Written by Dr. Laurence Jarvis (external contributor)
Horned frogs within the genus Ceratophrys are highly specialised frogs with particularly interesting ecology and behaviour. The eight recognised species are endemic to South America and inhabit grassland and rainforest habitats.
Their appearance is somewhat unusual with extremely wide heads and a relatively large gape. This allows adults to feed on a large range of prey including insects, small mammals and in many cases other frog species1. They are sit-and-wait predators spending many hours motionless, often concealed under leaf litter waiting for prey to move past.
For the majority of the year the adults burrow underground, often concealed in dry soil, unlike that of many other hibernating or aestivating amphibians which burrow into damp habitats2. During this period adult horned frogs develop a cocoon of dead skin around themselves to prevent desiccation. After the first seasonal rains, the adults emerge and breed with a highly explosive breeding strategy laying several thousand eggs in temporary water bodies. Unusually, the tadpoles are voracious carnivores, feeding on a range of other invertebrates, even members of its own species. This enables them to grow and metamorphose within a few short weeks before the water bodies dry.
Horned frogs have evolved a range of unique adaptations to enable them to catch fast moving and difficult to seize prey. Research by Lappin et al. (2017)3 showed that the bite force of Cranwell’s Horned Frog (C. cranwelli) can reach 500 N which is exceptional and rivals that of many medium-sized reptiles or mammals3. This enables them to subdue many species of frogs, lizards, snakes, birds and rodents. In addition, the teeth of horned frogs are sharp, pointed and curved and resemble those of predatory reptiles3. They also have a pair of long fangs enabling them to catch their prey.
In addition to strong jaws, horned frogs possess highly sticky tongues which allows them to latch onto rough substrates such as mammal fur and bird feathers. These allow the tongues of horned frogs to have extremely high adhesive forces which are beyond their own body weight and also outweigh any potential prey item4.
It appears that the high adhesive properties are due to a combination of adaptations. First, their tongues are covered in mucus, which enhances adhesion. However, this alone is not enough to explain the high adhesive properties. It appears that the mucus spreads across the surface of the tongue as contact is made with the prey which results in stronger adhesion4. Second, a high contact pressure, due to the fast movement of the tongue, allows an additional adhesive property. This is quite unlike the method of attachment of many reptiles such as geckos, which rely on small hair-like projections to adhere to the substrate’s surface4.
Furthermore, two species of horned frogs have also been observed carrying out complex pedal luring behaviour to capture their prey5. This involves the adult sitting partly concealed in the leaf litter and waving its back legs to attract moving insect or vertebrate prey.
In 2011 a novel discovery on the tadpoles of Bell’s Horned Frog (C. ornata) in Brazil was made by Natale et al. (2011)6. The researchers provide the first evidence for sound production by anuran larvae. Prior to 2011, sound production by anuran larvae was totally unknown6. It appears that the larvae of Bell’s Horned Frog produce underwater sounds only with physical contact with others of the same species. The reason for this is unclear but it may be that sound production reduces the chances of larval cannibalism in aggressive, rapidly developing predators.
Although four of the eight species of horned frog are relatively common across South America, two species are declining and threatened due to a combination of habitat loss, urbanisation and illegal collecting from the wild.
In addition, two species are Data Deficient with very little known of their ecology and distribution. Field-based research studies are beginning to reveal important aspects of their ecology1 but more information is required to enable effective conservation measures to be implemented. Horned frogs are a group of fascinating species and it would be a great loss to see them decline further throughout their range.
References:
1 da Silva Jorge J., Sales R.F.D., de Carvalho Kokubum M.N. and Freire E.M.X. (2015). On the natural history of the Caatinga horned frog, Ceratophrys joazeirensis (Anura: Ceratophryidae), a poorly known species of northeastern Brazil. Phyllomedusa, 14 (2): 147-156.
2 Faivovich J.L., Nicoli B.L., Blotto M.O., Pereyra D., Baldo J.S., Barrionuevo, M., Fabrezi, E.R. and Haddad, C.V.B. (2014). Big, bad, and beautiful: phylogenetic relationships of the Horned Frogs (Anura: Ceratophryidae). South American Journal of Herpetology, 9: 207–227.
3 Lappin A.K., Wilcox S.C., Moriarty D.J., Stoeppler S.A., Evans S.E. and Jones M.E. (2017). Bite force in the horned frog (Ceratophrys cranwelli) with implications for extinct giant frogs. Nature Scientific Reports, 7 (1): 11963.
4 Kleinteich T. and Gorb S.N. (2014). Tongue adhesion in the horned frog Ceratophrys sp. Nature Scientific Reports, 4 (1): 5225.
5 Radcliffe C.W., Chiszar D., Estep K., Murphy J.B. and Smith H.M. (1986). Observations on pedal luring and pedal movements in leptodactylid frogs. Journal of Herpetology, 20 (3): 300-306.
6 Natale G.S., Alcalde L., Herrera R., Cajade R., Schaefer E.F., Marangoni F. and Trudeau V.L. (2011). Underwater acoustic communication in the macrophagic carnivorous larvae of Ceratophrys ornata (Anura: Ceratophryidae). Acta Zoologica, 92 (1): 46-53.