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Ecological Observations On Amphibia in the Netherlands Ii. Triturus Helveticus Razoumowski: Migration, Hibernation and Neoteny

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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).

Between 1964 and 1969 a population of Triturus helveticus was studied in "de Hamert" (51 30' N, 6 10' E), property of the foundation "Het Limburgs Landschap". The animals were caught in the water by bownets and on land by pitfalls, which from 1968 onwards were used in combination with a fence. During the six years of the study more than 14.000 individuals were captured. Most larvae metamorphosed during August and September, but a certain proportion hibernated in the water and metamorphosed during their second year (June, July). If they failed to do so, then in their third year in the water they became sexually mature neotenic adults. Metamorphosed juveniles left the water to hibernate on land; they did not return to the water until they were mature. Adult animals migrated to the water during autumn (October, November), with a smaller number migrating in spring (March). In summer they left the water (June, July), but for unknown causes the number caught leaving the water was much smaller than the number caught entering the water (Fig. 5). The average length of the adults varied from year to year. Over six years of observation the average length of the males was 59.6 mm and of the females 64.2 mm. The largest male was 74 mm long and the largest female 81 mm. The members of this population appeared to be much smaller than those of others described in the literature. The length of neotenic individuals did not differ significantly from that of normal adults. The sex-ratio also varied from year to year, being in some years biased in favour of males and in other years in favour of females. The overall sex-ratio found during the six years of study was significantly greater than 1. The sex-ratio of neotenic adults was found to be much smaller than 1; this agrees with most published observations of neotenic populations of related species. First-year juveniles were on average about 5 mm shorter than second-year juveniles, which have an average length of 38.6 mm. The size-ranges of first- and second-year juveniles were 26-46 and 31-51 mm respectively. The length at which larvae metamorphosed, whether as first- or as second-year juveniles, appeared to increase during the course of the season. At the beginning of the period of metamorphosis the largest individuals of the larval population metamorphosed. The number of adults caught in autumn was smallest in 1964 and 1965, increasing thereafter to a maximum in 1969.

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


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