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Like snakes, frogs and toads can be hard to identify at a glance. Whereas with snakes it is their leglessness that seems to make them nearly identical to the eye, with frogs and toads it is their fairly similar body shape but also their small size. Add to that their habits of either lurking in the water or else hopping away from the observer, and it makes identification tricky.
Nevertheless there are means of distinguishing the species, and Europe does not have the huge number of very similar species that can be a headache for a frog-watcher in, say, the Amazon. Amphibians lack the scales that are used to identify reptile species, but they do have other characteristic marks.
What follows is therefore an attempt to introduce the reader into how we identify frogs and toads (hereafter referred to as anurans) in layman's terms. You can't get away from using anatomical details, but hopefully this article will throw some light on the subject and remove the mystery and panic.
Handling anurans
The small size of these animals will sometimes mean that you have to pick them up to check their details. This should be done carefully, and if possible with moist fingers as dry salty human skin can be an irritant to the animals. Thumb and forefinger can be used to grasp the anuran around its middle, but be aware that some can be slippery and may try to squeeze free. Once you have examined the animal, release it if possible where you caught it. As soon as you can, wash your hands afterwards as amphibian skin contains chemicals that can be irritating to human skin, especially to the mucous membranes, and even toxic if swallowed. The necessity of hand washing after handling amphibians should be impressed on children, and young children should be discouraged from picking amphibians up owing to the danger of small fingers going into mouths or eyes afterwards.
Note: Carrying infection between pools can cause serious illness to reptiles and amphibians, please keep any handling to a minimum and thoroughly wash hands.
General identification details
The differences between anuran species are quite subtle but present nevertheless. With a bit of practice it soon becomes possible to at least tell them apart at a genus level, if not individual species.
Shape is quite subtle, since all European anurans have a similar shape and pose, but there are areas of distinction. Firstly, look at the overall build of the animal. Some are quite stocky, like true toads (Bufo species) while others are more elongated. The shape of the snout is often a characteristic, since in some species it is blunt and rounded while in others it is elongated and pointed.
Colour is not always helpful, since some species may be found in different shades of brown or green. However, as a rule Rana in Iberia are either plain brown or green, sometimes with a darker patterning of spots and/or lines on the back and dark bands across the legs. Hyla tree frogs tend to be a grass green colour to match their surroundings. Bufo may be a clayish or earth brown, and in addition the Natterjack Toad Bufo calamita has a sort of camouflage pattern of darker patches. The other anurans in the area often have a pattern of smallish darker spots. Checking the colour of the underneath of the animal may also be helpful in identification.
Size is not particularly helpful with anurans, since the vast majority do fall within a narrow range of less than 30mm/1', even in the tropics. However, in some cases it can immediately eliminate some possibilities. The largest Iberian anurans are true toads (Bufo) which are both stocky and relatively large, females reaching up to 20cm although males may be about half this, with Rana (pond and water frogs) following. The smallest species are the various round-tongued, tree and parsley frogs, all in the 5cm long region, while the spadefoot toads are intermediate at about 11cm.
Skin texture is a helpful way of distinguishing true toads from other anurans. If it's very warty, it's a true toad; if it's smooth, it's not. Pond frogs tend to have rather smooth skin, as do tree frogs. The others are intermediate.
The shape of the pupil can be a very good indicator for which group a frog or toad belongs to. Pupil shapes are horizontal, vertical, heart-shaped (as in the card suit) or of the inverted tear drop shape. True toads and tree frogs have horizontal pupils; spadefoot toads (Pelobates) have vertical pupils, rather like cats in bright sunlight, while Painted Frogs (Discoglossus) have inverted tear-drop shaped pupils. In pond and water frogs (Rana) the pupil is almost or completely round.
Habitat and geographical location may also be a guide. Field guides will normally show the range of a species, so by process of elimination you may be able to arrive at a conclusion. For example in Spain the frog Rana dalmatina is only found in areas bordering the Pyrenees, while Rana iberica is not found in the east or south of Iberia. Habitat can also help to eliminate some possibilities, although less so than with reptiles since as a rule anurans tend to stay close to water. A frog sitting in a tree will almost inevitably be one of the Hyla (treefrog) species (if not then a Parsley Frog, Pelodytes punctatus), while Bufo tend to be found further away from water bodies than other European anurans. Some Spanish species are more tolerant of brackish water than others.
