Frogs spawn in spring and tadpoles emerge in summer…but not always

Tadpole from the Old Pond today on the point of metamorphosing

I hear reports of people worrying about having tadpoles not yet metamorphosing.

But this is not so unusual: for example, in a survey a few years ago of Glasgow parkland and garden ponds, 4 sites in a sample of 21 ponds had overwintering tadpoles.

Common Frog tadpoles usually take about 3 months to get through to metamorphosis – but they can take quite a lot longer where food is short or they are crowded (as they were in my pond). As my spawn was all laid by about the 21st March this means that the tadpoles,  still in the pond like the one above, have now been in the pond for about 5 months.

One recent investigation suggests that some tadpoles may quite early in the summer ‘decide’ that they are going to overwinter, delaying their metamorphosis. What triggers this  early decision is not yet entirely clear, though it seems more to do with the immediate environment rather than inbuilt frog genetics.

Early emerging tadpoles are usually bigger than the late emergers; and emerging from the water at a small size is a bad survival strategy. Being bigger gives you a better chance of surviving your first winter but how tadpoles know in July that they should sit out the winter is not yet known.

The animal above, having got its hind legs, might leave the pond this year or it may hang on until next year. But in August, this animal is still inside the ‘normal’ period for emergence for the Common Frog, which continues until the early autumn.