Headbutting Anurans

Something not out of line with a B-grade SCI-FI film has been discovered in Brazil, the venomous ability of the two previously described species has been unearthed.

Until the discovery of Aparasphenodon brunoi’s and Corythomantis greeningi’s unique venom delivery system there were no definitive examples of venomous frogs known to science, despite a plethora of poisonous species (Jared et al. 2015).  This unique system combines both the toxin and skull structure of the two species to form the venomous apparatus. Despite often being confused venoms and poisons are distinct from each other.Venoms must be produced in specialised tissues or glands and must be delivered through a mechanism such as stingers, fangs or spines (Meier & White. 1995).

Both C. greeningi and A. brunoi possess highly specialised glands in the skin, these folded structures produce a strong toxin. When threatened, spines in the head and lip area protrude through the epidermis and are coated in the venom upon exposure (Figure 1.).It is thought that the venom is used as a defense during phragmotic behaviour (When an animal uses its own body as a barrier whilst retreating in a burrow) (Jared et al. 2005).

Figure 1. Images of two venomous frogs, Aparasphenodon brunoi (A, C and E) and Corythomantis greeningi (B, D and F).(Jared et al. 2015).

The frogs have evolved a greater flexibility in the neck, allowing it to pierce the attacker with a side to side sweeping motion of the head.Whilst collecting specimens for toxin analysis, a researcher was jabbed in the hand by a C.greeningi.The envenomation resulted in intensive pain in the limb for 5 hours (Jared et al. 2015).

The lethal dose 50 (LD50 is a scale commonly used to compare toxicity, here all are intraperitoneally injected unless stated otherwise) of the toxin from the head of A. brunoi was found to be 3.12μg and 51.94 μg from C. greeningi (Jared et al. 2015).For comparison Some LD50’s of other amphibians and reptiles is included (Table 1.). Despite C. greeningi being less toxic, it was found to produce a greater volume of the toxin (Jared et al. 2015). 

Table 1. List of LD50 ratings for reptiles and amphibians.

Scientific Name	Common Name	LD50 (μg/20g)
Bitis atriens	Puff Adder	17.4
Naja haje	Egyptian Cobra	4.1
Oxyuranus microlepidotus	Inland Taipan	0.025
Phyllobates aurotaenia	Colombian Arrow Poison Frog	0.002 (subcutaneous)

Adapted from (Daly & Witkop. 1971), (Oukkache et al. 2014) & (Meier & White. 1995).

It is likely that more research of Hylidae frog species will likely expose their venomous nature (Jared et al. 2015).

References

Jared, C., Antoniazzi, M. M., Navas, C. A., Katchburian, E., Freymüller, E., Tambourgi, D. V., and Rodrigues, M. T. (2005), ''Head co-ossification, phragmosis and defence in the casque-headed tree frog Corythomantis greeningi.'' Journal of Zoology, Vol: 265, No: 1, pp: 1-8.

Jared, C., Mailho-Fontana, P.L., Antoniazzi, M.M., Mendes, V.A., Barbaro, K.C., Rodrigues, M.T. & Brodie, J., Edmund D. (2015), "Venomous Frogs Use Heads as Weapons", Current biology, Vol: 25, No:16, pp: 2166-2170.

Oukkache, N., Jaoudi, R.E., Ghalim, N., Chgoury, F., Bouhaouala, B., Mdaghri, N.E. and Sabatier, J.M. (2014), “Evaluation of the lethal potency of scorpion and snake venoms and comparison between intraperitoneal and intravenous injection routes.” Toxins, Vol: 6, No: 6, pp.1873-1881.

Daly, J. and Witkop, B. (1971), “Batrachotoxin, an extremely active cardio-and neurotoxin from the Colombian arrow poison frog Phyllobates aurotaenia”, Clinical toxicology, Vol: 4, No: 3, pp: 331-342.

Meier, J. & White, J. (1995), “Handbook of clinical toxicology of animal venoms and poisons.” CRC Press, Boca Raton.