The early frog gets the reproductive success

By Mark Brazil | Jun 21, 2001

Travel out of almost any of the major cities of Honshu on an overcast, rain-threatening evening, and head toward rice country.

There, in the darkness, where track, trail or road traverses the paddies, you will hear the astonishing spring-time and early summer chorus of countless frogs. The deep croaks and honks, the kero-kero and nee-deep, nee-deep of tree frogs and brown frogs can form a deafening chorus at times. It is a sound that one can become lost in, in fact a wonderfully restful sound to go to sleep to.

Travel anywhere in Japan, and the chorus comes from different species. The legacy of the Japanese archipelago’s ancient isolation is the separation of the links in the chain of islands of various sizes, with isolated mountain ranges, river plains isolated by mountains and separate peninsulas, all providing habitats for an amazing array of frogs from northern Hokkaido to the southern Yaeyama Islands, and from the Japan Alps to sea level.

In any one area, though, the diversity is limited, for species in Japan tend to be isolated by geographical barriers: one species in one region and a closely related, but different, species in another.

The largest frog in Japan is the introduced American bullfrog, whose bellowing call is an alien intrusion on the native amphibian chorus, and whose tadpoles are bigger than some of the native amphibian adults.

At mating time frogs gather, and the males compete over the females, fighting to establish mating rights, each to hold his chosen female in the tight grip known as amplexus. On dark early-spring nights one can find Hokkaido brown frogs (Ezo aka-gaeru) in this position, the grip of the male so strong that he is carried along on every leap the female takes.

The female carries within her the dense mass of eggs that eventually grace pond and paddy. How, one might well ask, can such a huge mass of eggs be accommodated in such a small body? A newly laid mass of spawn floating at the water surface may have the volume of a soccer ball, and surely no single female could possibly produce so much at once.

If one examines frog fatalities in the spring one will find, packed densely inside the body cavity, a mass of tiny dark eggs, each one smaller than a rice grain. The winning male fertilizes the eggs as they are laid, and the shimmering envelope of jelly comes in contact with fresh water and begins to swell. Tiny and dense at first, the eggs swell until they resemble small see-through grapes with a single visible pip at the center.

Frogs are astonishing in their diversity, some laying in wet rice fields, others in ponds, some even in temporary water held in hollow trees or the leaves of epiphytes high in trees.

As the late Hokkaido snow thaws, forming meltwater pools on the forest floor during late March or early April, the temperature still hovers close to freezing. An overnight frost can leave a skin of ice on such pools, and the arrival of spring still seems distant. Nonetheless, the Hokkaido brown frogs wake early from their winter torpor and gather at these ponds, fighting for breeding rights. Into water so cold as to numb one’s hand the pairs swim regardless, and egg-laying is completed before April is out.

Though not early by Kyushu or Honshu standards, April in Hokkaido is more of a winter month than a spring month, and the standing ponds are cold indeed. Nevertheless, spring does come. The rising air temperature warms the pools, and within the jelly-encased egg-masses the tiny tadpoles now develop quickly. Their nuptials over, the frogs themselves spread out into the forest and are only occasionally encountered, hopping beneath the sasa bamboo.

In the shallowest of the puddles and pools, when the tadpoles are well on their way toward hatching, new egg-masses suddenly appear. White at first, and glistening like the flesh of lychees, these eggs are not gathered into globular masses, but appear more like sausages strung in the water. Closer inspection reveals that they are wrapped around stems, stalks or twigs as anchor points, lest they be washed away in the floods that follow spring rains. These eggs are similar to those of the frogs: A round jelly mass about a centimeter across harbors a dark gray developing embryo.

Before these later egg-strings are half developed, the globular frog-egg masses hatch. Tadpoles emerge in their hundreds, having devoured the jelly of their eggs. It’s a race against time for the water-dependent tadpoles to turn into froglets that may hop away as their natal pool dries out. Even egg masses may meet this fate. Each year I find solidifying egg-masses in what once were ponds, with all the embryos dead.

At first sight, some of the newly emerged tadpoles, lucky enough to be in a longer-lasting pond, appear to be eating the later-laid egg-strings. Squint through a powerful hand lens, though, and you will find that the tadpoles are feeding on the green and brown algae that have discolored the egg-strings. The egg-jelly grows quite a garden of algae, and the tadpoles graze on it.

While the tadpoles are already rapidly turning into froglets, the egg-strings are only just hatching. They hold the young of a very different creature — the sansho-uo (salamander).

Coming from the salamander-less British Isles, I find it amazing that Japan, though only a little larger, is home to no fewer than 20 salamander species. They range in size from the enormous Japanese giant salamander, the world’s largest, which can tip the scales at 35 kg and measure 150 cm in length, to the little clouded salamander, which may only be 7 cm long and weigh as little as 3.5 grams.

Some species, such as the Japanese clawed salamander, are found widely throughout much of Honshu. Others are restricted to tiny areas, such as the Tokyo salamander, confined to a remnant ring of habitat around the capital.

What enables the brown frogs to steal the early march in Hokkaido is the fact that their tadpoles are vegetarian — algae grazers that benefit from the flush of growth in shallow water exposed to early spring sunlight. It is only once they have metamorphosed into tiny frogs that their diet changes and they become carnivorous creatures of the forest floor.

The salamanders, meanwhile, lay their eggs later, but they are able to coexist in the same pools because their hatchlings are hunters right from the start. In particular they take aquatic insects, among them the larvae of mosquitoes. So we can be grateful not only for the marvelous diversity of the rarely seen nocturnal salamanders, but also for their role in helping to keep mosquito numbers in check.