Using the bite force of Cranwell’s Horned Frog to calculate that of a Cretaceous Giant Frog.

The Vertebrates are a highly successful group, at least in part due to their specialised jaws, which have enabled them to adopt a wide range of feeding strategies. Many Vertebrates are specialised predators, making use of powerful jaws to overcome prey of a size that would be out of the range of most similar-sized invertebrates. Frogs are generally considered an exception to this rule, as while they are specialised predators, they typically have rather weak jaws, instead relying on a powerful sticky tongue to capture small invertebrate prey, which is typically swallowed whole. However, the Horned Frogs of South America, do possess powerful jaws, which they use in conjunction with their tongues to overcome much larger prey, including other Frogs, Snakes, Lizards, Birds and Mammals.

In a paper published in the journal Scientific Reports on 20 September 2017, Kristopher Lappin of the Biological Sciences Department at California State Polytechnic University, Pomona, Sean Wilcox, also of the Biological Sciences Department at California State Polytechnic University, Pomona, and of the Department of Evolution, Ecology, and Organismal Biology at the University of California, Riverside, David Moriarty and Stephanie Stoeppler, again of the Biological Sciences Department at California State Polytechnic University, Pomona, Susan Evans of the Department of Cell and Developmental Biology at University College London, and Marc Jones of the School of Biological Sciences at the University of Adelaide and the South Australian Museum, describe the results of a study in which they measured the bite force exerted by Cranwell’s Horned Frog, Ceratophrys cranwelli, and used this information to estimate the bite force exerted by other extinct Frogs, the living Brazilian Horned Frog, Ceratophrys aurita, another species of Horned Frog, and the Late Cretaceous Beelzebufo ampinga, an extinct Frog thought to have had a similar hunting strategy to the living Horned Frogs due to its well-developed jaws.

An individual Ceratophrys cranwelli biting a force transducer. Leather strips glued to ends of bite bars provide a natural surface that encourages high-effort biting and avoids damage to teeth and bones. The strips also indicate a bite point for standardization of bite-force performance. Lappin et al. (2017).

Lappin et al. measured the bite force of eight Cranwell’s Horned Frogs ranging in length from 39.8 to 95.6 mm and in mass from 8.9 to 147.8 grams. They found that the Frogs could exert a bite force of between 2.7 and 32.9 Newtons, with the greatest force exerted at the middle point of the jaw, rather than at the tips. Furthermore, different Frogs of the same size exerted similar bite forces, suggesting that size is a direct proxy for bite strength. 

 

Teeth of Ceratophrys cranwelli. As with almost all other extant frogs bearing teeth, Ceratophrys has teeth only on the upper jaw. Unusual among frogs, the teeth of Ceratophrys exhibit a derived non-pedicellate morphology and have sharp recurved tips situated upon robust, labiolingually expanded bases. (a) view of teeth with jaws closed; (b) view of teeth with mouth slightly open; (c) close-up of single tooth. Scale bars is 1 mm. Lappin et al. (2017). 

Using the measurements obtained from the Cranwell’s Frogs, Lappin et al. next attempted to calculate the bite force of the extant Ceratophrys aurita. A museum specimen of this species is known with a head width of 98.3 mm, which it is calculated would have been able to exert a bite force of 248.6 Newtons at the tip of its snout, and 497.1 Newtons at its mid-jaw. This is lower than the bite force exerted by similar sized Common Snapping Turtle, Chelydra serpentine (657 Newtons), American alligator, Alligator mississippiensis (689.3 Newtons), Saltwater Crocodile, Crocodylus porosus, (680.5 Newtons), or Freshwater Crocodile, Crocodylus johnstoni, (707.7 Newtons), but still formidable.

 A Brazilian Horned Frog, Ceratophrys aurita, consuming a smaller Frog, Trachycephalus mesophaeus. Axel Kwet in Solé et al. (2010).

Next Lappin et al. attempted to extrapolate the bite force of the extinct Cretaceous Frog, Beelzebufo ampinga, using two individuals, one with a jaw width of 111 mm, and the other with a jaw width of 154 mm. The smaller of these Frogs was estimated to be able to produce a bite force of 372.4 Newtons at its jaw tip and 744.8 at its mid jaw, while the larger was calculated to be able to exert a bite force of 1106.8 Newtons at its jaw tip and 2213.7 Newtons at the mid jaw. This is comparable to the bite force of an Common Snapping Turtle of similar size (2042 Newtons), but exceeds the bite force of similar sized Crocodilians, with predicted bite forces at this size of American alligator (1659.6 Newtons), Saltwater Crocodile (1836.5 Newtons), and Freshwater Crocodile (1863.6 Newtons). It also falls within the bite force range of Spotted Hyenas, Crocuta crocuta, with measurements from adult specimens recorded at between 1000 and 4500 Newtons, and exceeds that of an adult Lion, Panthera leo (2024 Newtons) or Tiger, Panthera tigris (2165 Newtons). 

Beelzebufo ampinga, Late Cretaceous of Madagascar. (A) Skull reconstruction showing parts preserved (white areas, Left) and distribution of pit-and-ridge ornament (stippling, Right). (B) Skeletal reconstruction and inferred body outline of average-sized (skull width, 200 mm; SVL, 425 mm) adult female Beelzebufo ampinga based mainly on Lepidobatrachus asper. White areas indicate parts represented by fossil specimens. For size comparison, dorsal view silhouettes of Ceratophrys aurita (the largest extant Ceratophryine) (C), and Mantidactylus guttulatus (the largest extant Malagasy Frog) (D), are shown. cp, crista parotica; fm, foramen magnum; frp, frontoparietal; mx, maxilla; n, nasal; pmx, premaxilla; qj, quadratojugal; qu, quadrate; sq, squamosal. Scale bars: 50 mm. Evans et al. (2008).

The deposits in Madagascar which produce Beelzebufo ampinga also produce small Crocodilians and non-Avian Dinosaurs, animals which Lappin et al. suggest could have been vulnerable to attacks by these large Frogs, particularly as larger specimens than the ones used in the study are known (though they were not included due to incomplete jaws).

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