The use of tritiated water (TOH) to estimate total body water (TBW) and total water turnover rate was validated in chukar partridges (Alectoris chukar) and sand partridges (Ammoperdix heyi). For six chukar partridges weighing between 315 to 475 g, TOH equilibration with body fluids was less than 45 min after intramuscular or intravenous injections. Mean TOH space in eight chukar partridges was 99.8% of the mean TBW measured by desiccation with individuals ranging between 97.9 and 103.2%. TOH space best approximated TBW when TBW was calculated by using the mean body mass for a bird weighed at TOH injection and at TOH equilibration. Total water intake as estimated by TOH ranged between 90.7 and 113.3% of measured water intake in three sand and three chukar partridges, birds ranging in mass from 145 to 446 g. We conclude that the TOH method provides accurate estimations of TBW and water turnover rates in birds. For birds of up to 500-g body mass, we recommend 45 min for TOH equilibration with body fluids, and intramuscular injections of 0.05 muCi TOH/g body mass for TBW estimations and 0.1 muCi TOH/g body mass for water turnover estimations.

Summary1. Two immunological tactics, a constitutive response that is always present, and an induced response that is only employed after an invader has been recognized, have evolved in animals as a defence against parasites. 2. The energy requirements, body mass change and blood parameters of flea-parasitized and nonparasitized gerbils ( Gerbillus andersoni ) were measured and compared with published data for a close relative ( Gerbillus dasyurus ). G. andersoni possesses a constitutive immune response that could require additional maintenance costs, whereas G. dasyurus develops an immune response only after being attacked by fleas. We therefore predicted that G. andersoni has higher energy requirements than G. dasyurus when both species are parasite-free. However, we also predicted that the immunological 'readiness' of G. andersoni makes it less susceptible to flea infestation than G. dasyurus. 3. Energy requirements for maintenance were estimated by offering different levels of metabolizable energy to parasitized and nonparasitized animals and measuring changes in their body mass. At the end of the experiment, blood samples were taken from all rodents and haematological and biochemical analyses were done. 4. Adjusted energy requirements per unit body mass of nonparasitized G. andersoni were higher than those reported for G. dasyurus . Also, there was no difference between energy requirements, body mass change and blood parameters of parasitized and nonparasitized G. andersoni in contrast to the pronounced differences found for parasitized and nonparasitized G. dasyurus . However, parasitized G. andersoni did not have lower energy requirements than parasitized G. dasyurus as predicted from the second prediction. 5. The results illustrate the energy costs and benefits related to each immunological strategy and suggest that parasites can cause other nonenergetic costs to their hosts. The two strategies are most likely a reflection of interspecific differences in probability of flea attacks.

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ABSTRACT1. Energy costs of eating were determined from the increased rates of oxygen uptake by five steers aged 18 to 20 months and weighing 298 to 407 kg.2. Five diets were tested: pelleted concentrate (500 g barley grain per kg, 400 g lucerne meal per kg, 90 g soya bean meal per kg, and 10 g NaCl, trace mineral and vitamin supplement per kg); pelleted lucerne; lucerne hay; chopped-grass hay (700 g brome per kg, 300 g fescue per kg); and chopped fresh turnips. The dry-matter concentration of the pellets and hays was approximately 900 g per kg while the turnips contained only 140 g per kg.3. The rates of ingestion differed markedly between diets during the (15 to 50 min) twice-daily (morning and evening) feeding periods. On a dry-matter basis, the pellets were consumed most rapidly at a rate of 130 to 138 g per min, while the hays were consumed at approximately 38 g per min and the turnips at 30 g per min.4. The energy costs per min spent eating were similar for all rations (27·6 to 35·6 J/ kg live weight). However, because of different rates of ingestion, the energy costs per kg DM ingested were different: 222 to 238 J/kg live weight for the pelleted foods, 1029 J/kg live weight for the hays and 1427 J/kg live weight for the turnips.5. The energy cost of eating is more a function of time spent eating than a function of the amount of food ingested. Thus, rate of ingestion and duration of the meal are key factors in determining the energy cost of eating in cattle.

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