All species of amphibians found in Britain hibernate, their metabolism slowing as temperatures drop from mid-October. While some individuals will overwinter among the silt and submerged water plants at the bottom of ponds, most adults leave the water and seek out damp, sheltered places beneath rocks and logs, in leaf piles and compost heaps, or in the crevices in muddy banks at the water’s edge. As their body temperature falls and breathing and heart rate slow, they’ll become dormant, though a mild spell can cause them to stir from their slumber and briefly emerge above ground to forage, before re-entering a state of torpor.
Having witnessed the mass emergence of froglets at a local lake in early June, I was surprised to discover that my pond still contains chunky late-stage tadpoles, presumably laid late in the breeding season. A couple have developed stubby hindlimbs, with bulges showing where their forelimbs will form, but most remain legless. Dainty smooth newt larvae are also lurking in the blanket weed, the feathery gills sprouting from their heads and absent hindlimbs indicating that they’re nowhere near ready to survive on land.
The development period between frogspawn or newt eggs being laid and the froglets or efts (the terrestrial juvenile phase of newts) leaving their natal pond is usually around 16 weeks. Most young amphibians are fully formed by the end of summer or early autumn, but these late bloomers won’t complete their metamorphosis until next spring. Metamorphosis is regulated by thyroid hormones, which trigger and coordinate development. Various factors can come into play when this process is delayed, including late spawning, food availability, overcrowding, suboptimal water temperature and water shading.
There are actually a number of advantages to overwintering as tadpoles or larvae rather than in adult form. The more time they spend eating at this stage of their development, the larger they’ll be when they eventually undergo metamorphosis. Emerging in spring will give them a head start on the next generation, and as larger froglets and efts, they’ll be better equipped to avoid predation and compete for territory and food. But first, they must survive the winter, evading the formidable almond-shaped, blackish-green great diving beetle that patrols the pond in search of prey.
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