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Here, we provide evidence for a wavelength-dependent effect of light on magnetic compass orientation in Pelophylax perezi (order Anura), similar to that observed in Rana catesbeiana (order Anura) and Notophthalmus viridescens (order Urodela), and confirm for the first time in an anuran amphibian that a 90° shift in the direction of magnetic compass orientation under long-wavelength light (≥ 500 nm) is due to a direct effect of light on the underlying magnetoreception mechanism. Although magnetic compass orientation in other animals (e.g., birds and some insects) has been shown to be influenced by the wavelength and/or intensity of light, these two amphibian orders are the only taxa for which there is direct evidence that the magnetic compass is light-dependent. The remarkable similarities in the light-dependent magnetic compasses of anurans and urodeles, which have evolved as separate clades for at least 250 million years, suggest that the light-dependent magnetoreception mechanism is likely to have evolved in the common ancestor of the Lissamphibia (Early Permian, ~294 million years) and, possibly, much earlier. Also, we discuss a number of similarities between the functional properties of the light-dependent magnetic compass in amphibians and blue light-dependent responses to magnetic stimuli in Drosophila melanogaster, which suggest that the wavelength-dependent 90° shift in amphibians may be due to light activation of different redox forms of a cryptochrome photopigment. Finally, we relate these findings to earlier studies showing that the pineal organ of newts is the site of the light-dependent magnetic compass and recent neurophysiological evidence showing magnetic field sensitivity in the frog frontal organ (an outgrowth of the pineal).

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Population density in the lacertid lizardPodarcis lilfordi on the Mediterranean islet of Nitge, Menorca, Balearic Islands, was found to be 12 190 ind · ha (SE, ±2135), exceeding densities reported for other island or mainland lizard populations. Field metabolic rates inP. lilfordi were measured by the doubly labeled water method, allowing estimation of a population metabolizable energy demand of 13.86 MJ · ha · day-only 9.8% of that for a theoretical mammal population of the same body mass and density. Energy demand was considerably higher than that estimated for other lizard populations, primarily due to high population density but also because of high individual daily energy expenditure (1255 KJ · day; body mass=5.13 g). Field metabolic rates were partitioned into maintenance and activity components by respirometry of captive animals at field body temperatures. Activity metabolism formed the main component (77.4%) of total respiratory metabolism resulting from a combination of long daily activity periods (ca. 12 h), and greatly elevated metabolism during activity (5.7 times greater than resting levels). It is hypothesized that low food availability per individual constrains the time-energy budget of this species, obligating long periods of intense foraging.