The “silent” half: diversity, function, and the critical knowledge gap on female frog vocalizations

Anuran vocalizations are crucial for species recognition and social interactions, particularly in reproduction. Historically, research has focused almost exclusively on male calls, leading to a male-biased perspective in anuran bioacoustics. Female calls have often been neglected due to their softness, making them difficult to detect. This review provides an overview of female calling behavior in anurans, addressing a critical gap in frog bioacoustics and sexual selection. Specifically, we aim to 1) provide an overview of the current state of knowledge of female calling in frogs; 2) propose a standardized classification for anuran call types; 3) identify general patterns and challenges; 4) recommend best practices; and 5) highlight areas for further exploration. Our literature review indicates that female calls have been documented in 112 species across 53 genera and 29 families, representing approximately 1.43% of all described anuran species. However, most descriptions are anecdotal or purely descriptive, with few functional analyses. Our findings underscore the widespread but largely overlooked presence of calling females in anurans. Expanding our research efforts on female vocalizations will improve our understanding of anuran communication. We hope this review motivates researchers to consider female frogs in future behavioral, ecological, and evolutionary studies.

Methods
(a) Literature review
We conducted a systematic literature review to identify studies on female calling behavior in amphibians beginning in July 2023 (Supplementary Material S1). We included peer-reviewed documents, including journal articles, theses, and conference abstracts (excluding preprints). Our search strategy included initial broad exploratory searches with subsequent targeted searches to maximize coverage while filtering irrelevant results. We used two primary databases: Google Scholar and Web of Science (WOS). Initial searches combined the term ‘amphibia’ with one or more of the following: ‘female vocalization,’ ‘female courtship call,’ ‘female reciprocal call,’ ‘female acoustic response,’ ‘female vocal interaction,’ ‘female mating vocalization,’ or ‘female call’. These returned 435 results in Google Scholar and 25,280 in WOS. 

Next, we conducted a more targeted search within WOS, adding the terms: ‘Anura’ OR ‘amphibian’ combined with ‘calling female’ OR ‘female acoustic response’ OR ‘female vocal interaction’ OR ‘female mating vocalization.’ To reduce irrelevant results, we excluded the terms ‘mammal’ and ‘bird.’ We also applied subject area exclusionary filters (i.e., subject categories not related to Zoology, Animal Ecology, Behavioral Sciences, Ecology, Biology, Evolutionary Biology, Neuroscience, and Reproductive Biology). This yielded 2,271 documents as of November 2024.

We supplemented these results by conducting backward and forward citation searches in Google Scholar using three key review papers on female anuran vocalizations (references [40,46,47]). Forward citation searches involved screening all articles citing these reviews. Backward citation searching involved examining the reference lists of the reviews to identify earlier relevant studies. Combining these results with those from the initial Google Scholar and WOS searches yielded 2,919 documents. We screened the titles and abstracts of all retrieved documents to assess relevance. Studies that explicitly mentioned female calls or showed potential to include relevant information (e.g., focusing on amphibian ecology, behavior, physiology, or bioacoustics) were read in full. To be included in our dataset, a source had to contain direct observations, spectrograms, or detailed descriptions of female vocalizations. We also included sources in French, German, Portuguese, and Spanish. This process yielded 90 records. For relevant cases within this set, we conducted additional backward and forward citation searches and identified 18 studies not captured by the initial search. Our final dataset comprised 108 documents. Data were compiled in a standardized spreadsheet with taxonomic names checked and duplicates removed (Supplementary Material S2). The search concluded in November 2024.

(b) Data extraction and family-level mapping
We compiled the data on female calls from the publications that specifically reported females calling. For each study, we extracted the species name, the type of call as defined by the authors, the description of the female call (if available), and any ecological or behavioral contextual information about the vocalization. We also recorded whether the authors conducted experiments or behavioral assays to test the function of the female call. Then, we compared the female call definitions from the original studies to our proposed definitions. Whenever our classification differed from the reported one, we emphasized that our assignments relied solely on the authors' information, without making any inferences beyond what was explicitly stated

To standardize the data, we coded instances of spontaneous calling, researcher manipulation (i.e., handling or physical restraint), and tests of functional significance as binary variables (yes = 1, no = 0). When information was unavailable, we designated it as NA (Not Available). All extracted information was then organized and summarized in a table (Supplementary Material S2). We mapped the different types of female calls onto a consensus amphibian phylogenetic tree [48], which includes 54 extant anuran families. To account for cases where female calling was reported but lacked sufficient detail to classify the call type, we included a 'No detail' category.

We recorded 132 instances of female calling behavior across 112 species (53 genera, 29 families) from all the studies examined for this review. This represents approximately 1.43% of all described anuran species. Female calling behavior in frogs is widespread across the phylogeny (Figure 3), occurring in both deeply divergent families like Bombinatoridae and Leiopelmatidae, as well as in more recently diverged families such as Craugastoridae and Rhacophoridae. Ranidae is the family with the greatest number of species (18 species), followed by Hylidae (15 species), Leptodactylidae (14 species), and Eleutherodactylidae (10 species). At the genus level, Leptodactylus has the highest number of species with female calls, with 12 species, followed closely by Eleutherodactylus with 10 species, and Xenopus with eight species reported to produce female calls. We want to emphasize that the lack of records of female calls in certain families or genera does not necessarily indicate the absence of this behavior. Instead, it reflects the insufficient knowledge of basic natural history and the lack of efforts in investigating female calling behavior in frogs.