The ovarian cycle in the natterjack toad, Bufo calamita, and its relation to breeding behaviour

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Abstract
The ovarian cycle in natterjack toads Bufo calamita from an island population with unpredictable breeding conditions was studied in an archipelago off the Swedish west coast. About 80% of the female population bred during spring and early summer, and the rest later in summer and early autumn. Time of breeding and female size (age) were not correlated. It was found that first growth phase (FGP) oocytes were recruited from oogonia mainly during early spring and late autumn, but that recruitment could occur at any time of the year. The FGP oocytes grew rapidly immediately after recruitment, and when transferred into second growth phase (SGP) oocytes. In between these two periods the FGP oocytes seemed to be in a resting stage. Females showed an asynchrony in ovarian developed. We found females with fully developed SGP oocytes, ready for ovulation, as well as females with ovaries in only an early vitellogenic growth phase at all times of the year. Females did not neccessarily ovulate as soon as the vitellogenic oocytes had reached full size. There were strong indications that some females only ovulated and laid a fraction of the egg mass in their first batch, and that they laid a second batch later in the season. This second batch was thought to originate from the same recruitment of oocytes as the first batch. There was a tendency for SGP oocytes to grow faster during summer than during spring. It was also concluded that SGP oocytes coulde be recruited all year around. Bimodal size frequency distribution patterns were found both among the FGP and the SGP oocytes; they were most pronounced among the latter, where they were found throughout the year.

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