Amphibians
Moor Frog
Rana arvalis · also called moor frog
For most of the year the moor frog is an unremarkable brown frog. Then, for a few days at the peak of spring breeding, the males turn a startling sky blue — a colour that fades within a week and that scientists still cannot fully explain.
Overview
Moor frogs live in damp meadows, marshes, and bogs across northern and central Europe eastward to Siberia. About 5.5–6 cm, normally brown with a dark eye mask and pointed snout, they eat insects, worms, and slugs on land.
Blue for a few days a year
At the height of spring breeding, males turn vivid pale blue, likely from lymph fluid scattering light under the skin, possibly to help males recognise each other in a crowded pond, then revert to brown within a week.
An explosive breeder
The whole population may spawn within a few days, filling ponds with a soft bubbling chorus in very early spring, sometimes while ice still lingers.
Status
The moor frog is Least Concern but decreasing in western Europe, where damp grassland and breeding pools are being drained.
Moor Frog: common questions
Why do male moor frogs turn blue?
The dramatic but very brief colour change, in which males shift from ordinary brown to a vivid pale blue or violet for just a few days at the height of spring breeding, is thought to be linked to the intense physical demands of the breeding period. One widely cited explanation is that a build-up of lymphatic fluid beneath the skin during the frantic breeding aggregation changes how light scatters through the skin, producing the blue appearance, rather than the frog developing any actual blue pigment. Functionally, the colour is generally believed to help males rapidly distinguish other males from the browner females in a crowded, chaotic breeding pond, reducing time wasted clasping the wrong frog.
How is the moor frog different from the common frog?
The two species look broadly similar and overlap across much of Europe, but the moor frog can usually be told apart by its more pointed snout, its typically slimmer build, a often more prominent pale ridge or stripe running down the centre of the back, and a relatively hard lump, called a metatarsal tubercle, on the hind foot that is larger and firmer than the common frog's. During the breeding season the temporary blue colouring of male moor frogs is an unmistakable identifier, since common frog males do not undergo any comparable colour change.
What does a moor frog chorus sound like?
During its short, explosive breeding period, large numbers of male moor frogs call together from a pond, producing a soft, low, bubbling or gurgling chorus that many observers compare to the sound of air bubbles rising through water, or to the noise made by running a finger along the teeth of a comb. Individually the calls are quiet, but a full breeding aggregation of many calling males creates a continuous, carrying background hum that is one of the characteristic sounds of northern European wetlands in early spring.
Why is the moor frog declining in western Europe?
While it remains widespread and common across large parts of northern and eastern Europe and into Siberia, populations at the western edge of its range, particularly in the Netherlands, Belgium, and nearby areas, have declined significantly and in some places disappeared. The main causes are the drainage of the damp meadows, bogs, and marshy grasslands the species depends on, along with the loss or pollution of the shallow, fish-free pools it needs for breeding, and agricultural intensification that removes suitable habitat and fragments the remaining populations, leaving them small and isolated.