Natterjack Toad (Epidalea calamita): Europe's loudest amphibian.
IT WAS DUSK WHEN I ARRIVED IN THE EAST NORFOLK VILLAGE; a calm, overcast evening, the low, featureless grey stratus cloud merging seamlessly into the nighttimes' darkening tones, but I could hear them already, only faintly, presently absorbed into their strident nocturnal activities. As I walked onto the heath and paused, I heard them quite distinctly, yet they were still a good 20-minute walk away, the voices of Europe's loudest amphibian carrying across the still night air as if they were literally tens-of-meters away, not the best part of a country mile. (An extraordinary feat accomplished by such a small animal, where air from the lungs is forced through the larynx, causing the vocal cords to vibrate, the resultant sound being amplified by the inflated vocal sac. During the whole performance, mouth and nostrils are kept tightly closed, the air used for this feat being pumped back and forth through the body by internal muscle power.) But how many people have ever seen one, a natterjack toad that is, such is their secrecy of lifestyle and infrequent, eruptive breeding patterns? Add to this the rapid decline and degradation of their preferred and essential breeding habitat -- dry, open heathland and coastal dune systems -- totalling less than 50 active sites across the entire United Kingdom (approximately 20,000 adults), it is easy to understand how their population crashed by 75% - 80% during the 20th Century and why they are particularly difficult to locate in the wild.


Back in 1992 the Council of Europe declared the natterjack toad as Europe's most vulnerable amphibian, an alarming statement, since which time serious conservation efforts have improved the short-term prospects of these anurans (the frogs and toads), who's global ranging is not fully understood (with its most westerly outpost of south-western Ireland somewhat of an ambiguity), spreading south into its Iberian stronghold, and then east, across into northern-central Europe, terminating around the Estonian, Belorussian and Ukrainian borders. But why should an amphibian -- a toad, not unlike the much more numerous and widespread common toad (Bufo bufo) -- suffer such species threatening declines when its cousin seems able to cope with modern living so much better?
 
Understanding the habits and necessities of the class of animals specified as Amphibia (possibly 5000 or more globally) is a useful starting point. They're thought to descend from an ancient tribe of fishes and possess thin, glandular skin that's evolved to be very permeable, promoting gaseous exchange and water transfer, allowing them to drink and partially breathe through their skin. They produce their eggs (spawn) wrapped inside a protective gelatinous substance, usually in open water. Their larvae (tadpoles) are aquatic, breathing through gills before developing lungs and metamorphosing into land dwellers -- air breathers. Norfolk is home to all seven species of native British amphibians (including the extremely rare pool frog), a status that no other county or Shire can claim, but even here the natterjack toad is clinging on to its highly developed existence which demands a cocktail of diminishing ingredients.



The toads choose open heathland and dune systems primarily for the lack of vegetative shade growing in these locations, where the light soils and exposure to the hot summer sunshine produces a heat and warmth absolutely essential to their breeding success. It is in these spacious regions that the toads come forth during the second-half of spring, when the ambient temperature has risen a few degrees and, most importantly, moisture is in the air. Emerging from their temporary burrows -- dug-out in moist, granular terrain -- the adult toads begin a nocturnal quest to locate two essential pieces of their reproductive jigsaw puzzle: a mate, and the ephemeral, shallow slack pools they cannot exist without. Both male and female toads will convene on humid, drizzling nights at these temporary, slight scrapes, formed during recent rains and lasting only a few weeks of the season before drying out in the ever increasing heat of the summer. The anurans may locate these places through memory, chance or a mixture of both, with juvenile toads the most likely to adopt a pioneering spirit and occupy the newest pools, even miniscule puddles if population densities are high, and competition for mates is fierce.

THE EXCELLENT AND EXTENSIVE ONGOING RESEARCH of Dr. T. J. C. Beebee (University of Sussex) and colleagues has revealed some fascinating breeding behaviour, suggesting that the male toads indulge in a number strategies to secure their place on the propagation ladder, most of which is inextricably linked to their celebrated vocalisations. It is a truism in nature that size often does matter, and this is an agreeable notion to apply to the natterjack toad, although not entirely correct. It is a fact, the bigger the toad, the louder the call. To a female toad, this is not of the strictest importance, for what she hears is based not on loudness, but closeness: intensity and resonance. She will collect the males' chorus on a thin membrane stretched behind her eyes, called the tympanum, which in turn vibrates onto a small bone (ossicle) known as the the columella, designed to relay these rumblings towards the inner ear and then onto the brain. Therefore, the closer the male can get to a female, the bigger and more attractive he will become in her world, significantly increasing his chances of mating with her. However, as the smaller, younger males can also detect the loudness, the true size of their rivals, they may not even bother to advance towards the favoured breeding pools, knowing that the larger toads have more stamina, choosing instead to avoid conflict and adopt a different strategy to secure a mate. Dr. Beebee discovered that these displaced males adopt a more passive technique, turning themselves into silent, amorous assassins, intercepting females on their way to established breeding pools and effecting amplexus before their arrival at the water's edge. Another, and even more effective strategy deduced by Dr. Beebee is that of the wandering, roaming male: the vagabond lover. These expert strategists will visit multiple pools during the elongated, sporadic breeding season, deploying a mixture of scrape-side, stand-and-shout bravado fused with predatory, ambush tactics: the biggest, strongest males combining these two techniques, for the length of the summer, have proven to be the most successful.       

