10th February 2008, Sunday

The perils for early breeding frogs were well illustrated in our garden this morning. The surface of the water, and four clumps of spawn were frozen.

Frozen frogspawn

So given that frogs spawn only once a year why do they take such risks with challenging weather?

The rather obvious answer is that in many circumstances it works, otherwise there would be natural selection for late-spawning frogs. Frogs have this urgent need to breed as soon as a pond gets warm enough. Once the water can sustain a chorus of males then other frogs are attracted there to breed. The eggs will be laid in the warmest water available, usually very shallow, and unshaded. When the first pair has laid eggs, further clutches of spawn are laid in the same location. To confuse the animals try moving the eggs to a different part of the pond – often they will change their spawning site!

This explains why some ponds are used, and others close by are not. When a cold deep pond lies adjacent to a warmer shallow pond all of the frogs will breed in the warmer pond before the other is warm enough to sustain breeding. If you want to attract frogs to a pond make sure it has a broad sunny margin with water only a few centimetres deep, that drains into the deeper water.

There is an advantage to laying eggs in large masses (the largest I have ever seen had more than 1,200 spawn clumps). The jelly in the centre of the spawn mass is marginally warmer than the surrounding water permittting the larvae to develop more rapidly. The speed of tadpole development is the key.

Early spawn can hatch sooner allowing the tadpoles to grow faster and the larger tadpoles are at a competative advantage. When they are over-crowded they suppress the growth-rates of the smaller tadpoles, making them more vulnerable to predators such as dragonflies and newts. Furthermore life as a tadpole is a race against time. Will the marsh they live in dry out? Having spawn laid over a period of weeks, as is often the case, is fortuitously a way of balancing the risk factors of premature desiccation, or early season freezing. Mind you a single nights frost does not necessarily kill them. They are tough things frogs!