Time for more spadefoot toads (that is, members of the anuran clade Pelobatoidea or Anomocoela). This time, we’re going to look at the two North American spadefoot toad genera (Spea and Scaphiopus). Like megophryids (the horned spadefoot toads discussed in the previous article) these have, conventionally, been classified as part of the Old World spadefoot group Pelobatidae and they’re highly pelobatid-like in possessing a specialised digging spade (an enlarged, keratinized metatarsal tubercle supported internally by the prehallux) [photo of Scaphiopus spade below either by Dawson or Fice].

However, American spadefoots have been considered distinctive enough by some authors to warrant classification in their own ‘family’: Scaphiopodidae. Phylogenetic studies generally support this view, since North American spadefoots don’t group within, or close to, pelobatids. I’m going to avoid saying any more about phylogeny right now – we’ll come back to this issue later.

Neither scaphiopodid taxon is especially speciose. There are four species of Spea (sometimes called the western spadefoots) and three species of Scaphiopus (sometimes called southern spadefoots). Some authors dispute the idea that these two should be separated as ‘genera’ and treat them as ‘subgenera’ within an inclusive Scaphiopus (this name has priority over Spea: the latter is Spea Cope, 1866, the former is Scaphiopus Holbrook, 1836). None are large, their SVLs ranging from 35-90 mm. They’re greenish, greyish or brownish, with subtle striping or spotting on their dorsal surfaces. As is the case across pelobatoids, they have vertical pupils that narrow to a slit in bright light. [Images of Scaphiopus species below by Stanley Trauth and Clinton and Charles Robertson.]

Fossils of both western and southern spadefoots – some referred to the living species – are known from the Miocene, Pliocene and Pleistocene, and extinct species of Scaphiopus have even been reported from rocks as old as those of the Oligocene and Eocene (Sanchiz 1998, Holman 2003). As per previous comments (see the article on parsley frogs), I think we should be sceptical of these Paleogene records. They’re often based on fairly unsatisfactory remains and the similarities that have led to the relevant classifications are often plesiomorphic, general ones.

Today, American spadefoots occur from southern Mexico to southern Canada and are mostly associated with arid and semi-arid habitats, including deserts, grasslands, scrub, chaparral, and deciduous woodland. Like the pelobatids of the Old World, they spend months at a time beneath ground, remaining buried during the dry part of the year and only emerging to the surface following the arrival of sufficient rainfall. These are the sorts of habitats where the pools, ponds and streams that appear after rainfall are frequently ephemeral, persisting only for a few weeks or even days. American spadefoots are specialised for this and both their eggs and tadpoles develop extremely rapidly: the eggs (laid in clutches ranging from 10-500) hatch within a day or so in some species and the tadpoles sometimes complete metamorphosis within just 14 days. This is one of the fastest rates of metamorphosis known for any amphibian. Of course, the pools often dry up before the tadpoles get to complete metamorphosis. Now, about those tadpoles...

Remarkable polymorphic - or polyphenic - tadpoles

One of the best known things about American spadefoots is the morphological plasticity present in their tadpoles. As anyone familiar with anuran biology will know, tadpoles are absurdly diverse in anatomy, ecology, lifestyle and growth patterns. I really should make an effort some time to provide some sort of Tet Zoo review of tadpole diversity. While the tadpoles of American spadefoots aren’t especially remarkable in anatomical terms, the great speed with which they can complete metamorphosis (discussed above), and the phenotypic variation they express make them particularly interesting.

‘Normal’ tadpoles are round-bodied omnivores that have jaw muscles and mouthparts that are ‘normally’ proportioned when compared to the rest of the animal. These are called ‘omnivore-morph’ tadpoles. But then there are predatory, wholly carnivorous ‘carnivore-morph’ tadpoles that – despite being the same age as their ‘normal’ siblings – are larger overall, have a substantially bigger, broader head, a proportionally far slimmer body with smaller guts, hypertrophied cranial muscles, and mouthparts more strongly adapted for predation: the keratinised mouthparts (or beaks) are bigger, and have serrated edges, with a dorsal hook on the midline that fits into a deep notch on the lower beak (Bragg & Bragg 1959, Pfennig & Murphy 2002). The two morphs are different socially: omnivore-morph tadpoles congregate in schools while carnivore-morph tadpoles tend to be solitary. Carnivore-morph tadpoles tend to develop more quickly.

