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About 2 weeks after metamorphosis, midwife toads Alytes obstetricans judge the size of a prey object mainly in scales of visual angle. For distant objects they show a tendency to respond to absolute object size. About 6 months later the juvenile animals are able to judge absolute prey size in relation to the stimulus distance (''size constancy'' phenomenon). This ability is not necessarily based on learning or previous experience.

The aim of the present study is to answer the following questions: (1) Is the ability of toads to distinguish between different configurational prey stimuli based on learning during ontogeny? (2) Is the configurational meaning of a stimulus influenced by its distance from the animal? (3) Are there differences in configurational prey selection among different anuran species? (1) Experiments with postmetamorphic juveniles of Alytes obstetricans indicate that preferences for worm-like objects can be obtained immediately after metamorphosis. This preference undergoes some specialization during ontogeny. (2) The configurational meaning of ''worm-like'' and ‘antiworm-like’ objects is – within broad ranges – independent of the distance from the animal so far as has been investigated in A. obstetricans and Bufo bufo. (3) The configurational discrimination ability (between wormlike and antiworm-like objects) has common basic components in different anuran species, so far investigated in the following species: B. bufo, Hyla arborea, Hyla cinerea, A. obstetricans, Bombina variegata. There are remarkable species differences for preferred absolute prey sizes, which may depend on different body sizes and in particular upon the size of the mouth.

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