In the world of amphibians, sexual dichromatism, where one sex appears a different colour, is an unusual phenomenon.
Dichromatism can be further split into ‘ontogenetic’, where the sexes remain different colours, and ‘dynamic’, where the colour change occurs temporarily – something considered rare as hen’s teeth among anurans.
Fighting this froggy trend to a spectacular degree is the moor frog, a species that, for most of the year, appears unobtrusive and underwhelming.
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The moor frog occurs in a broad swathe from northern continental Europe across to western Siberia, and in a diverse range of forested and swampy habitats, only finally thinning out when treeless tundra properly takes hold.
For most of the year (when not hibernating), this small, brown frog perfectly matches its boggy surroundings. But during the short breeding season, the males make a sudden and dramatic transformation, from their default camo colours to a vibrant and conspicuous blue.
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Emerging in early spring from hibernacula on dry land, the males are invariably the first to arrive at the breeding sites, with the frogs highly faithful to the primarily acidic ponds that they will have used in previous years and possibly even hatched from when tadpoles.
Precisely how the male frogs turn blue has not been determined, but the colour change occurs rapidly, with the males reverting to type once mating opportunities become exhausted.
Often gathering either at the water’s edge or in the shallows, the males form loud choruses to advertise for females.
Their distinctive call has been likened to the sound of bubbling air escaping from a bottle submerged underwater. The main calling period in central Europe takes place in late March and early April, while mating further east may not take place until as late as June.
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Driven by an intense desire to mate and drawn in by the males’ calls, freshly arrived females are often intercepted by unattached suitors, who latch on in that froggy embrace known as amplexus.
At most spawning sites males tend to be substantially more numerous, resulting in both intense competition and unseemly scrambles for any available females.
Having successfully snaffled a female, the incumbent male’s next job is to fend off rival suitors. It’s then a case of ‘to the victor the spoils’, as he proceeds to externally fertilise between 500 and 3,000 eggs released by the female into the water.
The entire breeding season may last little more than a week, as the males then readopt their cloak of anonymity, heading off inland to feed on anything from beetles and bugs to slugs and snails.
Like many amphibians, moor frogs face an uncertain future, with pollution, habitat loss and fragmentation all contributing towards a decline, particularly in peripheral parts of the species’ range, such as France and Belgium.
As ever, this species’ fate is in our hands – imagine how blue we’d all feel if we let this incredible European spectacle slip away without a fight.
Why do male moor frogs turn blue?
Initially it was assumed that the blue colouration evolved as a means of attracting females, with the intensity of colour an indication of fitness and fertility.
However, faced with a potential rugby scrum of suitors, each female, in reality, has very little choice over which male she’ll mate with, leading to the conclusion that sexual selection might not be behind the colour change.
This was indeed confirmed in 2012 by researchers from the University of Vienna. Using brown and blue models of frogs, they deduced that the colour change enabled the males to quickly and visually pick out females from a froggy fracas, instead of wasting valuable time and effort with amorous overtures towards other males.
