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Abstract
Recently developed acoustic technologies - like automatic recording units - allow the recording of long sequences in natural environments. These devices are used for biodiversity survey but they could also help researchers to estimate global signal variability at various (individual, population, species) scales. While sexually-selected signals are expected to show a low intra-individual variability at relatively short time scale, this variability has never been estimated so far. Yet, measuring signal variability in controlled conditions should prove useful to understand sexual selection processes and should help design acoustic sampling schedules and to analyse long call recordings. We here use the overall call production of 36 male treefrogs (Hyla arborea) during one night to evaluate within-individual variability in call dominant frequency and to test the efficiency of different sampling methods at capturing such variability. Our results confirm that using low number of calls underestimates call dominant frequency variation of about 35% in the tree frog and suggest that the assessment of this variability is better by using 2 or 3 short and well-distributed records than by using samples made of consecutive calls. Hence, 3 well-distributed 2-minutes records (beginning, middle and end of the calling period) are sufficient to capture on average all the nightly variability, whereas a sample of 10 000 consecutive calls captures only 86% of it. From a biological point of view, the call dominant frequency variability observed in H. arborea (116Hz on average but up to 470 Hz of variability during the course of the night for one male) challenge about its reliability in mate quality assessment. Automatic acoustic recording units will provide long call sequences in the near future and it will be then possible to confirm such results on large samples recorded in more complex field conditions.
Highlights
During the last four decades, acoustic communication has been extensively studied to understand the behavioural processes associated to sound emission and reception
Within-individual CVs computed on dominant frequency measures from the entire night were 4.57 ± 2.00%
When using consecutive calls, the CV ratio increased with the logarithm of window size, resulting in a better estimation of the total CV (Table 2, Fig 1a)
Summary
During the last four decades, acoustic communication has been extensively studied to understand the behavioural processes associated to sound emission and reception. Intra-Individual Variability in Acoustic Signals manipulated signals, many studies have determined how environmental constraints influence signal design and evolution They have identified the relevant acoustic features and their functions in key behavioural processes like species/individual recognition or mate choice [1]–[3], and the trade-off between natural and sexual selection [4], [5]. Many behavioural contexts assume a low intra-individual variation over short temporal scales (days or weeks) for acoustic features involved in individual signature [6] or in honest signalling of individual quality [4] This theoretical assumption has so far been supported by short recordings, since long acoustic sequences have long been difficult to acquire and tedious to analyse. These limitations are overcome by recently developed automatic sensors which allow recording long acoustic sequences in natural environments, mainly used for biodiversity survey or endangered species monitoring [7], [8], and by the automation of signal analysis and classification, which facilitates the exploitation of long sequences [9], [10]
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