Gregarious sexual segregation: the unusual social organization of the Malagasy narrow-striped mongoose (Mungotictis decemlineata)

Abstract

Sex-specific costs and benefits of sociality are rarely evaluated, even though the main fitness determinants differ between the sexes. The Carnivora include some of the few mammalian species in which the sexes differ in social organization, providing an opportunity to study male and female sociality separately. Anecdotal reports indicated that Malagasy narrow-striped mongooses (Mungotictis decemlineata) appear to have unusual and flexible association patterns. We therefore conducted a 3-year field study in Western Madagascar to delineate the social organization of this forest-dwelling species and to obtain insights into sex-specific determinants of sociality. We conducted systematic radio tracking on 40 adult individuals (20 males, 20 females) and collected additional data during regular censuses and behavioral observations. We found males and females to live in small, same-sex social units. Males formed stable associations of up to four adult individuals, except for the short annual mating season, when they roamed by themselves. Male home ranges exhibited high mutual overlap and encompassed those of up to four female social unit ranges. Female social units were on average composed of two adult females, one juvenile, and one infant offspring of the dominant female and occupied exclusive territories. Female units were stable year-round and their size was unrelated to home range size, but home range sizes were significantly larger during the dry season. Our analyses indicate that both male and female associations are likely stabilized by antipredator benefits, with females accruing additional benefits from joint resource defense. Males trade off the safety in associations for temporary reproductive opportunities. Thus, antipredator benefits appear to favor group living in both sexes, but sex-specific reproductive fitness limiting factors affect the stability of such units differently. This study presents new detailed results on the sex-specific social organization of a previously unstudied endemic Malagasy carnivore, the narrow-striped mongoose. We relate our findings to important hypotheses of sociobiological theory. In particular, we evaluate the results in comparison to the closest related taxonomic group, the mongooses (Herpestidae), in which long-term studies have contributed substantially to the recent development of theories. Thus, this study on a representative of a largely unknown group of carnivores can contribute to revealing evolutionary transitions in carnivore sociality as well as insights into the mechanisms driving mammalian social evolution in general.