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Written by Laurence Jarvis (external contributor)

Communication in frogs and toads (anurans) has been studied by numerous researchers for decades. Despite this, new methods of communication in anurans, especially the use of multiple cues, have recently been discovered. For example, recent research has demonstrated unique and novel methods of communication among anurans including the use of ultrasonic calls (Feng et al., 2006), multiple modes of communication such as foot waving and twitching (Zina and Haddad, 2007), as well as cryptic mating systems involving complex female mate choice (Bruning et al., 2010).

One method of communication that has received relatively little attention is the use of seismic waves. Seismic communication may be defined as the “exchange of information using self-generated vibrational signals transmitted via a substrate such as the soil, a plant stem, or even a blade of grass” (Narins, 1990). Studies have shown that an increasing number of anurans may use seismic communication to supplement vocal cues, particularly in mate choice and acquisition.

Lewis and Narins (1985) were two of the first researchers to note the potential use of seismic communication in their investigations of the white-lipped frog (Leptodactylus albilabris) which inhabits the Luquillo Mountains and surrounding lowlands of Puerto Rico. They initially observed that isolated calling males alter their calling patterns in response to distant, very light footfalls which suggested that these frogs have a heightened sensitivity to vibrations transmitted through the soil (Lewis and Narins, 1985).

They subsequently found that whilst males performed mating calls using their vocal sacs, these would contact with the soil or grass creating small, but detectable, seismic waves. The receiving males would respond to these seismic waves by shifting and changing their call structure. Subsequent studies have demonstrated further use of seismic waves by anurans which may also influence mate choice in females.

The Menwig Frog (Physalaemus albonotatus) and Cope’s Swamp Frog (Pseudopaludicola mystacalis) are two poorly known frogs of Brazil which breed in small puddles and small water bodies. During the breeding season males use their vocal sacs to help project their calls to alert other males, as well as attract females. Males establish small territories, which they defend against intruding males.

Forti & Encarnação (2012) have observed both species using their vocal sacs to send water-waves across the water surface. In P. albonotatus, a resident male was observed to send water-waves when an intruding male approached. The intruding male was not deterred and a physical encounter resulted. The authors suggested that the production of water-waves by the resident male served to alert the intruding male of his presence and to augment his vocal cues. In addition, Forti & Encarnação (2012) observed similar behaviour in P. mystacalis. On this occasion, the resultant water-waves towards an intruder led this male to turn away and avoid the resident male. Forti & Encarnação (2012) proposed that the use of seismic communication may supplement the calls of males and aid in spacing of males across their small breeding ponds.

Golden rocket frogs (Anomaloglossus beebei) are native to the Kaieteur National Park in Guyana. Like many dendrobatid frogs, they breed in small bodies of water which have accumulated in the leaves of bromeliads. During the breeding season males defend suitable water bodies from intruding males by sitting and calling from the bromeliad leaves.

Narins et al. (2018) tested the hypothesis that males not only use vocal cues but also rely on vibrations sent through the leaves on which they perched. To test this the researchers sent small vibrations through leaves towards those of a resident calling male. These vibrations mimicked the calls of an approaching intruding male. Narins et al. (2018) found clear responses in the resident male who turned towards the vibrations as well as altering the timing and duration of their own call. These experiments demonstrate that these frogs may well use multiple cues to deter intruders using both sound and vibrations to communicate their presence.

Similar experimental findings have been found in the red-eyed treefrog Agalychnis callidryas from Central America. This popular and much studied tropical species forms large mating aggregations during the breeding season. Males defend small patches, occupying leaves over-hanging water bodies. Males will aggressively chase off intruders and make loud mating calls. In addition, males perform ‘tremulations’, which involves raising their bodies off the leaf and rapidly contracting and extending their hind limbs (Caldwell et al., 2010).

These create unique vibrations which are transmitted through the leaves on which they perch. Caldwell et al. (2010) performed mimicking experiments and found that, like golden rocket frogs, the males of the red-eyed treefrog responded to the vibrations of calling males. In addition, the vibrations conveyed information relating to the status of the male. More victorious and successful males produced more vigorous tremulation vibrations. Male size did not itself alter the vibrations produced (Caldwell et al., 2010).

It appears that intruding males do not just respond to any vibrations as these occur frequently due to the mass movement of frogs during breeding. Instead, it appears that males respond specifically to the vibrational cues conveyed in tremulation displays. This study adds to the current research that frogs may use multiple modes of communication during the breeding season to deter intruders and potentially to attract females.

As well as producing seismic cues during communication whilst breeding, research has shown that some amphibians may detect forthcoming seismic events. Research by Grant & Halliday (2010) suggests that the common toad (Bufo bufo) may detect tiny cues produced prior to an earthquake.

During fieldwork at a common toad breeding pond in Italy in April 2009, the researchers observed over 95% of the population to abandon breeding, five days before a large earthquake hit the region. This behaviour is highly unusual for common toads, which normally remain at breeding sites for several days until spawning has been completed.

At this site, the population of common toads had only just started. The sudden abandonment of breeding by the common toads was therefore very unusual. The researchers suggest that the common toads were possibly responding to small perturbations to the gravitational field in the ionosphere, which alter preceding an earthquake (Grant & Halliday, 2010).

Why the toads should abandon breeding is not clear, but could potentially act as an ’early warning system’ for an approaching seismic event. Although this behaviour has not previously been documented in amphibians, unusual behaviour prior to seismic activity has been observed in several species of bird and mammal (see Grant & Halliday, 2010). Further research will show if this was an isolated phenomenon of whether other amphibian species may exhibit such interesting behaviour.

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