So, why are they called glassfrogs, hm? Ventral surface of Hyalinobatrachium aureoguttatum; photo by Martín Bustamante.

Readers with supernaturally good memories might remember the two articles, published here back in January and February 2013, on glassfrogs, a highly unusual and poorly known group of Neotropical frogs, so named due to their incredible translucent or transparent ventral skin.

Glassfrogs (properly: Centrolenidae) are weird and fascinating for all sorts of reasons (if you need a refresher on the group, go check those 2013 Tet Zoo articles). But it's one thing in particular that's caught my attention: this being that the males of species belonging to several of the genera (there are about 152 species as of right now) have prominent bony spines or knobs on their upper arm bones that they seemingly use when fighting with other males.

The humeri (upper arm bones) of select male glassfrogs (belonging to Centrolene, Chimerella and Ikakogi) showing the giant bony spines (and bony flanges) that project from the bone's shaft. The proximal ends of the bones are at the top of the page. Image from Guayasamin et al. (2009).

Observations and photos demonstrate that male glassfrogs really do fight, grappling and wrestling while clinging to leaves, sometimes even hanging upside-down by their feet while engaged in combat. Incidentally, the ‘hanging upside-down’ form of combat was thought for a while to be unique to just one of the glassfrog clades (Centroleninae), the members of this group using the hanging form of combat and not the less peculiar wrestling one. This turns out not to be accurate, since the centrolenine species Centrolene daidaleum has recently been shown to use both inverted hanging combat as well as a wrestling style that resembles amplexus (Rojas-Runjaic & Cabello 2011).

Hutter et al. (2013) recently reported a substantial amount of new behavioural and natural history data on the Red-spotted glassfrog Nymphargus grandisonae, much of it pertaining to vocalisations and male aggression. They also presented data on site fidelity: that is, they showed that male individuals of N. grandisonae do seem to exhibit high site fidelity, preferring to stick to and call from a single chosen perch (Hutter et al. 2013).

But the main reason we’re here is that their paper also included copious discussion of combat behaviour. Individuals were seen to fight while dangling belly to belly, in an amplexus-like pose where one male grabbed the other from behind, but also in a dangling, amplexus-like pose and a never-before-reported ‘head-to-vent’ wrestle where each male grabbed the opponent around the waist. The bottom line from these observations is that combat behaviour is more flexible in glassfrogs than previously implied, and that N. grandisonae, at least, isn’t restricted to just one or two fighting styles.

Glassfrog fighting: now with more poses! Hutter et al. (2013) reported at least five different kinds of wrestling combat in male Red-spotted glassfrogs, all of which are shown in the photo montage here.

So what of those formidable humeral spines? I mean: how are they deployed, and what sort of damage do they inflict? Back in February 2013, I said "So far as I know, injuries caused by [glassfrog humeral spines] have yet to be reported or described (please say if you know otherwise)". Hutter et al. (2013) thought that, in some of the combat events they observed, one of the fighting frogs was squeezing his opponent, an action “which appeared to drive the humeral spines into the opposing male”. Are we – at last – seeing deployment of the spines, and did injuries result?

Male Red-spotted glassfrog photographed 3 days before he was known to have engaged in combat. From Hutter et al. (2013).

While Hutter et al. (2013) didn’t, alas, report any cases where a bleeding, limping male Red-spotted glassfrog retreating from battle, a great gaping wound visible across his back, they did observe a male who, 31 hours after being involved in a battle, had wounds across his dorsal surface that weren’t there before he engaged in the fight. As you can see from the photos here, the wounds (which seem to have mostly healed) aren’t debilitating, but they're reasonable large and conspicuous relative to the size of the animal. Because the incident leading to the injuries wasn’t observed, we can’t be sure that the injuries were caused by a humeral spine. Nevertheless, this is the first record of injuries probably caused by humeral spines, and as such it potentially validates predictions that (1) humeral spines are used offensively during male-male combat, and (2) soft tissue injuries can be the result.

The same individual as that shown above, photographed 31 hours after a fight, and with injuries visible on his dorsal surface. Images from Hutter et al. (2013).

Finally, obviously, the paper concerned was published back in 2013 – I meant to write about it many months ago but, then, I mean to do lots of things. My apologies to Carl Hutter and his colleagues for taking about two years to write about what’s no longer a cutting-edge paper. So, why cover it now? Well, I have anurans on my mind quite a lot right now... stay tuned for thoughts on where we’re going with that. And more glassfrogs are always a good thing.

Many thanks to Carl Hutter for assistance and use of images.

UPDATE (added 7th Jan 2015): I’ve just learnt about Krohn & Voyles’s (2014) paper ‘A short note on the use of humeral spines in combat in Espadarana prosoblepon (Anura: Centrolenidae)’. Therein, they feature photos of two battling individuals of E. prosoblepon. While injuries resulting from the fight were not observed, it definitely seems that the two were using the humeral spines as weapons. In one photo, the spines of the two animals appear to be interlocked. The paper is open-access so you can see this for yourself. Thanks to Rob Gandola for bringing my attention to this paper.

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

Refs - -

Guayasamin, J. M., S. Castroviejo-Fisher, L. Trueb, J. Ayarzagüena, M. Rada, C. Vilá. 2009. Phylogenetic systematics of Glassfrogs (Amphibia: Centrolenidae) and their sister taxon Allophryne ruthveni. Zootaxa 2100, 1-97.

Hutter, C. R., Esobar-Lasso, S., Rojas-Morales, J. A., Gutiérrez-Cárdenas, P. D. A., Imba, H. & Guayasamin, J. M. 2013. The territoriality, vocalizations and aggressive interactions of the red-spotted glassfrog, Nymphargus grandisonae, Cochran and Goin, 1970 (Anura: Centrolenidae). Journal of Natural History 47, 3011-3032.

Krohn, A. R. & Voyles, J. 2014. A short note on the use of humeral spines in combat in Espadarana prosoblepon (Anura: Centrolenidae). Alytes 31, 83-85.

Rojas-Runjaic, F. J. M. & Cabello, P. 2011. Centrolene daidaleum (Ruiz-Carranza & Lynch, 1991) (Anura, Centrolenidae): a glassfrog with primitive and derived combat behavior. Zootaxa 2833, 60-64.