However, the frog, by reason of its fertility, holds its own. Early in the year, sometimes while there is still ice and snow, the frogs leave their hibernating places (mostly holes in the ground, under moss, or in the mud), and they begin to pair in standing or slowly flowing, mostly shallow, waters.
They are not always very careful in the selection of the spawning locality, many of them lay their eggs in a ditch, or even in the shallowest puddle, which is sure to dry up, and thus to cause the destruction of the whole brood. This carelessness is all the more surprising when there are large pools or lakes in the immediate vicinity, perhaps only one hundred yards to the other side of the road. The Natterjack is, by the way, equally careless, while other toads and the tree-frogs are very circumspect.
Fig. 49.–Rana temporaria. Eight successive stages in the development from the egg to the almost complete Frog, × 1.
Both sexes can croak, and this sound is frequently produced under water; but there are no regular concerts, although many collect in the same pond or spring, which is perhaps the only suitable place for miles around. The male puts its arms around the chest of the female, behind her arms, and the embrace is so firm that nothing will induce him to loosen his hold. The process becomes an involuntary reflex-action, a cramp which may last for days, or even for weeks, if sudden cold weather sets in, until the female is ready to expel the eggs, an act which is quick and soon over. The usual time of spawning in Middle Europe is the month of March, earlier in warm, later in cold seasons; in southern countries, February or even January, but in Norway not until May. Although the males of this species are not more numerous than the females, and therefore should be able to mate without much trouble, their ardour is so great that they occasionally get hold not only of the wrong kind of frogs, but of toads or even fishes, and, if taken off by force, they fasten on to anything else, a log or on to your own fingers. The eggs measure 2-3 mm. in diameter, are black with a whitish spot on the lower pole, number from 1000 to 2000, and sink at first to the bottom. Their gelatinous cover soon swells to a large globe more than 10 cm. in width, and the whole mass, as large as a man's head, floats on the surface, often stained with mud and other impurities. During the cold weather which often prevails in the spring, the dark brown larvae are slow in their development; and provided with rather large branched external gills and a well-developed tail, they wriggle about in the dissolving slime for three or four weeks. Fischer Sigwart[[114]] has timed and measured them as follows.–The eggs were laid on the 10th of March. On the 15th the larvae were 4 mm. long and began to leave the eggs. On the 19th they measured, body 4, tail 9, total 13 mm.; on the 5th of April 10, 16, and 26 mm. respectively. On the 13th of May they were 40 mm. long and the hind-limbs appeared; the fore-legs burst through on the 25th, when the tadpoles had reached their greatest length, namely 45 mm., the body measuring 15 mm. On the 31st of May they left the water, still provided with a rather long tail of 20 mm., the total length being reduced to 35 mm. The larvae of this set developed unusually fast, perhaps owing to artificial conditions. The whole development is, however, mostly finished in three months, so that the little stump-tailed baby-frogs swarm about well before midsummer, and have time enough to grow to the size of 20 mm. or ¾ inch before they begin to hibernate in October.
In higher localities and in northern countries the tadpoles are sometimes obliged to winter in the unfinished condition.
In spite of the unusually hot summer of 1899 I found plenty of tadpoles on the 10th of September in the tarns of the hills of North Wales, 1500 feet above the level of the sea; while thousands of little frogs, with and without stumpy tails, were hopping about in the surrounding bogs. The water of these tarns is always very cool. Cold and rainy weather set in by the middle of the month, and on the 26th the tadpoles, all rather small, measuring only 35 mm., with the four limbs developed, but still with a broad fin on the tail, had all settled down under stones at the bottom of the now very cold water, prepared for hibernation. A few were taken home and kept in a glass vessel with water, cool, but less so than that of their native tarns. Within two days they lost the fins on their tails; before the end of a week they left the water, and crawled on to the moss, and the tails were reduced to little stumps. By the 10th of October the metamorphosis was complete, the little frogs measured only 13 mm. in length and showed no desire to hibernate in the genial atmosphere of the greenhouse.
This species has a very wide distribution. It ranges from the west of Ireland to the islands of Saghalin and Yezzo, being found everywhere in the enormous stretch of intervening countries, practically the whole of Central and Northern Europe and the middle belt of Asia. Its most northern extent is the whole of Sweden and Norway. I have found it to the east of the Dovrefjeld, at an elevation of 4000 feet, well-nigh the snow-line. In conformity herewith it ascends the Italian Alps up to 10,000 feet. The southern limit in Europe is the Cantabrian range and the hilly province of Galicia. In the rest of the peninsula, in Italy and Lombardy, Greece and Turkey, and on the Mediterranean islands it is absent.
R. arvalis is often confounded with R. temporaria, as it differs from the latter only by the following characters. The snout is rather more pointed and is narrower; the inner metatarsal tubercle is large, compressed, and hard; the dorso-lateral glandular folds are more prominent and the belly is white and immaculate; lastly, it scarcely reaches 3 inches in length, a size which is not rarely surpassed by the other species. There are also some differences in habits. R. arvalis prefers moist, boggy, open localities, and does not ascend beyond 2000 feet in Central Europe. It pairs as a rule later in the spring and the eggs are smaller, only 1½-2 mm. in diameter; they do not swell up so much, and the whole mass does not float but remains at the bottom of the shallow water. The coloration much resembles that of R. temporaria, and is likewise subject to much variation, except that the pale vertebral stripe is perhaps more common. This species is distributed over the whole of Central Europe, Russia, and Western Siberia, south of the 60th degree of latitude, living side by side with R. temporaria. Between the rivers Elbe and Rhine it becomes decidedly rare, and the latter river is practically its western boundary, while the Bavarian Alps and the Danube form its southern limits.
R. agilis is still more frequently confounded with both the two former species. It differs from either by the absence of the two internal vocal sacs of the male, and by the decidedly longer hind-limbs, the tibio-tarsal joint reaching often a little beyond the tip of the snout. The inner metatarsal tubercle is as prominent as in R. arvalis. Total length up to 3 inches. The prevailing colour of the upper parts is rather yellow or pink-brown with few and small blackish spots; a Λ-shaped dark mark on the neck is often present, and the large dark patch on the temporal regions is always conspicuous. The under parts are white, inclining to lemon yellow on the flanks and thighs. The iris is golden yellow in its upper half, dark brown in the lower half.