Better Smaller, But More

In the city, frogs do not feel as comfortable as in the wild nature because of dirty water, a lack of food, and dangers at every turn. That is why the life of frogs in urban areas is shorter. However, they do not leave these habitats, but adapt to them. Apparently, there are two ways to adapt: either become more tolerant or increase the number of progeny.

Every spring from 1998 to 2001, Elena A. Severtseva and her colleagues from the Biological Faculty of the Moscow State University studied the spawn of two frog species (Rana temporaria and Rana arvalis) most common in Moscow parks and ponds. The scientists counted the number of layings and the quantity of eggs in each laying and measured the diameter of eggs and their yolks using a microscope.

In average, Moscow frogs have smaller eggs than their sisters in the countryside, but each urban frog lays several hundred eggs more than the rural one. The egg diameter is about one millimetre, and the difference between egg sizes in the city and suburbs constitutes several decimal fractions of millimetre, but that is sufficient to gain in quantity. At the same time, urban conditions do not change the yolk size in relation to that of the whole egg; sometimes yolks of Moscow frogs are even larger. Therefore, the embryo has a sufficient food supply to grow into the tadpole, though tadpoles from small eggs need a longer time for development.

The amphibians were studied in several places within Moscow: near the Gorodnya River, in the vicinity of the Kapotnya Oil Plant (district Brateevo), in Vostryakovo (in a pond adjacent to the Moscow Ring Highway and in other three ponds in the park near the cemetery), in the area of Matweevskoe (the Setun River floodplain), and near the railway station Ramenskoe. Similar observations were done at the biological stationary near town Zvenigorod.

Both frog species spend most of their life on the land and come into water pools only to spawn. City frogs are very reticent and almost silent at the breeding time. The researchers have heard their sounds only in Matveevskoe. While the spawn of suburb frogs is assembled in large clusters, that of Moscow ones appears in small clusters all over the spawning area. Oil products and chemicals dissolved in water kill many embryos. The rate of embryo death in Brateevo is five times as high as in natural conditions and reaches 12%. Hence, to survive in cities, the high quantity of eggs is more important than a large size of embryos, the scientists conclude.