Tetrapod Zoology

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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)].

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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].

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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.

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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.

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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…

For previous articles on hyloid anurans see…

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.

Comments

  1. #1 David Marjanović
    October 21, 2009

    Noradrenaline is pretty much the same – another such hormone.

  2. #2 Anonymous
    October 21, 2009

    “Whereas anurans typically have small teeth lining their upper jaws, toads are entirely toothless.”

    We’re talking about the “true toads” of the Bufonidae, right? Because I thought I read somewhere that some of the spade-foot toads possessed teeth or something too. Anyway, if toads don’t have any teeth, then how do they snag their food? Do they just grab it with their tongue and hope for the best? Or do their toothless jawbones play a role?

    Secondly, do you think the rough skin and relative tolerance of dry climates in toads is the reason why we don’t see many lizards evolving a toad-like shape, or are there lizards who share the same niche, but have just deigned not to evolve a similar body shape.

  3. #3 Sven DiMilo
    October 21, 2009

    Also, adrenaline and noradrenaline are now referred to nearly exclusively as epinephrine and norepinephrine (I heard the switch resulted from a pharmcorp patenting the word “adrenaline” but have never checked that story out).

    But of course neurons that secrete norepinephrine are still called “adrenergic.” *shrug*

  4. #4 Sven DiMilo
    October 21, 2009

    Bufonid skin does not protect against water loss at all–evaporative water loss rates from toads approximates that of a free-water surface of equal area. Although many bufonids prefer to walk rather than hop, their body design reflects the hopping specialization of their ancestors. Lizards, in contrast, do have dry skin with (generally) low water conductance, and have always been walk/runners.

  5. #5 Rajita
    October 21, 2009

    It is interesting to note that dehydrobufotenine is also a derivative of a neurotransmitter serotonin. Thus along with nor/epinephrine it is also synthesized as a part of the animal neurotransmitter pathway from amino acids. A common strategy in the evolution of venoms is the use of neurotransmitters or their derivatives as you have a molecule with a ready target. The most interesting in this class is a toxin secreted by certain toads by the big gland behind the ear — 5-methoxy-dimethyltryptamine. It causes an interesting range of hallucinations. In humans it is supposed to cause a particularly strong reaction in combination with a monoamine inhibitor. I suspect this is what the 5MeODMT and dehydrobufotenine combination does to a potential tetrapod predator.

    Marinobufagin differs from them in being a steroid related to the Digitalis steroids. As several anurans acquire their toxins from insects they feed it makes one wonder if this toxin is made by an insect or acquired by the insect from a plant and then concentrated by the toad.
    Cheers
    Rajita

  6. #6 Rajita
    October 21, 2009

    BTW – in our secondary school zoology we would have a weird question: What are the differences between frogs and toads :-)
    The text book used to list Bidder’s organs as one of them and the teacher would characteristically state that a great open scientific problem was to figure out its function. Later in life we were fascinated by an apicomplexan parasite that would only infect the Bidder’s organ of toads. It resembled Eimeria or Isospora. I latter found an obscure paper by an American naturalist who noticed some parasite in the Bidder’s organ but she never illustrated so I am not sure if she was seeing the same thing as me. I would like to know if any of you all have further leads on this line of research.

  7. #7 David Marjanović
    October 22, 2009

    We’re talking about the “true toads” of the Bufonidae, right? Because I thought I read somewhere that some of the spade-foot toads possessed teeth or something too.

    Of course.

    The most interesting in this class is a toxin secreted by certain toads by the big gland behind the ear — 5-methoxy-dimethyltryptamine. It causes an interesting range of hallucinations. In humans it is supposed to cause a particularly strong reaction in combination with a monoamine inhibitor. I suspect this is what the 5MeODMT and dehydrobufotenine combination does to a potential tetrapod predator.

    Somewhere in the Amazon, a certain anuran species is used for inducing powerful religious experiences. Just tie it to something, poke it gently, and take the white stuff off its skin.

  8. #8 Owlmirror
    October 22, 2009

    The most interesting in this class is a toxin secreted by certain toads by the big gland behind the ear — 5-methoxy-dimethyltryptamine. It causes an interesting range of hallucinations. In humans it is supposed to cause a particularly strong reaction in combination with a monoamine inhibitor. I suspect this is what the 5MeODMT and dehydrobufotenine combination does to a potential tetrapod predator.

    And some tetrapods besides humans find the effect interesting enough to want to repeat the experience — frequently.

    The canid who deliberately sought out bufonids so as to ingest their hallucinogenic secretions.

  9. #9 Nathan Myers
    October 22, 2009

    Thank you, Rajita, that’s just the information I was hoping to elicit.

