ORIGINAL PAPER Selective aggressiveness in European free-tailed bats (Tadarida teniotis ): influence of familiarity, age and sex Leonardo Ancillotto & Danilo Russo Received: 10 December 2013 /Revised: 9 January 2014 /Accepted: 10 January 2014 /Published online: 26 January 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract Bats are highly social mammals that often form large groups and represent good models to test the role played by individual status in shaping social relationships. Social cohesion relies on the ability of group and individual recog- nition, which is mediated by a range of sensorial cues. In this study, we selected the European free-tailed bat Tadarida teniotis as a model species to test the effects of familiarity, sex and age on aggressiveness and mutual tolerance. We hypothesize that T. teniotis is able to recognize group members and exhibit selective aggressiveness, and thus we predict fewer aggressive events and more amicable encounters be- tween colony mates than between strangers. As female bats are generally more sociable and perform prolonged parental care to juveniles even after weaning, we hypothesize that sex and age of bats have significant influences on aggressive behaviours and thus predict that females will perform more amicable behaviours than males and that adults of both sexes will be less aggressive towards juveniles. Our results confirm that T. teniotis is able to discriminate between familiar and stranger individuals, showing higher rates of aggressive behaviours towards the latter. Females are more prone to exhibit amicable behaviours, particularly during same-sex interactions, while males show higher level of aggressiveness. Juveniles are subjected to fewer aggressive behaviours by adults of both sexes. Familiarity appears crucial for T. teniotis in determining the degree of aggressiveness during social interactions but the rate of aggressive events is also influenced by intrinsic individual factors such as sex and age. Keywords Chiroptera . Colony . GLMM . Roost . Sociality . xenophobia Introduction Aggression, i.e. the set of behaviours used with the intent to cause direct physical harm (Hinde 1974) or indirect detrimen- tal effects (Wingfield et al. 1987) to an opponent is an impor- tant determinant in the characterization of mating and social systems of animals (Trivers 1971). Most social species evolved ritualized displays and non-physical communication (e.g. posturing and acoustic signals) to channel aggression during conflictual situations (Lorenz 1963). For social species, i.e. those living in groups with high frequencies of interactions among individuals, aggressive behaviours and tolerance be- tween group members are two diverging forces which play a major role in determining the social structure of the group, as well as of their mating system (Trivers 1971). Such forces are also shaped by a species’ ecological and biological traits, as demonstrated by comparative studies on rodents (Patris et al. 2002). The degree of aggressiveness exhibited or elicited by an individual is, in general, also influenced by intrinsic factors such as individual’ s age and sex. Juvenile animals are known to elicit and perform fewer aggressive interac- tions, thus being more tolerated (e.g. Scott et al. 2005; Hirsch Communicated by: Sven Thatje Electronic supplementary material The online version of this article (doi:10.1007/s00114-014-1146-6) contains supplementary material, which is available to authorized users. L. Ancillotto Dipartimento di Biologia e Biotecnologie “Charles Darwin”, Università degli Studi di Roma La Sapienza, Rome, Italy L. Ancillotto : D. Russo (*) Wildlife Research Unit, Laboratorio di Ecologia Applicata, Sezione di Biologia e Protezione dei Sistemi Agrari e Forestali, Dipartimento di Agraria, Università degli Studi di Napoli Federico II, via Università 100, 80055 Portici, Napoli, Italy e-mail: danrusso@unina.it D. Russo School of Biological Sciences, University of Bristol, Bristol, UK Naturwissenschaften (2014) 101:221–228 DOI 10.1007/s00114-014-1146-6