A number of frogs in the hylodes family have taken amphibian mating behavior to a new level, with limb and toe movements that can be likened to a disappointing break dance routine. Amphibians are often used as a model for studying vocal communication because of their relatively simple calling behavior – males typically produce two distinct calls, an advertisement call to attract females and an aggressive call to protect territory from male competitors. More recently, researchers have begun to investigate the possibility of visual signaling in frog mating behavior. Whereas most neotropical frogs are nocturnal and use ponds as breeding grounds, hylodine frogs are active during the day and reproduce beside noisy streams. These two factors are thought to have favored the evolution of visual signaling – background noise reduces the efficacy of acoustic signals, while sunlight increases the detectability of visual signals. In this study, Furtado et. al (2019) gave a novel field report on the visual displays of Hylodes meridionalis, a neotropical frog native to the southern Atlantic Forest in Brazil. This frog may not look like a stud in comparison to its colorful relatives, but more than makes up for its lacking fashion sense with style on the dance floor (stream-side rocks and logs).
Hylodes meridionalis
Because this species is so shy around humans, the team was only able to observe a small number of natural interactions between frogs in the wild. In place of studying interactions between frogs, Furtado et. al cleverly placed mirrors in front of stream-side males to simulate the presence of a rival male. Video recordings were taken for eighteen males and visual signals were counted under six categories – toe flagging, foot flagging, arm waving, arm lifting, leg lifting, and throat display. Overall, the use of acoustic signals was less frequent than the use of these visual signals. Toe-flagging was defined as slow up and down movement of one or more toes (essentially jazz hands), and foot flagging as a slow raise of one hind-limb in a semicircular movement. Arm waving was defined as moving an arm in an arc beside the head, while arm lifting and leg lifting consisted of rapidly moving a limb up and down without extending it. In the throat display, males inflate one of two paired vocal sacs without producing calls. This last behavior is particularly interesting in that the frogs appear to be reappropriating the vocal sac, a structure that originally evolved for vocal communication, to produce visual signals. A seventh behavior, both legs kicking, was observed exclusively in females during courtship and had never before been observed in the hylodes family. However, because this species is so shy around humans, only one male – female interaction was observed. Some of the other visual displays were observed only in male-male encounters, but because only one male-female encounter was recorded, it would be a stretch to contextualize or assign meaning to any of the visual displays at this stage.
A male Hylodes asper exhibits a leg lift
Perhaps the most exciting finding in this study was the discovery of an unrecognized courtship call during the one male – female encounter. Though all of the throat displays performed in male – male encounters were silent, one courting male was observed producing an unrecognized call while inflating a single vocal sac. Unfortunately the team did not get a recording of the call, but the observation calls for further investigation of hylodine courtship calls.
A male Hylodes meridionales dances alone, with a remarkably similar companion
The authors are quick to acknowledge that the findings of this study are only preliminary – there are a number of theories as to why these frogs dance, though at present no studies have attempted to unravel the function of each visual display. For another closely related species Hylodes japi, De Sá et al. (2016) suggested that by rapidly raising their arms and quickly uncovering the more colorful skin beneath, frogs are producing a flashing signal for receivers. Bright coloration under the arm is ubiquitous to hylodines, so this hypothesis should apply to Hylodes meridionalis as well. Unfortunately it would be difficult to explore the impact of coloration on the efficacy of these visual displays, as 1) these frogs are notoriously shy and 2) frogs easily absorb chemicals through the skin and could not be artificially colored as researchers have done for other animals with colorful visual displays.
The authors also note the possibility that visual displays are not actually signals, and are just unintentional behaviors with no relevance to the animal’s situation (stretching perhaps?). Because signals can evolve from unintentional behaviors, it is also possible that some of the displays are visual signals while others carry no information at all. However, given the prevalence of the seven visual displays described in the study, it seems likely that at least some of the displays carry information and can be characterized as visual signals.
If this frog’s dance moves are indeed visual signals and not just compulsive tics or stretching routines, then this study provides evidence that yet another member of the hylodidae family uses visual signals in addition to acoustic signaling. Not only are these dancing frogs adorable, but their behavior reflects the power of environmental constraints over the evolution of mating behavior. It’s as if the frogs were to say, “I can’t hear you, let’s just dance!”
Furtado, R., Lermen, L.N., Márquez, R. et al. J Ethol (2019) Neotropical dancing frog: the rich repertoire of visual displays in a hylodine species. Journal of Ethology 37: 291-300.
de Sá FP, Zina J, Haddad CFB (2016) Sophisticated communication in the Brazilian torrent frog Hylodes japi. PLoS One 11(1):e0145444