    Street lore is that the way to prepare Bufo(?) toxin is to rub the inverted toad on your windshield and let it (the deposit, not the toad) dry, then scrape it off and vaporize it in a pipe with a lighter. It seems like it ought to matter which species to use.

  10. #10 brooks
    October 22, 2009

    5MeoDMT is found in only one toad that i know of (the Sonoran Desert or Colorado River toad, Bufo alvarius), and several central and south american plants.

    bufotenine’s (5OH-DMT) short buzz is bought at a higher price, as it is also pretty damn toxic (note that you’re smoking a substance capable of crippling or killing all of australia’s predatory vertebrate species). ick.

  11. #11 Sven DiMilo
    October 22, 2009

    How about Them Toad Suckers,
    Ain’t they clods?
    Sittin’ there suckin’
    Them green toady-frogs.

    Suckin’ them hop-toads,
    Suckin’ them chunkers,
    Suckin’ them leapy types,
    Suckin’ them plunkers.

    Look at Them Toad Suckers,
    Ain’t they snappy?
    Suckin’ them bog-frogs
    Sure makes’em happy.

    Them huggermugger Toad Suckers,
    Way down south,
    Stickin’ them sucky-toads
    In they mouth.

    How to be a Toad Sucker?
    No way to duck it.
    Gittchyself a toad,
    Rare back and suck it!

    -Mason Williams

  12. #12 John H
    October 24, 2009

    Cool stuff. I wonder how much the gland secretions help toads against the chytrid fungus that is wiping out frogs and other species; are more glandular toads less susceptible to it?

  13. #13 William Miller
    October 25, 2009

    “Puerto Rican crested toad Peltophryne lemur

    Why is a Puerto Rican toad named after a primate from Madagascar? Or is it named after the Roman evil spirits?

  14. #14 Dartian
    October 26, 2009

    William:

    Why is a Puerto Rican toad named after a primate from Madagascar? Or is it named after the Roman evil spirits?

    I suspect that only Edward Drinker Cope would know, and he’s probably taken the information with him to the grave. He named the species Peltophryne lemur, but he did not explain the etymology.

    But did you notice that Darren also mentioned the toad species Rhamphophryne* tenrec in the article? Now that South American species is definitely known to have been named after ‘the curious mammals with long noses’ from Madagascar (Lynch & Renjifo, 1990:367).

    * That’s the genus it was originally put in, although Darren seems to be suggesting that this toad should now be included in Rhinella.

    References:

    Cope, E.D. 1868. Sixth contribution to the herpetology of tropical America. Proceedings of the Academy of Natural Sciences of Philadelphia 20, 305-313.

    Lynch, J.D. & Renjifo, J.M. 1990. Two new toads (Bufonidae: Rhamphophryne) from the northern Andes of Colombia. Journal of Herpetology 24, 364-371.

  15. #15 Daniella Perea
    October 27, 2009

    Antioquia beaked toad, origially _Rhamphophryne tenrec_ Lynch and Renjifo, 1990, has since been included within _Rhinella_ by Chaparro, Pramuk, and Gluesenkamp (2007, Herpetologica, 63: 211). I know that Darren is writing abut beaked toads later in this series so am sure will hear about it then.

  16. #16 Jenny Pramuk
    November 27, 2009

    I am so happy to see folks discussing bufonid morphology in such a thorough, passionate, and enlightened manner!! If you keep studying “lowly” bufonids, you’ll realize that they really determine the fate of the universe.

    In response to the lemur etymology comment: I think that the specific epithet in this case refers to a forest sprite (lemur=Latin for a ghost or sprite) and thus, must apply equally to the furry primitive primates as well as the Puerto Rican Peltophryne.

    Finally: Peltophryne (West Indian bufonids) should be Peltophryne and I sincerely apologize for ever publishing anything that contradicts this.

  17. #17 DDeden
    December 7, 2010

    Wow, the missing link. The chemical that induces hallucinations is critical for bone construction, REM sleep allows bone accumulation as a way to isolate that nasty toxin somehow, not found in tadpoles or sharks except minimally I guess, no hibernation/estivation. Serotonin -> melatonin -> osteoblasts
    Only bony animals dream, thin boned fish only a bit. http://www.eurekalert.org/pub_releases/2004-04/jhmi-csc041604.php

  18. #18 DDeden
    December 7, 2010

    I’ll have to write about this at my blog, connecting REM and bone and the square/triangle/arsenic-phile forms of hypersaline archaebacteria when I get time. See my comments:

    http://scienceblogs.com/tetrapodzoology/2010/01/redbellies_harlequins_plump_toads.php

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