The common toad, also known as European toad or simply the toad in Anglophone parts of Europe (Bufo bufo, from Latin bufo "toad"), is a toad found throughout most of Europe (excluding Iceland, Ireland, parts of Scandinavia, and some Mediterranean islands), western North Asia and a small part of Northwest Africa. It belongs to a group of closely related species descended from a common ancestral line of toads and forming a species complex. The toad is an inconspicuous animal, usually lying hidden during the day. It becomes active at dusk, hunting the invertebrates on which it feeds at night. It moves with a slow, ungainly walk or by jumping short distances, and has greyish-brown skin covered with wart-like lumps.
Although toads are usually solitary animals, large numbers of them converge on certain breeding ponds during the breeding season, where the males compete to mate with the females. The females lay their eggs in gelatinous strings in the water, which later hatch into tadpoles. After several months of growth and development, the tadpoles sprout limbs and undergo metamorphosis to become tiny toads. The juveniles then leave the water and remain largely terrestrial for the rest of their lives.
The common toad appears to be in decline in some areas, but is currently classified as being of "least concern" on the IUCN Red List of Threatened Species. It is under threat due to habitat loss, particularly the drainage of its breeding grounds, and some toads are killed on roads during their annual migrations. The toad has long been associated with witchcraft in popular culture and literature. Taxonomy
The common toad was first given the name Rana bufo by the Swedish biologist Carl Linnaeus in the 10th edition of Systema Naturae in 1758. In this work, he placed all the frogs and toads in the single genus Rana. It later became apparent that this genus should be divided, and in 1768, the Austrian naturalist Josephus Nicolaus Laurenti placed the common toad in the genus Bufo, naming it Bufo bufo. The toads in this genus are included in the family Bufonidae, the true toads.
Various subspecies of B. bufo have been recognised over the years. The Caucasian toad is found in the mountainous regions of the Caucasus and was at one time classified as B. b. verrucosissima. It has a larger genome and differs from B. bufo morphologically and is now accepted as Bufo verrucosissimus. The spiny toad was classified as B. b. spinosus. It is found in France, the Iberian Peninsula and the Maghreb and grows to a larger size and has a spinier skin than its more northern counterparts with which it intergrades. It is now accepted as Bufo spinosus. The Gredos toad, B. b. gredosicola, is restricted to the Sierra de Gredos, a mountain range in central Spain. It has exceptionally large paratoid glands and its colour tends to be blotched rather than uniform. It is now considered to be a synonym of Bufo spinosus.
B. bufo is part of a species complex, a group of closely related species that cannot be clearly demarcated. Several modern species are believed to form an ancient group of related taxa from preglacial times. These are the spiny toad (B. spinosus), the Caucasian toad (B. verrucosissimus) and the Japanese common toad (B. japonicus). The European common toad (Bufo bufo) seems to have arisen more recently. It is believed that the range of the ancestral form extended into Asia but that isolation between the eastern and western species complexes occurred as a result of the development of the Central Asian Deserts during the Middle Miocene. The exact taxonomic relationships between these species remains unclear. A serological investigation into toad populations in Turkey undertaken in 2001 examined the blood serum proteins of Bufo verrucosissimus and Bufo spinosus. It found that the differences between the two were not significant and that therefore the former should be synonymized with the latter.
A study published in 2012 examined the phylogenetic relationships between the Eurasian and North African species in the Bufo bufo group and indicated a long evolutionary history for the group. Nine to thirteen million years ago, Bufo eichwaldi, a recently described species from south Azerbaijan and Iran, split from the main lineage. Further divisions occurred with Bufo spinosus splitting off about five million years ago when the Pyrenees were being uplifted, an event which isolated the populations in the Iberian Peninsula from those in the rest of Europe. The remaining European lineage split into Bufo bufo and Bufo verrucosissimus less than three million years ago during the Pleistocene. Very occasionally the common toad hybridizes with the natterjack toad (Epidalea calamita) or the European green toad (Bufotes viridis).
Description
The common toad can grow to around in length. Females are usually stockier than males, and southern specimens tend to be larger than northern ones. Its head is broad, with a wide mouth below the terminal snout, which has two small nostrils. There are no teeth. The bulbous, protruding eyes have yellow or copper-coloured irises and horizontal, slit-shaped pupils. Just behind the eyes are two bulging regions: the paratoid glands, which are positioned obliquely. These glands secrete a noxious substance called bufotoxin, which is used to deter potential predators. The head joins the body without a noticeable neck, and there is no external vocal sac. The broad. squat body is positioned close to the ground. The forelimbs are short, with the toes of the front feet turning inwards.
During the breeding season, the male develops nuptial pads on his first three fingers. He uses these to grasp the female during mating. Its hind legs are shorter than those of other frogs and its hind feet have long, unwebbed toes. There is no tail. The skin is dry and covered in small, wart-like lumps. It is a fairly uniform shade of brown, olive-brown or greyish-brown, and is sometimes partly blotched or banded with a darker shade. The common toad tends to be sexually dimorphic, with the females being browner and the males greyer. The underside is a dirty white speckled with grey and black patches.
