Temporal and spatial variations in local sex ratios in a suburban population of the European green toad Bufotes viridis

Abstract

Background: Sex ratios of animal populations are important factors of population demographics. In pond-breeding amphibians, the operational sex ratio (OSR) among the breeding population is usually male-biased. Also, in European green toads (Bufotes viridis), males usually outnumber females at breeding sites, while the sex ratio of the total adult population (ASR) is assumed to be balanced. It has been suggested that sex-specific breeding behavior causes male-predominance at the breeding sites. We used a dataset of 5 years of street patrols to test this hypothesis. For this we analyzed local sex ratios of green toads in terrestrial habitats and at two artificial breeding ponds. We expected temporal and/or spatial changes of local sex ratios which would indicate sex dependent differences in breeding behavior.

Results: Overall observed ASR among 2111 green toads, counted in the course of street patrols from 2016 to 2020, was slightly male-biased (ASR = 0.56, annual ASRs = 0.49-0.63). Based on the data of 1631 toads (920 males, 711 females) captured within a radius of 300 m around nine main breeding sites, temporal and spatial variations in local ASRs were evaluated. Resulting values were compared to the calculated OSR at two artificial breeding ponds in 2021 (645 adult: 553 males, 92 females). Estimates predict more equally distributed females and males prior to the main breeding season. During breeding season, males predominated at both breeding sites (B1: 0.83, B2: 0.89), whereas females are estimated to outnumber males in terrestrial habitats. Proportions of females highly significantly increased with advancing time of the year and increasing distance to the breeding sites. While males tended to accumulate in proximity to water bodies, females dispersed soon after breeding to more distant areas.

Conclusions: Observed sex ratios in the studied green toad population changed with time and sampling site, deviating from the population-wide sex ratio. Expanding sampling effort in amphibian conservation assessments in time and space, i.e., outside the main breeding season and away from the breeding sites, would be important to encompass such variations.

Keywords: Amphibian migration; Anura; Austria; City; Pond-breeding; Population demographics; Prolonged breeder.

Fig. 1

Study site and spatial distribution of adult green toads Bufotes viridis during breeding seasons 2016–2020. (A) Map of Vienna, Austria (B) Map of the “Simmeringer Haide” including individual observations (n = 1631) within 300 m of 9 main breeding sites (B1 and B2 sampled in 2021). Gray circles around each of the water bodies indicate 300 m radiuses split up in 50 m intervals. Maps prepared in QGIS [42]

Fig. 2

Observed proportions of green toad Bufotes viridis males during breeding seasons 2016–2020 per month (A) and per 50 m distance interval to the closest breeding site (C) compared to predicted trends in the proportion of females over time (B) and distance (D)