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Ecological Observations On Amphibia in the Netherlands

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image of Netherlands Journal of Zoology
For more content, see Archives Néerlandaises de Zoologie (Vol 1-17) and Animal Biology (Vol 53 and onwards).

A population of Rana arvalis Nilsson was studied weekly during five subsequent years in the conservation area "de Hamert", which is part of a region of sand dunes covered with heather in the upper parts, and with Molinia and Erica in the lower parts. In total 2, 203 frogs were trapped mainly in traps which were arranged in a so-called "trapping-square" on the margin of the Heerenven. Additional direct observations were also made. Breeding choruses and spawning were restricted to a very short period in early spring; in the successive years it could be observed between March 10 and April 2 at temperatures between 4 and 10° C. During two or three days after spawning females showed copulation scars. Depending on the temperature, the larval period lasted 11-15 weeks; normally it took 13-14 weeks. In all the years of observation the entire larval population completed its metamorphosis within one week. The tail of a young frog was resorbed within two days. At metamorphosis young frogs measured 15-19 mm. At the end of the year, first year frogs normally had a length of 26-34 mm, while second year frogs exceeded 40 mm. Third year individuals were sexually active. Average length of males and females (third years and older ones) was equal (47-48 mm; maxima: 65 mm for males and 69 mm for females). The sex ratio was approximately 0.6. Adults were trapped mainly in spring; during summer they were obtained in very small numbers; in autumn their number increased again. The great number trapped in spring is due to migration to and from the spawning site; part of them were third year frogs. As soon as spawning was completed, the frogs left the water if weather conditions were favourable. In our opinion weather has a great influence upon population size. In 1964 (extremely dry) the population was reduced, whereas in 1966 (a wet year) an increase started.

Affiliations: 1: Department of Animal Ecology, University of Nijmegen, The Netherlands


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