Frog
Since childhood, ponds and their inhabitants have fascinated me: dipping a jam-jar or net into a neighbourhood pond was to enter a world completely unlike that on dry land. Reading reports of the legendary ‘marches’ when frogs would simultaneously return to their birthplace in their thousands to re-commence the annual life-cycle only added to the mystery. Sadly, I never witnessed this phenomenon which I assume is now a thing of the past. Aquatic habitats are constantly under threat as ponds have been filled in, fields drained and watercourses confined by culverts, and yet even with the added threat of pesticide pollution amphibian species such as frogs survive.
Having survived the winter months in a secure hiding place, with cold temperatures reducing their bodily functions to a bare minimum, frogs emerge during warmer periods motivated entirely by the urge to return to find a mate and to spawn. Only after this process has been carried out will they start to feed, leaving the pond to search for food in the moist undergrowth. The date at which this happens is determined by the winter temperatures; in a mild season I have known the first spawn to appear in early January, which is why I start looking for activity as soon as the Christmas festivities are finished. In recent years spawn has been recorded in Cornwall as early as late November, admittedly in the almost frost-free area of West Penwith. Sowing early vegetable seeds in the (relative) warmth of a greenhouse while listening to a chorus of frogs in the adjacent pond certainly feels like being close to nature!
Prior to mating, the female appears to be twice the size of the male because she has already formed her batch of 1500 or so eggs ready for release. The male rides piggy-back, holding on tightly with his front legs so that the eggs can be fertilized as soon as they are released from the female, critically before the jelly swells on contact with the water. There being no 'parental care', the eggs are left to the mercy of the elements, although the jelly does protect them from the attention of most predators. The black dot embryo will rapidly change into the characteristic tadpole shape, and then progress can be followed with the naked eye. However, if the water freezes, the egg will die, turn white and rot away. Once the tadpoles have hatched and are able to move around freely a little-known law of physics can save them; between 0 and 4 degrees Celcius cold water rises while warm water sinks so ponds freeze from the top down (above this temperature the normal rule applies and water is hottest at the surface). This phenomenon enables all living things to survive at the bottom of a pond in winter.
It is surprising how a small pond or ditch can be adequate to attract the attention of mating frogs, although they run the risk of the habitat drying out. Many years ago, we found clumps of spawn left high and dry on an area of high Dartmoor. We must have spent an hour or so scooping them up in our bare hands and dropping them into the remaining pools and even a slow moving leat. I assume we did the right thing! Throughout the rest of their life in the water, further hazards will await the young tadpoles in the form of predators including many fish and the primeval-looking dragonfly larvae which lie in wait to catch them in their fearsome jaws. From above many birds will swallow them up and later in the year I have seen herons unusually feeding on 'dry' land and picking off the emerging froglets as they leave their watery birthplace.
Many animal species produce large quantities of eggs and young so in spite of the odds seeming to be stacked against the survival of any one tadpole, enough will survive to ensure the continuation of the next generation of adult frogs. But significantly we must help by retaining and protecting their watery habitats and when possible creating new ponds free of polluting chemicals. In our gardens, amphibia of all species are excellent pest controllers feeding on slugs in particular. And in East Devon and North Cornwall the newly introduced beaver colonies are busy creating new aquatic habitats which will help amphibians such as the frog and many other species to thrive.
Pete Mayston