Nevertheless, prosperous breeding takes more than boy meeting girl on 'Planet Natterjack' -- a ticking time-capsule where the clock is always counting down -- a place where terrestrial and environmental perils are ever present and disaster is always lurking in the shadows. Why so? Shallow pools, the liquid basket in which the toads have placed all of their eggs, dry out quickly in such warm, open, windy environments as dunes and heaths, demanding an approach to maturation of the extremely rapid kind if the toad's spawn is to survive. Laid as a single string of eggs, the larvae (tadpoles) will hatch within 5 - 10 days (by comparison, 2-weeks or more for the common toad) and must feed and grow with vigour to escape not only the receding waters of their natal pool, but the imminent predations of numerous executioners -- birds, snakes, but mostly large, carnivorous invertebrates. Ironically, it is the shallowness of the pools, warming as they hastily evaporate in these exposed environs, that promotes the rapid development of the juveniles in such perfect growing conditions, providing them with a thin, wobbling tightrope of opportunity to reach adulthood.

But beware adult natterjacks, you have other dilemmas to solve: -- What if your chosen scrape is too deep, allowing the invertebrate enemy of your spawn the ideal habitat to breed, to proliferate and gorge on your investments? The water will be too cold for too long, your youngsters will not develop quickly enough to evade these hungry mouths.  -- What if tadpoles are already in situ at your chosen location, invaded weeks earlier by common frogs (Rana temporaria) and toads, who's hungry and well-developed larvae will make hearty meals of your progeny? (Natterjack toads are reluctant to use pools containing eggs or tadpoles from another species of amphibian.) -- What about the pH level of the water, too low (acidic) and it spells disaster for eggs and spawn? (A further threat to the natterjack toad's future; the increased amounts of acid rainfall we receive each year.) There's a lot more to being an endangered toad than one might think.


 


It's odd to think that even the natterjack's own cousins are a direct threat to their existence, considering they are so closely linked (the common toad is also from the family Bufonidae, whilst the common frog is from the family Ranidae), yet this is the way the evolutionary cookie crumbles at times; being outnumbered and overpowered by hardier, more rugged, less fussy competitors is commonplace in natural life. It becomes apparent, nonetheless, after spending a period of time studying these animals at their spawning grounds, why the rarest of this tripartite is the most delicately poised.

Although not impossible to confuse the natterjack toad with a common frog, it is much more understandable to muddle it with its common toad relative. The appearance of the two is not dissimilar, with Epidalea calamita being somewhat smaller and sporting its instantly recognisable yellow dorsal stripe, offset against exquisitely pigmented skin tones of olive, grey and beige, deft inflections of pink and red; by comparison, B.bufo is much more drab. But it is the behaviour of the two that clashes most. The common beast is more of a ruffian, displaying behaviours not apparent in the natterjack, such as the mob-handed scrum that ensues when numerous males attempt amplexus with a female at their breeding pond, often leading to her suffocation or drowning; the natterjack is far more genteel -- one at a time please. It is also more subtle (until it starts to sing) with shorter legs allowing it to run surprisingly fast, rather than crawl with an awkward, stiff-legged gait, as is the form with its kindred kind.



 


Natterjack toad standing in slack pool.
Natterjack toads in amplexus.
Natterjack toad croaking with inflated throat sack.
Natterjack toad stalking a fly.
Newly arrived at a breeding pool, an adult natterjack toad pauses to assess the scene. By the time midnight has struck, the shallow scrape will be busy with toads, the males noisily croaking for the attentions of the females.
Firmly grasping a female, a successful male natterjack toad practices a manoeuvre known as amplexus. The pair may stay locked together for some time as he releases sperm, fertilising the string of eggs laid down by his mate.
After a night of being nose-to-nose with raucous toads, I was left in no doubt as to why male natterjacks are known as being the loudest amphibian in Europe. My head was numb from their ringing song by the time I returned to my vehicle.
When not vying for a mate or supremacy over a breeding scrape, natterjack toads will take time out to grab a meal. Purely carnivorous, they predominantly eat invertebrates. This one is stalking a fly in the bottom right of the picture.
Where to see these rare toads in Norfolk then? We have four known sites in the county, one of which is on private land, another is on inland heath, leaving two coastal locations that prove to be the most reliable and frequently used by E. calamita. Bearing in mind they are such a vulnerable creature -- protected by law  (the Wildlife and Countryside Act 1981) it is a criminal offence to handle or disturb them without licence -- and easily trodden upon during the dimming crepuscular light, even when searching for them with modern torches (such is the effectiveness of their camouflage), it would be foolhardy to suggest unsupervised visits to their localities. Instead, a visit to NWT Holme Dunes is in order, where a chat with the extremely knowledgeable warden will prove to be most rewarding.

Interesting fact...
Recent research carried out by Dr. Peter Minting of Amphibian and Reptile Conservation has revealed that the relationship of dorsal stripe and warts on individual toads is unique to each animal, a means of identification already being used to distinguish adders -- using the black inverted-V and head patterns -- in the field; knowledge that will surely lead to a better understanding of natterjack toad conservation in the future.
Natterjack toad staring at camera.
Glaring at me as I crouch down on the ground to gain a closer view of the animal, a natterjack toad displays its incredibly colourful eyes. The intricate patterning of both eyes and skin make them a very attractive amphibian.

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