Frogs are cold-blooded. When the temperature falls their vitality is lowered. Although they have lungs, they also breathe through their skins – provided they can be kept damp – and the small amount of oxygen they absorb in hibernation is enough to keep them ticking over.
During the long winter months, gradual changes have been taking place in their body chemistry. Hormones have been at work. On awakening in the spring, the frogs are immediately ready to breed.
The males have developed a large, swollen pad on the ball of each “thumb”. With this the male grips the unfortunate female in a cold, clammy and muscular embrace which may last for days on end. In their frenzy to mate the males are none too choosy; they have even been known to grapple a good-sized goldfish, and clumps of half a dozen frogs or more can form which would even make the author of the Kama Sutra blush.
The males cling on, waiting for the females to exude the streams of spawn, which are fertilized as they emerge. A slimy coating on the eggs swells almost immediately on contact with water, so that the spawn floats close to the surface, close to light and warmth.
A fortnight later the tadpoles begin to hatch. At first they resemble the fry of tiny fish, minute black slivers clinging in clusters to fronds of weed. Within a few days the mouth is fully developed and they start to forage on minute algae and other vegetable matter. They breathe using two pairs of feathery gills, external at first but later internal. The tail grows longer, and the body fatter.
Then, a few weeks after hatching, two buds appear which develop into hind limbs; the body becomes flatter and the eyes migrate to the top of the head. Next the forelimbs emerge. At this stage the lungs are already well developed and the gills, like the remains of the tail, are absorbed. The metamorphosis from larva to adult is complete. Three months after hatching, the tadpoles have become froglets.
The casualty rate among spawn and tadpoles is high. Besides fish, their enemies include many sorts of insects. Among the most ferocious of these are dragonfly nymphs, sometimes called the “Tigers of the Pond”, and the even more ferocious and aggressive backswimmers, bugs with the scientific name Notonecta.
By the time the young frogs leave the water and are ready to start exploring the damp vegetation nearby, their chances have improved somewhat, but they are still at risk, although from a different set of enemies. A number of water birds – including herons and ducks – are partial to frogs, and so are brown rats, foxes, and water shrews.
For defence the frogs rely on their powerful hind legs, with which they can spring out of danger, and on the cryptic colouring of their skin. The spots and blotches help to break up the shape of the frog and make it harder to see, and the colours themselves – green and brown and dull yellow – blend in well with the background.
In addition the frog can change colour to some extent, like a chameleon. There are special cells in the skin containing dark pigment. In dark, wet, or cold conditions the cells expand, making the frog look darker; in hot, dry conditions they contract and the frog can appear almost sand-coloured.
Adult frogs have two modes of feeding, both quite effective in their way. The upper jaw, but not the lower, is equipped with a set of backwards-pointing teeth. The prey – mainly worms, slugs, and large insects – is seized and held with the jaws and then swallowed whole.
The other mode of feeding is more colourful and makes use of the frog’s excellent eyesight. Glands in the roof of the mouth exude a sticky goo; the tongue is long and forked and rolled back on itself when at rest. Should a suitable insect alight nearby, or even cruise past, the tongue is uncoiled at lightning speed, brushes past the glands, and the insect is glued to the tip, whereupon it is brought back to the mouth and swallowed with a complacent blink of those big, bulbous eyes.
Frogs have set migration routes and a sort of inbuilt memory of ancestral breeding grounds, returning year after year to the same spot, even when the pond or ditch has long since been filled in. They have suffered badly in this district from the activities of man, and tadpoles should never be kept in jamjars: much better to leave them in the wild.
For all their promiscuity at breeding time, frogs are to be welcomed in the garden, as they destroy vast numbers of slugs; and anyway, March would not be March without their quiet, lascivious croakings, the plop as they dive into the goldfish pond, or the tapioca-like masses of spawn which signal the start of another spring and another summer.
(Introduction to these pieces; see all)
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