactate concentration of the blood was determined at the beginning and end of the run. We considered that an animal had achieved Vo2max when Vo2 no longer increased with increasing tread-speed and the additional energy consumed by the muscles could be accounted for by anaerobic glycolysis. We found that Vo2 increased linearly with tread-speed up to a maximal rate and then remained unchanged with further increases in speed in nine species. When the animals ran at speeds faster than that where Vo2max was reached, they refused to continue when blood lactate concentrations reached 18-28 mmol . kg-1. Values for Vo2max obtained with this procedure were found to be about 20% greater than values obtained with the 'cold exposure' procedure for determining Vo2max. We conclude that our treadmill procedure is valid for determining the allometric relationship between Vo2max and Mb.