Noise and Soundscapes
Communication can be hindered from noise, and in multimodal signaling animals like the túngara frog, noise may be both acoustic and visual. Derek Coss explored how female túngara frogs perceive the attractiveness of male advertisement calls amidst multimodal noise.
Robotic túngara frogs with inflatable silicon vocal sacs (designed by SaviMade) and a controller (designed by Paul Clements at Salisbury University) are used to test the reactions of female frogs to the multimodal courtship display of male frogs.
Derek also used soundscapes to understand how the amount of background noise could potentially affect a female túngara frog's choice in a mate. The jungle is a noisy place with other frog species and countless other critters making sounds that overlap with a male's mating call, so how a female might (or might not) be able to distinguish a male from all of this background sound is a point of interest. One particular method we use to visualize the sounds of a night in the Panamanian rainforest are false-color spectrograms.
To watch Derek's thesis defense on this research, click here.
Although lek mating has been investigated in frogs for decades - the phrase 'frogs gather at the pond to mate in leks' is routine in the relevant literature - researchers know little about frog movements. Where do these gathering frogs come from? Do females visit the same pond every night? Can a model of these movements in one species be generalized to make predictions in other species? Such questions are integral to our understanding of metapopulation dynamics in frogs and may be relevant to generating conservation management plans.
Our laboratory has investigated these questions by using miniaturized radio transmitters and silicone "backpacks". Such backpacks can be safely attached to a frog, whose movements can be monitored actively with a handheld receiver or passively with a stationary receiver array.
Multimodal Signaling and the Cocktail Party Effect
Many species of frogs mate in large aggregations called leks. Leks insulate individuals from predation through the dilution effect. Though affording protection, leks are difficult environments in which to communicate. Temporal and spectral masking generated by overlapping signals from multiple signalers make isolating an individual signaler challenging. This is analogous to the acoustic scene at a cocktail party. We are able to circumvent the cocktail party problem by utilizing visual cues (i.e. lipreading) to improve our perception of speech. Interestingly, female green treefrogs prefer multimodal (visual + acoustic) to unimodal male signals.
How does the addition of a visual cue - here, the males' inflating vocal sac - impact females' ability to isolate calling males? Our laboratory has used the robofrog system coupled with frog call playback to determine whether multimodal signals improve female frogs' ability to perceive individual mates. See Matthew Murphy's video below for more information on this experiment.
As with a number of other species, green treefrogs are territorial and males show site fidelity. The distribution of males is such that, as noted in Multimodal signaling and the cocktail party effect, calls overlap with one another. This proximity leads to fierce competition between individuals. Part of this competition is the 'duel'. During a duel, two frogs will call at one another until the loser of the duel falls silent. It is thought that duels prevent the loser from wasting energy on females that are much more likely to choose the winner of the competition.
Do duels follow a pattern? Smaller males have high-pitched voices and are generally more likely to concede defeat to low-pitched males. Could relative assessment of the difference in frequency or in call rate between a frog and his opponent influence whether he begins a duel? What about the duration of a duel? Or could males instead use self-assessment; in other words, are high-pitched males simply more likely to give up a fight?