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-and-error process of selecting lower temperatures, which are favourable for the reduction of metabolism and therefore of the fre- quency of arousals. Arousal and movement involve an enormous in- crease in energy expenditure: this seems to be worth-while in species which feed in winter in milder climates, e.g., in

In: Netherlands Journal of Zoology

system has to fulfil one function or when one function pre- 124 dominates, the second strategy is applicable. Starting from the real function, a model is constructed that functions with least cost of energy or material (RASHEVSKY, 1960). The optimized model is then compared with the real structure

In: Netherlands Journal of Zoology

scales isometrically. REISS (1989) has suggested a related model of the form: energy avail- able for growth = energy assimilated - energy required for average daily metabolic rate. Empirical data show that both energy assimilated and metabolic rate usually scale allometrically with weight. Whatever the

In: Netherlands Journal of Zoology

only a mere 37 % . The touch-hunting/eye-hunting ratio is 10:1 for C. alpina and 7:1 for C. alba, while C. maritima has a completely different ratio: 1:2. The prolonged probing in the touch-hunting technique obviously requires more energy per unit of time than the more extensive probing in eye

In: Netherlands Journal of Zoology

tip, but also lateral to the tip, building up less com- pression ahead of the tip and allowing deeper penetration while using the same expenditure of energy. An even more pointed shape builds up little compression ahead of the tip, and the penetration energy required depends upon the friction between

In: Netherlands Journal of Zoology

high rate of growth, above all in the females and dur- ing the initial months of life. Together with a high reproductive effort (FERNANDEZ-DELGADO et al., 1988, see below), this leads to a consider- able expenditure of energy which can not be maintained for a long time. This was reflected in the 1983

In: Netherlands Journal of Zoology

is indicated by o. respect to the energy that may have to be expended to obtain deforma- tion of the muscle perpendicular to the direction of muscle force exer- tion. As a result, architectural efficiency of muscles (i. e. the maximal external work performed by the muscle force divided by the

In: Netherlands Journal of Zoology

in the homeo- thermic state, when involved in maintaining such minimal temper- ature differences with the environment (e.g., HENSHAW, 1968). It has long been realized that it is this reduction in energy expenditure, that endows survival value on the whole complicated set of physiological and

In: Netherlands Journal of Zoology

schistosomes in the snail host resemble aspecific stress responses in mammals, e.g. upon a bacterial challenge. KEY WORDS: schistosomin, cytokinine-like factor, neuroendocrine system, stress responses, schistosomes, snail host. INTRODUC'1'ION In order to obtain energy and space it has been shown that Tri

In: Netherlands Journal of Zoology

amount of the length changes that occur during muscle movement. They can absorb and release elas- tic strain energy which increases the efficiency during energetically costly movements such as in jumping (ALEXANDER & VERNON, 1975; CAVAGNA, 1977). HOFFER et al. (1989) and GRIFFITHS (1991) have shown that

In: Netherlands Journal of Zoology