Some of this year’s tadpoles

Dear Readers, I have sometimes noticed that although most of the tadpoles in my pond are tiny frogs by now, there are still some tadpoles, who seem to have not gotten the memo about that metamorphosis business. I always wondered what would happen to them – would they just die when winter comes, or would they just complete their growth in the spring? So, I had a little dig around in the literature to see what I could find out.

A 2016 study by Walsh et al found, that in Common Frogs at least, the main factors for growth were food availability and temperature. Low temperatures lead to slower growth, and I’ve often noticed that tadpoles tend to congregate in the warmer, sunnier, shallower parts of the pond. Furthermore, whenever I’ve reared tadpoles in an aquarium they were several weeks ahead of their siblings who were in the colder conditions of the pond. As you might expect, lots of food equals faster growth too, so a warm spring, which tends to promote the growth of algae ( a tadpole’s first food once the contents of its egg have been ingested) should lead to fast development and soon the patter of tiny webbed feet. In the Common Frog, the normal period from hatching to leaving the pond is about fourteen weeks, but this is much slower in cold, high-altitude ponds, such as those in the Scottish Highlands.

So far, so good. But then I read this enigmatic sentence in an earlier paper by Walsh et al”

In addition, the decision on whether to over‐winter as a tadpole appears to be made relatively early in the season (Walsh et al., 2008)

So, at some point in the spring, tadpoles seem to ‘decide’ whether to go hell for leather for development, or to take their time in the hope that things are better during the following spring. But it can’t be as simple as this – if I look closely at my tadpole population there is a wide range of different sizes and developmental stages. One explanation is that there is only a limited supply of food in any given pond. Some tadpoles will start off with a size advantage, and these will hog the food supply and grow quickly. Once they’ve become froglets and left the pond, there will be more left for the remaining tadpoles, but quite possibly not enough for them to complete development before winter comes. Furthermore, there is some evidence that overcrowding in a pond leads to the tadpoles producing a hormone which inhibits growth until the concentration of animals goes down.

The real danger for a tadpole is if it develops into a froglet at a time when there is no insect food for it to eat, so if metamorphosis can’t be completed before the end of summer, the tadpoles will remain as tadpoles and hibernate at the bottom of the pond until spring. At this point, they’ll have an advantage over any new tadpoles, and should be able to grow into froglets with the whole of summer before them to fatten up.

The variation between tadpoles that develop quickly, and those that stay put as tadpoles for a full year, illustrates how adaptable these supposedly ‘simple’ animals are, and how advantageous such plasticity could be, enabling the offspring from a single mating to have two very different paths through life.