Sunday, 25 November 2012

Amphibious activity …

Plaistow, 8 March 2012

Overnight and as if by magic my pond is once again awash with frogspawn. Every year I keep a close watch for returning Common Frogs (Rana temporaria), their presence often betrayed by their calling.

Generally emerging from hibernation in late February spawning typically takes place in my pond in early March, though in some seasons the frogs emerge sooner and spawn as early as January. The females are ready to spawn immediately after hibernation and the animals enter into amplexus (the term describing the act of the smaller male clasping the female underneath in a nuptial embrace) very soon after arriving at their breeding sites. The spawn is laid in clumps and typically consists of 300-400 gelatinous eggs containing tiny black embryos. As the female lays her eggs the male expels sperm to fertilize them. The very young tadpoles are black but soon become speckled brown in colouration making them distinguishable from the permanently black tadpoles of the Common Toad (Bufo bufo). The tadpoles develop throughout the summer and emerge as tiny froglets in wet weather during August or September.

I recently read an article describing R. temporaria activity in the French Alps. I was particularly interested to note that cold-climate frogs grow far more slowly than their relatives in temperate areas, but typically live much longer (12 years, compared to 5 for lowland frogs) and grow somewhat larger. They are also active during the warmer daylight hours unlike their cousins elsewhere.  Though egg laying occurs in spring, frog pairs in mountain habitats can begin hibernation in amplexus which may possibly provide a reproductive advantage by allowing mating as quickly as possible once warm weather arrives. Eggs of high-elevation frogs may also be 30% larger than those of lowland females, giving the tadpoles a head start.

Another sign of spring ...

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