One of my favourite markers for seasonal change is the appearance of frogspawn in our ponds and tarns, and it usually happens about now. This winter has been a very unusual one in terms of temperature and weather, so I am particularly interested in this year’s records. My sister in Scotland confidently expects frogs to arrive in her garden the last week in February but this year they have delayed. A friend in Baildon spotted one clump of spawn three weeks ago and three or four since (perhaps a rather reckless gene at work here?) but the main frenzy when the smaller male frogs bob their white-chinned faces above the water as they croak their invitation to incoming females is yet to come – a sensible reaction to the recent heavy frosts.
The amazing metamorphosis of spawn to tadpole to air-breathing, four-legged frog is something we are all familiar with. It takes place in plain sight, unlike the hidden transformation of moths and butterflies. Not long ago, every primary school would have its classroom aquarium of spawn, a practice now discouraged because of the declining frog population. I certainly felt confident that I knew all about it. I was wrong.
A friend passed on to me a scientific article about tadpole development. Apparently tadpoles develop their legs internally but back legs emerge much earlier than front– a fact I vaguely recognised as true. Also one front leg usually erupts several hours before its partner. I’d certainly never noticed that. Being scientists, these observers had, of course, asked themselves why. There must, they thought, be some evolutionary advantage to the sequence. It seemed likely that this would relate to the need to escape predation. Tadpoles are on the menu not only for underwater hunters like dragonfly larvae but also have been observed being scooped up from the shallows by blackbirds!
The theory was exhaustively tested on a range of tadpole species. The results were baffling. Tadpoles with four legs actually were nippier than when they only had the back two. And as for the three-legged ones, who, you might think, would swim in circles, they were actually faster still. All those experiments, all that racing and timing of tadpoles, and the evolutionary WHY? remained unanswered. It seems to me that sprouting new legs must take a lot of effort, perhaps they just have to do the best they can. Anyhow, I shall enjoy looking out for three-legged tadpoles in the coming weeks.