Females of solitary- and group-living sister species of African striped mice show a similar social structure following experimentally imposed group-living

Abstract

Social systems vary within and between species, ranging along a continuum between solitary-living to complex societies. Social systems are emergent properties of interactions between individuals, and these interactions are often modulated by hormones. We investigated the behavioral and hormonal consequences of experimentally imposed social grouping in two sister species of the African striped mouse Rhabdomys, one of which (R. pumilio) is group-living in its natural xeric environment, while the other (R. dilectus) is solitary-living in its mesic environment. We compared social behavior of triads of age-matched, unfamiliar females of each species, and measured their serum corticosterone and testosterone concentrations in comparison to solitarily housed controls. On day 1, triads of both species displayed mainly amicable behavior. Aggression was rare, but R. pumilio females showed more aggression than R. dilectus females. Triads of both species shared nests from day 2 onwards, and amicable behavior increased over time. Neither species differed in hormonal concentration compared to their solitary controls, but R. dilectus had 1.3 × higher serum corticosterone than R. pumilio. Serum testosterone did not differ between species or treatments. Neither corticosterone nor testosterone concentration measured at the end of the experiment was related to the duration of amicable behavior shown between triad members at the end of the study. Both species established a similar social structure when housed in non-kin triads in captivity. Both displayed high amicability and little aggression, indicating that environmental factors, such as resource distribution and duration of the breeding season, may be more important determinants of social organization in striped mice than social tolerance. Variation in social systems occurs both between and within species, and to date, it is still poorly understood how differences in social tolerance relate to inter-specific variation in social systems. We experimentally imposed social grouping in females of two sister species of the African striped mouse, one naturally group-living and one solitary-living species. We studied interactions between individuals in these experimental groups over 21 days and measured testosterone and corticosterone concentration at the end of the experiment. Both species mainly showed socio-positive behavior and little aggression occurred. Individuals from experimental groups of both species had similar testosterone and corticosterone concentrations compared to solitary-housed control females of the same species. Our results show that both species are tolerant of conspecifics, at least in captivity, indicating that environmental factors might be more important determinants of social organization than social tolerance.