However, eradication or restoration efforts in certain habitats are useless if new bullfrogs are introduced into nature at the same time. Preventing new 'introductions' is always better and cheaper than eradicating populations afterwards. It is therefore important to keep track of new observations of American bullfrogs. Have you seen or heard of an American bull frog? Then pass it on at www.waarnemingen.be.
You can quite easily distinguish the bull frog from our native frog species if you look at the right characteristics. Their call, size, eardrums, throat and back are distinguishing characteristics. Bullfrog tadpoles are more difficult to distinguish from their native counterparts, but their size is usually a good indicator.
The call is by far the easiest characteristic to recognize American bullfrogs. The male bull frog bellows like a bull, hence the species’ name. The sound is sometimes described as if someone shouts the word “rum” into an empty barrel. Please note: 'green frogs' such as the edible frog and the marsh frog can also make a lot of noise. They are therefore sometimes confused with bullfrogs. The sound can also sometimes be confused with the call of the bittern.
A calling male bullfrog. The yellow throat and large eardrums are clearly visible and are characteristics by which the species is easily identified.
Bull frogs are by far the largest frogs you can find in our regions. Adults can reach a size of more than 20 cm, with a weight of more than 500 g. In comparison, the marsh frog (Pelophylax ridibundus), our largest native frog, is a lot smaller with an average size of 10 to 14 cm.
A sizeable bullfrog. Animals of this size are pretty common.
The other aforementioned characteristics are a bit more subtle, but still quite easy to recognize for the trained eye. Bullfrogs have a large tympanic membrane, which is about the same size as the eye in females, and much larger in males. In our native frogs, the eardrum is always the same size or smaller compared to the eye. Bullfrog males have a single throat sac, which is yellow in color. Native green frog males have two cheek sacs. The moor frog, common frog and European tree frog also have a throat sac, but this is never colored yellow and you will not easily mistake these species for a bull frog. Finally, bullfrogs usually have an olive green to brown back, with no dorsal ridges (raised lines on the back) or longitudinal stripes. Native green frogs have two dorsal ridges and almost always a lighter longitudinal stripe centrally on the back.
On the left a bullfrog, and on the right a native frog. The notable characteristics for species identification are highlighted: eardums (A), longitudinal stripe (B) and dorsal ridges (C).
A native green frog, with clearly visible dorsal ridges and longitudinal stripe. The eardrum is also smaller compared to the eye.
Tadpoles are always harder to identify correctly. Here the only really useful feature is their size. Bullfrog tadpoles become very large, up to 16 cm long. It normally takes two to three years for a bullfrog tadpole to develop into a frog. They overwinter in the water near the bottom. Again, it is important not to confuse bullfrog tadpoles with the tadpoles of the green frog, which can also take up to two years to become a frog but typically remain much smaller.
A tadpole of an American bullfrog
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