Sonar signals of big brown bats, Eptesicus fuscus, contain information about individual identity, age and family affiliation

We examined the possibility that the echolocation (sonar) signals used by big brown bats convey information beyond species identity (e.g. individual identity, family membership, age class, sex). The echolocation signals of six adult females, their 12 offspring, and eight additional offspring were recorded as bats performed a standardized laboratory target-detection task. Sonar signals were analysed for consistency within individuals, and for similarity within families, within age classes (adult versus juvenile), and within sex for juveniles. Evidence was found for individual distinctiveness of emissions, for resemblance of the sonar emissions used by different individuals within a family, and for differences between the emissions of adults and juveniles. No evidence was found for differences between the echolocation calls of juvenile males and females. Individual differences in sonar signals may allow a bat to identify other bats by their sonar calls and may also prevent jamming of the bat's sonar receiver. Similarity within families suggests a heritable component to call variation, although learning cannot be ruled out and is indeed suggested by the findings that the sonar signals of family members (a mother and her offspring) are more evenly distributed in cross-correlation space than expected by chance, and that heritability estimates are higher based on mother-offspring comparisons than on sibling-sibling comparisons. The finding that the emissions of juveniles differ from those of adults and that the repeatability of sonar signals improves with the age of the juvenile suggests that acquisition of a stable, adult-like signal requires several months, well beyond the time young start flying, at about 1 month. The ability to recognize other individuals or classify them by age and family on the basis of their echolocation calls could be important to bats in a number of social interactions, including mother-offspring recognition, territoriality and altruism.