Skulls, crests, snouts and giant poison glands: the heads of toads

Yes! MORE TOADS. You surely know what a toad's head looks like. But there's a lot about toad skulls that you almost certainly don't know, and the aim of this article is to review toad skull anatomy. This might seem like an arcane subject, but - as we'll see - the diversity of toad skulls is really quite remarkable and much of toad success can be put down to various of their cranial features (such as their parotoid glands and strong degree of cranial ossification)...

Whereas anurans typically have small teeth lining their upper jaws, toads are entirely toothless. However, tooth-like structures (termed odontoids) are present on the palate of the Colorado river toad Incilius alvarius (Mendelson & Pramuk 1998), though they're small and - so far as we know - don't play any special role in the feeding ecology or behaviour of this species (which raises the question of why the toads have them). Odontoids are widespread in anurans, but their recent discovery in a toad was still a bit of a surprise.

The degree of cranial ossification is highly variable within toads: members of the group are best known for having strongly ossified, robust skulls where the overlying skin is fused to the bones. Some species, however, have lightly ossified, 'open plan' skulls where crests and ridges are absent, the interorbital part of the skull is narrow, and all the bones are thin. It used to be thought that the toads classified together in Bufo sensu lato [see this previous article for an introduction to the 'Bufo problem'] could be divided into 'narrow-skulled' and 'broad-skulled' groups though, confusingly, referral to either one of these groups was sometimes determined by molecular data or style of vocalisation (Blair 1972). A toad with a broad skull might, actually, be a member of the narrow-skulled group under this scheme. Recent phylogenetic work has shown that narrow-skulled taxa are scattered all about the tree and do not form a clade. Short, rounded snouts are typical for toads, but the configuration of snout bones is pretty variable within the group, and novel ossifications and even long, pointed snouts have evolved in some taxa.

Crests and weird protruding noses

Toads exhibit a variety of cranial crests and accompanying bony bosses, though these are not present in all taxa. Formed from dermal bone that accretes during ontogeny, these are diagnostic to the species level and, in areas where several toads occur together, the pattern and distribution of the crests and bosses provides one of the most reliable means of distinguishing the species. The cranial crests are frequently larger in females than in males. Among the more important crests are the frontoparietal or interorbital or supraorbital ones (they extend in parallel fashion along the dorsal surface of the skull, medial to the eyeballs), and the postorbital ones (these extend transversely across the skull surface, in between the eyeballs and parotoid glands). Various different terminologies have been used for the cranial crests: a helpful explanatory diagram of one system of nomenclature is shown here [from Pramuk & Kadivar (2003)].

Bony crests have been done away with in some lineages and are replaced by fleshy ones. This is the case in the Bom Jardin toad Rhinella dapsilis of Colombia, Brazil, Peru and Ecuador: this species also possesses a soft, fleshy extension to its snout tip. Extended snout tips are also present in other members of Rhinella (such as R. sclerocephalus from Venezuela, and in the beaked toads [one of which, R. tenrec, is shown at lower left in the montage at the very top]), but are hard and bony, rather than soft. Outside of Rhinella, the weird noses of the Puerto Rican crested toad Peltophryne lemur and Cuban long-nosed toad P. longinasus curve upwards and give them a pretty comical appearance [P. lemur photo below from the Crested Toad Species Survival Programme site].

In keeping with the high degree of variation within skull ossification, the configuration of the postorbital region is pretty variable within toads. In some species the squamosal is ventrally forked, with only the posterior fork descending as far ventrally as the quadratojugal. In others (such as the West Indian Peltophryne species and the members of the 'Bufo granulosus group'*), a heavily ossified anterior fork forms a robust sheet that extends down to the maxilla (Pramuk 2000, 2002). In the Cuban toad P. empusa the postorbital part of the skull is uncharacteriscally massive for an anuran, and the deep maxillae and stout cheek regions of some West Indian toads make their skulls look like those of miniature turtles [assemblage of tropical American toad skulls shown below: you can see the ventrally forked squamosal in the species at bottom right [Incilius valliceps], and can then imagine that the two forks have united to form a massive sheet in some of the other species [like H, P. empusus, and I, P. taladai; from Pramuk (2002)].