Other species that could be confused with the common toad include the natterjack toad (Bufo calamita) and the European green toad (Bufo viridis). The former is usually smaller and has a yellow band running down its back, whereas the latter has a distinctive mottled pattern. The paratoid glands of both species run parallel to each other, rather than slanting as in the common toad. The common frog (Rana temporaria) is also similar in appearance but it has a less rounded snout, damp smooth skin, and usually moves by leaping.
Common toads can live for many years and have survived for fifty years in captivity. In the wild, common toads are thought to live for about ten to twelve years. Their age can be determined by counting the number of annual growth rings in the bones of their phalanges.
Distribution and habitat
After the common frog (Rana temporaria), the edible frog (Pelophylax esculentus) and the smooth newt (Lissotriton vulgaris), the common toad is the fourth most common amphibian in Europe. It is found throughout the continent except in Iceland, the cold northern parts of Scandinavia, and a number of Mediterranean islands. These include Malta, Crete, Corsica, Sardinia and the Balearic Islands. It has been introduced to a number of places in Ireland, and appears to be thriving. Its easterly range extends to Irkutsk in Siberia and its southerly range includes parts of northwestern Africa in the northern mountain ranges of Morocco, Algeria and Tunisia. A closely related variant lives in eastern Asia including Japan. The common toad is found at altitudes of up to in the southern part of its range. It is largely found in forested areas with coniferous, deciduous and mixed woodland, especially in wet locations. It also inhabits open countryside, fields, copses, parks and gardens, and is often found in dry areas far from standing water.
Behaviour and lifecycle
The common toad usually moves by walking rather slowly or making short shuffling jumps with all four legs. It spends the day concealed in a hollow it has made under foliage, a root or a stone, where its colouring makes it inconspicuous. It emerges at dusk and may travel some distance while hunting in the dark. It is most active in wet weather. By morning, it has returned to its lair, where it may remain for several months. It is voracious and eats woodlice, slugs, beetles, caterpillars, flies, ants, spiders, earthworms and even small mice. Small, fast-moving prey may be caught with a flick of the tongue while larger items are grabbed with the jaws. As it has no teeth, the species swallows food whole in a series of gulps. It does not recognise its prey as such, but will try to consume any small, dark-coloured, moving object it encounters at night. A research study showed that it would snap at a moving piece of black paper as if it were prey, but disregarded a larger moving piece. Toads seem to use visual cues for feeding and can see their prey at low light intensities where humans are unable to discern anything. Periodically, the common toad sheds its skin. This comes away in tattered pieces and is then consumed.
In 2007, researchers using a remotely operated underwater vehicle to survey Loch Ness in Scotland, observed a common toad moving along the lake bed at a depth of approximately . They were surprised to find that an air-breathing animal could survive in such a location.
The annual life cycle of the common toad is divided into three periods: the winter sleep, the time of mating and feeding period.
Predators and parasites When attacked, the common toad adopts a distinctive defensive posture, inflating its body and standing with its hindquarters raised and its head lowered. Its chief means of defence lies in the foul-tasting secretion that is produced by its paratoid glands and other glands on its skin. This secretion contains a toxin called bufagin, which is enough to deter many predators, although grass snakes seem to be unaffected by it. Other predators of adult toads include hedgehogs, rats, mink, and even domestic cats. Birds that feed on toads include herons, crows and birds of prey. Crows have been observed puncturing the skin with their beaks and pecking out the toad's liver to avoid the toxin. The tadpoles also secrete noxious substances to deter fish, but not great crested newts. Aquatic invertebrates that feed on toad tadpoles include dragonfly larvae, diving beetles and water boatmen. They usually avoid the noxious secretion by puncturing the tadpole's skin and sucking out its juices.
A parasitic fly, Lucilia bufonivora, attacks adult common toads. The fly lays its eggs on the toad's skin, and when they hatch, the larvae crawl into the toad's nostrils and eat its flesh from inside, with lethal consequences. The European fingernail clam (Sphaerium corneum) is unusual in that it can climb up water plants and move around on its muscular foot. It sometimes clings to the toe of a common toad and this is believed to be one of the means by which it disperses to new locations.
Reproduction
The common toad emerges from hibernation in spring, migrating en masse to the breeding sites. They converge on certain favoured ponds while avoiding other seemingly suitable stretches of water. Adults use the same location year after year, and over 80% of males marked as juveniles have been found to return to the pond where they were spawned. They find their way to these by using a suite of orientation cues, including olfactory and magnetic cues, but also visual cues help guide their journeys. Toads experimentally moved elsewhere and fitted with tracking devices have been found to be able to locate their chosen breeding pond when the displacement exceeded three kilometres (two miles).