Behaviour may also be a key. The most obvious example of this is the genus Alytes (Midwife Toads), whose males at a certain time of the year may be encountered carrying a string of eggs wrapped around their rear limbs. An anuran digging itself backwards into soil or sand will usually be the Spadefoot Toad Pelobates cultripes.
Sexual identification
As a rule male anurans tend to be smaller than their female counterparts. An anuran calling and/or blowing its throat or corners of the mouth out is a male. If two anurans are seen clasped in a piggyback-like embrace (amplexus), the uppermost one is the male. There are different forms of amplexus according to how the male embraces the female: in European species it is usually either just behind the front limbs (axillary amplexus) or in front of the rear limbs (lumbar amplexus). The presence of rough, dark pads on the feet and hands (nuptial pads) also indicate a male.
Identifying tadpoles
This is really a whole different ball-game and one that would take a lot more space to cover, not to mention the difficulties in being able to see the detail necessary in such a small creature. For those who are interested in pursuing this, a good field guide to Iberian or European reptiles and/or amphibians will often have characteristics of the tadpoles for each species.
Anatomical characteristics
Anatomical details on European anurans can be quite helpful in discerning a species. The main areas to be considered are dorsal (essentially the back or upper side, from head to rear) and the limbs and digits.
Dorsal area
In some genera or species there is a clear line or ridge along the back where the back joins the sides of the body. This is the dorsolateral fold.
Look to the rear of the eyes. There may be a raised area on both sides of the head containing pore-like openings. These areas are known as the parotoid glands. Not only can their presence be a key to the species, but also their shape, since in some species the gland area is straight and in others bent inwards.
As noted earlier, the shape of the snout differs between species.
Look at the sides of the frog to see whether there is a longitudinal row of warts. Further forward, behind the eye, look for a round disk-like structure. This is the tympanum or ear, which is often quite visible in Rana species, visible in Hyla, Discoglossus and Alytes, somewhat so in Bufo and not at all in Pelobates. The diameter of the tympanum relative to that of the eye is sometimes used as an identifying characteristic.
Limbs and digits
In European anurans the hindlegs are always longer than the forelegs, often greatly so. One way to use the limbs in identifying an anuran is to gently turn the rear leg forwards along the body and press the “hump” on the back down. Then check if the heel (where the foot joins the leg) reaches the snout or not. Do not perform this procedure if you feel uncomfortable doing so.
On foot close to the heel, check for an enlarged growth or tubercle. If present then the animal is probably a Spadefoot Toad, Pelobates cultripes. (This is the origin of the “Spadefoot” name!).
Treefrogs have expanded discs on the end of their fingers and toes to aid them in climbing.
In herpetology, anuran digits are numbered from the inside towards the outside. Thus the innermost finger on the hand of a frog will be 1, while the outermost finger will be 4 (since anurans have only 4 fingers). The outermost toe on the anuran foot will always be 5, while the middle one will be 3. Comparing the relative lengths of digits on an anuran hand or foot is another way of identifying a species.
On the underneath of the fingers and toes are often further small hard tubercles. These may occur in pairs or singly, and their location, shape and layout is a further characteristic for many species. For example, one species of Alytes (Midwife Toads) in Spain is distinguished from another simply by the number of tubercles on its palm (3 as opposed to 2).
Other characteristics
Measurements are often used by experts to tell anuran species apart, but are included here mainly for completeness rather than necessity. Measurements include: distance from the tip of the snout to the nostril; distance from nostril to the front of the eye; diameter of nostril; diameter of eye; distance from the back of the eye to the tympanum; diameter of tympanum; length of head (from tip of snout to rear edge of tympanum); length of snout from tip to front edge of eye, including nostrils; distance between nostrils; closest distance across the top of the head between the eyes; width of head as measure from side to side touching the rear edge of the tympanum. These measurements are often done with calipers and in Spain and Portugal should not be necessary.
In summary
The above guide is as much a guide to the conventions and terms used in describing frogs and toads as a guide to their identification in Spain. It can thus be used anywhere in the world, especially those countries where a multitude of different anurans can cause confusion. It should be noted however that the situation in tropical areas, especially South America, can be rather more difficult owing to the vast numbers of very similar-looking species.
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