Adult Common Toads Bufo bufo are brown, grey or olive above, often plainly coloured but sometimes with dark markings. The hind legs are not so large and powerful as the Common Frog Rana temporaria. Adult toads are variable in size, with females being larger, sometimes much larger, than males, 85 mm in length compared with 55 mm. They are not usually very variable in colour. In some populations young toads can be quite distinctive in colour, even though the adults have normal colouration. Brick-red is quite common in north Northumberland and in the Chester-le-Street/Washington areas. Blue-green adults are very occasionally seen, and have been photographed at Hetton Bogs and at Horden. Very pale or quite dark toadlets also occur, either as individuals or as the majority colour.
The “warty” skin is a distinctive feature. It contains glands which release a toxic and distasteful fluid if the toad is bitten by a predator. Pores can also release the toxin on to the skin if the toad is attacked without the skin being broken. Despite this, some foxes, badgers and hedgehogs learn to split the toad open, skinning it and eating the innards without being affected by the skin.
The Common Toad is much more terrestrial than the Common Frog, usually visiting ponds only for the few days of the intensive mass spawning. This preference probably accounts for the brief spawning period. The rest of the year is spent on land, up to two km from the breeding pond, often in fairly dry habitats. Studies in Holland and Germany have shown that the migration towards the breeding ponds starts in the autumn, until it is interrupted by cold weather. The animals then hibernate in crevices or small mammal burrows and resume their journey in the spring. It is not known if this happens in northeast England. Spring migrations of toads can be very visible, with single-minded animals travelling en masse by straight-line routes, sometimes with high casualties where they cross roads.
Toads spawn in the spring, several weeks after frogs, and are often quite noticeable because of their numbers and the calling of the males. Males develop “mating pads” on the inner three digits (“middle finger” to “thumb”) of their front legs in the breeding season (Beebee and Griffiths, 2000). The males mature a year earlier than females, so they outnumber the females at the spawning pond, producing intense competition for mates. This can result in large balls of struggling males with a single female at the centre. They spawn in deeper water than frogs, often in the centre of the pond, laying strings of eggs rather than clumps. These strings are wrapped around water plants and like frogspawn they have transparent jelly that expands on contact with the water. The dark-coloured eggs are in two rows along the string, averaging from 1,200 to 1,500 per female (Beebee and Griffiths, 2000).
The tadpoles are almost jet black and often congregate together in large shoals, moving around the pond through the shallows keeping to where the water is warmed by the sun. They can group together because they already have toxins in their skin and are safe (or at least, the second one is safe!) from being bitten or swallowed whole by birds or fish. They can still be killed by some species of aquatic invertebrates, such as dragonfly and beetle larvae, which have piercing mouth parts and are able to suck out the tadpole’s flesh through a hole in the skin. The tadpoles feed on algae and small invertebrates but also on frogs and toads that have died in the pond after spawning. In 2009, at Quarryhouse Moor ponds in Northumberland, a late frost killed several hundred adult toads and subsequently most of the tadpoles fed in large clusters around each of the decaying bodies of the adults.
The metamorphosed tadpoles emerge from the pond en masse and seek out sheltered niches with cover and a supply of invertebrate food. At this stage, mortality, which has been low in the pond, becomes much higher. Once a young toad has become established in a suitable terrestrial niche, they can remain quite faithful to that location for many years.
Toads are widespread in the region and only absent from the heather moors, the Northumberland islands and from some of the urban areas. Apart from these areas, gaps in the distribution map are very likely to be unrecorded areas, rather than areas where toads are absent (Durkin, 2010A).
Toad tadpoles are distasteful to most fish and waterfowl, so toads are able to breed successfully in the larger ponds, lakes and reservoirs, unlike frogs. Small ponds and shallow ponds, including most garden ponds, are rarely used for spawning by toads and as a result they are rarely caught out by ponds drying out in summer.
Toads frequently spawn in rivers if conditions are right. The ideal situation seems to be one of the smaller rivers at a point where the channel is “braided” and there are side channels with more slowly flowing water. There are a number of regular toad spawning sites in the Rivers Greta, Derwent and Coquet. Inedibility to fish is probably a significant feature in this behaviour. One consequence of spawning in rivers is that the tadpoles are dispersed for several miles downstream. Toads and their tadpoles are also able to tolerate slightly brackish water where other amphibians cannot. They can be found in sand dune pools and other ponds that receive sea spray, such as cliff top ponds, salt marshes and where freshwater streams reach the sea, such as at Castle Eden Dene mouth.
The Natural England Site of Special Scientific Interest (SSSI) assessment criteria for amphibians ranks toads populations by two measures, “estimated” and “counted” spawning adults, because of the difficulty of an accurate count when toads are spawning in deep water some distance from the shore. A “Low” population is for the number of spawning adults to be under 100 counted, or 500 estimated. “Good” is up to 1,000 counted, or 5,000 estimated. “Exceptional” is over 1,000 counted, or over 5,000 estimated (Nature Conservancy Council, 1998). Note that these figures are about 10 times the numbers for Common Frogs, as toads tend to be more concentrated in a smaller number of breeding sites than frogs. In our region there are a number of “exceptional” sites, including Quarryhouse Moor, Seaton Dunes, Newton Pool, Pockerley Farm Pond, Whitburn Observatory Pond, Caistron and Rothley Lake. Some reservoirs, such as Fontburn, Scaling Dam and Tunstall, may also have over 1,000, but there are no proper counts yet.
Toads can be the only amphibian species in some of their habitats, such as slightly saline ponds, large water bodies with predatory fish populations and in rivers. Where they do share the habitat with other amphibians it is usually in larger ponds without fish, where all five of our amphibian species may be present.
by John Durkin