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Monomorphic call structure and dimorphic vocal phenology in a sex-role reversed frog

Abstract

Sexual signals in different animals are expected to be dimorphic when both sexes signal, but cases of monomorphism are known to occur, and we lack a clear understanding about the factors that modulate the level of sexual dimorphism in signals. In this study, we evaluated the hypothesis that the lack of dimorphism in sexual signals might evolve in systems experiencing temporal changing conditions of intra-sexual competition. We used the Darwin’s frog (Rhinoderma darwinii), a species with paternal care, as a model. We compared advertisement calls and examined call distinctiveness among females, pregnant and non-pregnant males in a wild population from Chiloé island, Chile. We also recorded the vocal activity of both sexes along the reproductive season. Additionally, we compared the acoustic properties of their advertisement calls in terms of sexual distinctiveness and individual repeatability. We found that the proportion of females and pregnant males vocalizing changed over time following distinct patterns. Females produced calls with lower dominant frequency and longer note and call durations than males, and these acoustic differences were related to body size differences between sexes, but only dominant frequency contributed significantly to the distinctiveness of calls between sexes. Also, individual repeatability was high, indicating that calling can be relevant for social recognition. Overall, our results suggest that mutual selective pressures could be involved in the limited dimorphism of the advertisement calls in Darwin’s frogs, as the sex ratio of individuals vocalizing (i.e. females vs. reproductive males) is reversed along the breeding period.

Significance statement

Whether sexually monomorphic signals are evidence of adaptive mutual choice or a by-product of genetic constraints on females remains as an open question. In species with exclusive parental care of males, it would be expected that males and females alternate their reproductive availability while performing slightly differentiated sexual signals. Using acoustic recordings and capture-recapture data of the Darwin’s frog, we found that advertisement calls of this frog tend to be monomorphic. Interestingly, the males performing parental care were calling actively and the population had a clear bias in the number of males. Males and females of this endangered frog called actively, but the vocalization rate of each sex peaked at different times along the breeding season. These findings open new questions about the mechanisms of sexual recognition under restricted signal dimorphism.

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Acknowledgments

Nicolette Thompson, Mara Santoyo, María Luisa Estay, Jaime Beltrand and the Palacios family helped in obtaining data. Comments from two anonymous reviewers significantly improved the manuscript. This manuscript is dedicated to the people of Inio, Chiloé, and their unbeatable spirit (Este manuscrito está dedicado al pueblo de Inio, Chiloé, y su espíritu imbatible).

Funding

JMS was funded by Becas al extranjero Consejo Nacional de Ciencia y Tecnología (N° 312160) and Beca Nacional para Extranjeros Comisión Nacional de Investigación Científica y Tecnológica (N° 63130134). CA was funded by Fondecyt Project N° 1181758. During manuscript writing, JMS was supported by a postdoct grant at UCM, and AV-S was supported by grant FONDECYT de postdoctorado N° 3180107.

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JMS designed the study, collected and analysed the data and wrote manuscript drafts. MP contributed to the experimental design, data collection and manuscript writing. CA and MAM facilitated fieldwork, funding and manuscript writing. AV-S contributed to the manuscript writing.

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Correspondence to José M. Serrano.

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This study was carried out in accordance with the Chilean law (Servicio Agrícola y Ganadero permit N° 9822/2015). The Ethics Committee of the Faculty of Sciences of the University of Chile approved the guidelines followed to carry out this study.  All applicable international, national, and/or institutional guidelines for the use of animals were followed.

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Serrano, J.M., Penna, M., Valenzuela-Sánchez, A. et al. Monomorphic call structure and dimorphic vocal phenology in a sex-role reversed frog. Behav Ecol Sociobiol 74, 127 (2020). https://doi.org/10.1007/s00265-020-02903-3

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Keywords

  • Sexual monomorphism
  • Sexual role reversal
  • Vocal phenology
  • Male pregnancy
  • Anuran communication
  • Rhinoderma darwinii