* The exact affinities of the 'Bufo granulosus group' are controversial, but it's probably part of Rhinella.

Both of these toad groups also exhibit extra ossification in the snout region that isn't present in other groups: in the 'Bufo granulosus group' there are new 'prenasal bones' that take the place normally occupied by the premaxillae, while in the West Indian toads the maxillae extend anteriorly to take the place of the premaxillae (the premaxillae themselves now lie in the roof of the mouth) (Pramuk 2000). In the West Indian toads, the anterior extensions of the maxillae were originally misidentified as separate ossifications dubbed 'rostral bones' by Pregill (1981). There's some indication that, like other anurans with extra skull ossification (like casque-headed tree frogs), these West Indian toads might be what's termed phragmotic: that is, they might use their enlarged, bony snouts to help prevent desiccation when they hide in burrows or the axils of bromeliads. This is supported by the fact that, like casque-headed tree frogs, some of these species (like the Cuban toad P. empusus) are able to flex the head at a 90° angle relative to the body (Pramuk 2000).

Poison and poison glands

The parotoid glands contain large poison sacs connected to the surface via ducts [image below shows Cane toad head: the parotoid glands are pretty obvious. From wikipedia]. When a toad is bitten or otherwise molested, milky white, viscous poison oozes out from the glands onto the skin; under extreme provocation at least some species can actually squirt the poison for a distance of a metre or so. The poison of the Cane toad (about the only species for which detailed information is available) is a complex cocktail, containing a whole list of chemical substances including marinobufagin, dehydrobufotenine, adrenaline and noradrenaline and, no, I have no idea what these names mean either (well, I know what adrenaline is). There's some suggestion that the toxins might not just act as a very persuasive deterrent to predators (dogs, marsupials and other predators are routinely killed by Cane toad poison), but that they might also protect the toad against bacteria and fungi (Tyler 1994). Several toads belonging to the same group as the Cane toad (Rhinella) also have large poison glands on their limbs. I'll be showing examples later on in the series.

Despite their effectiveness, parotoid glands are absent in many toad taxa, most notably many of the South American forms. It seems likely that the presence of the glands is derived within the group, and that many of the taxa that lack them do so because these toads occupy a basal position in the phylogeny. Having said that, reduction or loss of the parotoids also seems to have occurred on several occasions.

Coming next: Toads of the north!

For previous articles in the monumental, ground-breaking toad series see...

• Toadtastic - the invasion begins!
• Bidder's organ and the holy quest for synapomorphies
• Our sex lives in words and pictures (or, On the reproductive biology of the Bufonidae)

For previous articles on hyloid anurans see...

• Britain's lost tree frogs: sigh, not another 'neglected native'
• Ghost frogs, hyloids, arcifery.. what more could you want?
• Green-boned glass frogs, monkey frogs, toothless toads
• It's the Helmeted water toad!
• Horn-headed biting frogs and pouches and false teeth
• More wide-mouthed South American horned frogs
• We need MORE FROGS

Refs - -

Blair, W. F. 1972. Summary. In Blair, W. F. (ed) Evolution in the genus Bufo. University of Texas Press, Austin, pp. 329-343.

Pramuk, J. B. 2000. Prenasal bones and snout morphology in West Indian bufonids and the Bufo granulosus species group. Journal of Herpetology 34, 334-340.

- . 2002. Combined evidence and cladistic relationships of West Indian toads (Anura: Bufonidae). Herpetological Monographs 16, 121-151.

- . & Kadivar, F. 2003. A new species of Bufo (Anura: Bufonidae) from southern Ecuador. Herpetologica 59, 270-283.

Pregill, G. 1981. Cranial morphology and the evolution of West Indian toads (Salientia: Bufonidae): resurrection of the genus Peltophryne Fitzinger. Copeia 1981, 273-285.

Tyler, M. J. 1994. Australian Frogs: A Natural History. Reed Books, Chatswood, New South Wales.