Using Approximate Bayesian Computation to infer sex ratios from acoustic data

Abstract

Population sex ratios are of high ecological relevance, but are challenging to determine in species lacking conspicuous external cues indicating their sex. Acoustic sexing is an option if vocalizations differ between sexes, but is precluded by overlapping distributions of the values of male and female vocalizations in many species. A method allowing the inference of sex ratios despite such an overlap will therefore greatly increase the information extractable from acoustic data. To meet this demand, we developed a novel approach using Approximate Bayesian Computation (ABC) to infer the sex ratio of populations from acoustic data. Additionally, parameters characterizing the male and female distribution of acoustic values (mean and standard deviation) are inferred. This information is then used to probabilistically assign a sex to a single acoustic signal. We furthermore develop a simpler means of sex ratio estimation based on the exclusion of calls from the overlap zone. Applying our methods to simulated data demonstrates that sex ratio and acoustic parameter characteristics of males and females are reliably inferred by the ABC approach. Applying both the ABC and the exclusion method to empirical datasets (echolocation calls recorded in colonies of lesser horseshoe bats, Rhinolophus hipposideros) provides similar sex ratios as molecular sexing. Our methods aim to facilitate evidence-based conservation, and to benefit scientists investigating ecological or conservation questions related to sex- or group specific behaviour across a wide range of organisms emitting acoustic signals. The developed methodology is non-invasive, low-cost and time-efficient, thus allowing the study of many sites and individuals. We provide an R-script for the easy application of the method and discuss potential future extensions and fields of applications. The script can be easily adapted to account for numerous biological systems by adjusting the type and number of groups to be distinguished (e.g. age, social rank, cryptic species) and the acoustic parameters investigated.

The In-Flight Social Calls of Insectivorous Bats: Species Specific Behaviors and Contexts of Social Call Production

“…Sexual segregation of foraging habitats has been observed in some bat species (Senior et al, 2005;. The use of Bayesian statistics to infer sex from echolocation call characteristics has been suggested as a method for discerning sex ratios from passive acoustic monitoring (Lehnen et al, 2018), however some bat species, including E. fuscus, do not exhibit sex-specific differences in echolocation call characteristics (Heller et al, 1989;Masters et al, 1995).…”

How to quantify factors degrading DNA in the environment and predict degradation for effective sampling design

“…We excluded temperature and humidity measures from the first (newspaper deployment) and the last (sample collection) day, because exact deployment and collection times were not always recorded. We stored all droppings of each RV separately in airtight plastic boxes with silica‐gel‐beads to dry the droppings immediately upon collection (Lehnen et al, 2018). Boxes with samples were kept at room temperature for 1–20 days after collection, and subsequently stored at −20°C prior to extraction to achieve optimal cold and dry storage conditions (Wasser et al, 1997).…”

“…We excluded temperature and humidity measures from the first (newspaper deployment) and the last (sample collection) day, because exact deployment and collection times were not always recorded. We stored all droppings of each RV separately in airtight plastic boxes with silica-gel-beads to dry the droppings immediately upon collection (Lehnen et al, 2018).…”

Lesser Horseshoe Bat Rhinolophus hipposideros (André, 1797)

“…The total costs of the study amounted to €7.25 per sample for consumables and two months of lab work and analysis, demonstrating that non-invasive genetic sampling can now be readily expanded to large datasets and in an affordable manner in terms of time and finances. This protocol has already been used to validate a novel approach using Approximate Bayesian Computation to accurately and rapidly infer the sex ratio of populations from acoustic data alone (Lehnen et al, 2018); which is possible due to the sex-based differences in lesser horseshoe bat echolocation calls (Jones et al, 1992). The high throughput protocol was also used to develop a new approach based on non-invasive CMR data to estimate adult survival, fecundity and juvenile survival -key parameters of population growth (Jan et al, 2019a).…”