In ecological modeling the interaction between a predator and its prey, is usually implemented as a linear or saturated function of the prey density. The main advantages of such a ''functional response'' are its simplicity, general applicability, and well understood mechanistic basis. In systems where predators compete directly for the available prey however, the functional response should depend not only on the prey density but also on the predator density. We aim here to devise a simple and generic ''predator-dependent'' functional response. We derive such a functional response by making quasi-steady-state assumptions for models, in which we allow predators and prey to form interaction complexes. We end up with the-previously proposed-''Beddington'' functional response. Because of our formal derivation this simple predator-dependent functional response is now based on clear mechanistic reasoning. The direct predator interference of this functional response emerges from the interaction between a predator and a prey, and not from direct predator-predator interactions. We conclude however that, although the Beddington functional response is generic for a two-dimensional system of one prey interacting with one predator, it is difficult to generalize it to higher dimensional systems consisting of several prey and predator species.7 1997 Academic Press Limited

In beagle dogs swimming, in contrast to treadmill running, was found to cause an increase in urine flow and urinary protein excretion. Renal blood flow measured by electromagnetic flow probes decreased by 13.0 +/- 4.9% when the treadmill gradient was 15% and arterial pressure was elevated by 11.6 +/- 4.9%. Immersion resulted in an immediate decrease in renal blood flow of 8.8 +/- 5.1% and a 24.6 +/- 6.9% increase in arterial pressure. Acid-base status indicated a respiratory alkalosis in all running experiments, no net change in five swimming experiments in which hyperventilation occurred, but a metabolic acidosis in eight swimming experiments without hyperventilation. During running there was a threefold increase in oxygen consumption. We conclude that swimming possibly induces more sympathetic nervous activity than treadmill running in dogs, while an alkalosis is consistently present during running, but acid-base response is variable during swimming.

The influence of splenectomy in the dog on plasma catecholamine levels and plasma renin activity during treadmill running and swimming was investigated. Plasma catecholamines were measured by a radioenzymatic assay and plasma renin activity by a radioimmunoassay. Exercise consistently increased plasma catecholamine levels before and after splenectomy (range of increase:3--38 pmol X ml-1). Swimming, however, was a stronger stimulus than running. No change in the ratio between noradrenaline and adrenaline was found. In intact dogs exercise results in a marked increase in hematocrit due to splenic contraction (range of increase 3--8 volume %), while renal blood flow and plasma renin activity remain virtually constant. In splenectomized dogs, exercise has been reported to induce a decrease in renal blood flow. In contrast to this known effect on renal blood flow, splenectomy did not affect plasma renin activity in treadmill running dogs. In swimming dogs, however, plasma renin activity was increased after splenectomy (range of increase 3.3--6.9 ng X Ang I X ml-1 X h-1). Possibly, a threshold in sympathetic tone is required to increase renin release in the dog.

Determination of the dog's splenic storage volume without surgical procedures requires measurement of both circulating and total red cell volumes. The estimation of circulating red cell volume by radioisotope techniques is impeded by the rapid uptake of tagged cells in the spleen. The circulating cell volume might be calculated from plasma volume and large vessel hematocrit, provided that the latter is corrected for the unequal distribution of red cells in the circulatory system. However, the correction factor can only be estimated in the splenectomized dog. We describe here a method to determine the factor in the intact dog, "physiologically splenectomized" by the severe exercise. The values obtained by this method slightly exceed those in the resting dog, as shown by studies in splenectomized exercising beagles in which splenic function was simulated by infusion of packed cells. The method was tested in beagles exercised by swimming and treadmill running and it was concluded that in the unanesthetized resting beagle about one-third of all erythrocytes is stored in the spleen. Labeled cells are equilibrated with about one-half of the splenic storage volume within 10 min after their injection. During maximal exertion the mean increase in large vessel hematocrit was 38.6 +/- 3.3%, the mean decrease in plasma volume 13.6 +/- 1.7% and the mean increase in plasma osmolarity 2.8 +/- 0.9% (percentages of control values).

1. The interaction between the effects of food restriction and cold stress on the development of body temperature, homeothermy index, metabolic rate and body weight were studied in Muscovy ducklings from hatching to 21 d of age. 2. The control group (ad libitum fed) and the food restricted group (fed to zero growth rate for 9 d) both became homeothermic when they were 2 d old with moderate (10 degrees C) cold stress. At severe cold stress (0 degrees C) the control group was homeothermic 5 days after hatching. However the food restricted group did not reach homeothermy at 0 degrees C and showed a large decrease (to hatching level) in homeothermy index at 10 degrees C and 0 degrees C after 9 d of food restriction. 3. Body temperature was lower in the food-restricted group during restriction and increased by 1 degree C after 24 h of ad libitum feeding. During food restriction, resting metabolic rate did not increase with age and was lower than the basal and existence metabolic rate predicted by Aschoff and Pohl (1970) and Kendeigh (1970) respectively. 4. The ratio of metabolisable energy (ME) intake to resting metabolic rate was 3 times lower in the food-restricted group than in the control group (0.09 and 0.27 respectively) on day 9. The availability of ME was more important than age for the development of thermoregulation in Muscovy ducklings. It is concluded that small improvements in the feeding regimen of young ducklings enhance the endurance and consequently reduce mortality from to environmental cold stress in a scavenger poultry system.