It should be noted that variation within the tadpoles is not black and white but that various intermediate morphs are present as well, the precise lifestyles and diets of which remain somewhat uncertain. Remarkably, carnivore-morph tadpoles are not locked into this morphotype but can morph back into omnivore-morph versions when they stop predating on crustaceans and/or other tadpoles (Pfennig 1992a, b).

A substantial number of questions have arisen from these observations and there’s an extensive literature on scaphiopodid tadpole polymorphism (or polyphenism; both terms get used in the literature). What, for example, controls these dietary and morphological shifts? While it used to be inferred that niche shifts and phenotypic changes in spadefoot tadpoles were influenced by crowding or pool size, it now seems that an (initially opportunistic) early ingestion of fairy shrimp prey causes the switch (Pfennig 1992a). They then take to eating other tadpoles as well as shrimps, and may thus be cannibalistic. While this cannibalism is a well known and oft-mentioned phenomenon, less well known is that the tadpoles practise kin discrimination, opting not to eat siblings when given a choice (Pfennig 1999).

Incidentally, intraspecific cannibalism among tadpoles is not unique to Spea and Scaphiopus: it’s also practised by some American treefrogs (namely Osteophilus septentrionalis).

It’s also been shown that American spadefoot species differ as goes their propensity to develop carnivore-morph tadpoles and, furthermore, that this propensity is partly controlled by the presence of other spadefoot species. Within western spadefoots, the Plains spadefoot Spea bombifrons is evidently a ‘better’ shrimp predator than the New Mexico spadefoot S. multiplicata, and its tadpoles are far more likely to become carnivores when living alongside S. multiplicata (Pfennig & Murphy 2002). Conversely, S. multiplicata tadpoles are less likely to become carnivores when living along S. bombifrons.

It should also be noted that – within these species – all populations don’t behave the same. Lower-elevation populations of S. multiplicata have a substantially reduced tendency to produce carnivore-morph tadpoles than higher-elevation ones. In fact, some lower-elevation S. multiplicata populations have apparently lost the ability to produce ‘carnivore-morph’ tadpoles entirely, all (seemingly) because of competition from S. bombifrons. This variation in tadpole plasticity seems to match mate preference in adults of the S. multiplicata populations concerned, suggesting that speciation is occurring in these frogs – speciation that’s being driven by selection imposed by S. bombifrons! (Pfennig 2000, Pfennig & Murphy 2002).

In fact, so much neat research has been done on the development, biology and evolution of western spadefoots – much of it published within recent years by David Pfennig of the University of North Carolina – that I can’t begin to adequately summarise it in this article.

I do have to give honorary mention to the amazing discovery that the females of S. bombifrons [adjacent photo by Stanley-Trauth] will preferentially mate with the males of S. multiplicata [adjacent photo by Sarah Beckwith] where the two occur in sympatry, and where this sympatry involves shallow, rapidly drying pools. The conclusion from this discovery is that hybridisation is adaptive in S. bombifrons: that is, that females are able to give their offspring a survival advantage by hybridising with members of another species (Pfennig 2007). Oh, and the Pfenning who published that paper was Karin Pfennig, not David.

So, there we have it: the first ever Tetrapod Zoology article devoted to American spadefoot toads. What could be next?

For previous Tet Zoo articles on frogs and toads, see...

Refs - -

Bragg, A. N. & Bragg, W. N. 1959. Variation in the mouth parts in tadpoles of Scaphiopus (Spea) bombifrons Cope (Amphibia: Salientia). The Southwestern Naturalist 3, 55-69.

Holman, J. A. 2003. Fossil Frogs and Toads of North America. Indiana University Press, Bloomington and Indianapolis.

Pfennig, D. W. 1992a. Polyphenism in spadefoot toad tadpoles as a locally adjusted evolutionarily stable strategy. Evolution 46, 1408-1420.

- . 1992b. Proximate and functional causes of polyphenism in an anuran tadpole. Functional Ecology 6, 167-174.

- . 1999. Cannibalistic tadpoles that pose the greatest threat to kin are most likely to discriminate kin. Proceedings of the Royal Society of London B 266, 57-61.

- . 2000. Effect of predator-prey phylogenetic similarity on the fitness consequences of predation: a trade-off between nutrition and disease? American Naturalist 155, 335-345.

- . & Murphy, P. J. 2002. How fluctuating competition and phenotypic plasticity mediate species divergence. Evolution 56, 1217-1228.

Pfennig, K S. 2007. Facultative mate choice drives adaptive hybridization. Science 318 965-967.

Sanchiz, B. 1998. Salientia. Handbuch der Paleäoherpetologie. Dr. F. Pfeil, Munich.