Spring is a time when pond-breeding amphibians within temperate areas return to breeding sites. Although amphibians do not migrate as far as birds and mammals, they often have to traverse difficult terrain, such as dense scrub or long grass and they may have few distinctive visual cues. Individual amphibians have been observed to return to the same breeding site year after year which demonstrates an ability to use external cues to navigate successfully back to their breeding ponds.
Within Europe, after leaving breeding ponds in the summer or autumn, amphibians will often travel considerable distances away from breeding ponds, which may take many months. For example the common toad may migrate between 50 m and 5 km from breeding sites. Research has shown that amphibians probably use a range of methods to navigate which may include: visual, olfactory, auditory, celestial, lunar and magnetic cues. However, not all species can use all techniques. For example, some species are able to use magnetic cues, while others are not.
Research on the smooth newt (Lissotriton vulgaris) and great crested newt (Triturus cristatus) suggests that individuals use predominantly olfactory cues (e.g. pond chemical cues) to navigate towards breeding ponds. If displaced outside their normal migratory range of a few hundred metres individuals are unable to return. These species seem unable to navigate using celestial or magnetic cues and suggests that individuals lack an internal spatial map. This is in contrast to the palmate newt (L. helveticus) and alpine newt (Ichthyosaura alpestris) which in addition to using olfactory cues, also use a geomagnetic compass to find their breeding ponds. Individual palmate newts can successfully navigate back to breeding ponds even if displaced up to 19 km outside their natural range.
Recent research suggests that auditory cues from calling anurans (frog and toads) may also play a role in orientation and navigation in newts. In a study on the palmate newt, individuals were able to orient towards breeding ponds based on the calls of the sympatric common toad (Bufo bufo), but not common frog (Rana temporaria). In addition, research on the great crested newt also indicates that adults are able to navigate using calls of the common toad but not the common frog, which is probably due to the non-overlapping breeding season of the common frog with great crested newts. Individuals also appear to avoid white noise and the ability to distinguish between the calls of different anurans may be learned.
Methods of orientation may not be the same within individuals of the same species. Male natterjack toads appear to use different methods of orientation compared to females. This species breeds in highly ephemeral water bodies, the location of which is not entirely predictable from year to year due to variations in rainfall over the winter. After the winter, males will start searching for ponds using predominantly visual cues; olfactory cues appear to be less important. Magnetic cues may be used if individuals are a considerable distance from a pond. Once at water bodies, males start calling for females. In contrast to males, the females do not employ visual, olfactory or magnetic cues but orient using the calls of the males. Ponds with louder calls attract more females.
There is increasing evidence that the moon may play a role in navigation as the large arrival and amplexus events and large spawning events of common frogs and toads are more frequent around full moon at a range of European sites.
Overall it appears that amphibians use a range of methods to correctly orientate towards breeding sites and the techniques employed vary depending on species, sex, breeding strategy and migratory behaviour. However we still have a lot to learn about how these, especially lunar cues, are used in many amphibian species and how increased artificial light levels or sound levels may negatively impact on their ability to correctly navigate back to breeding ponds in the spring.
Laurence Jarvis, Froglife Conservation Co-ordinator
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