Better off alone! Reproductive competition and ecological constraints determine sociality in the African striped mouse (Rhabdomys pumilio)

Abstract

Summary1. While the reasons for group-living have been studied for decades, little is known about why individuals become solitary. 2. Several previous experimental studies could demonstrate that group-living can arises as a consequence of ecological constraints. 3. It has been argued that reproductive competition between group members leads to significant costs of group-living, being a main reason of solitary-living. However, so far, no studies tested experimentally whether reproductive competition can explain solitary-living. 4. Using a socially flexible species, the African striped mouse (Rhabdomys pumilio), we tested experimentally in the field whether dispersal and solitary-living are more likely to occur when reproductive competition is present. 5. We investigated ecological constraints, here expressed as a function of population density, by removing groups of striped mice and creating vacant territories. To control for the effect of reproductive competition, which occurs only during the breeding season, we performed experiments during both the breeding and the non-breeding season. This is the first removal experiment performed in a species with communal breeding during the non-breeding season. 6. During the breeding season, when population density was low, more striped mice from experimental groups moved into the vacant territories and became solitary than striped mice from control groups. This is in support of the ecological constraints hypothesis. 7. During the non-breeding season, striped mice remained group-living despite the availability of free territories. Significantly, more striped mice became solitary-living during the breeding than during the non-breeding season. This is the first experimental support for the reproductive competition hypothesis explaining solitary-living. 8. Analysis of the sexual maturity of males showed that males which became solitary had a higher reproductive potential than males that remained group-living. Analysis of the body mass data of females showed that more solitary females reproduced than group-living females. These results indicate that by becoming solitary individuals of both sexes avoided costs of reproductive competition within groups. 9. Our study provides experimental evidence that reproductive competition within groups can lead to dispersal and solitary-living.

Differential investment into testes and sperm production in alternative male reproductive tactics of the African striped mouse (Rhabdomys pumilio)

“…philopatric to roamer to 560 breeder; Schradin et al, 2012). Our present study indicates that these switches between tactics 561 are accompanied by physiological switches, such as changes in hormone levels, testes 562 development and functionality.…”

“…At this age, all males were fully scrotal. This represented the 216 age at which males would have dispersed and become solitary roamers under field conditions 217 of very low population density (Schoepf and Schradin, 2012;Schradin, 2005). Their body 218 mass (to the closest 0.1 g) was determined before they were anaesthetized and a blood sample 219 of 300 µl was taken using sublingual blood sampling (Heimann et al, 2009).…”

Intraspecific variation in social organization by genetic variation, developmental plasticity, social flexibility or entirely extrinsic factors

“…If conflict occurs, no changes in dominance hierarchy are possible nor do submissive behaviours exist to resolve conflict. Instead, striped mice switch to a solitary lifestyle if conflict increases [45,48]. Social flexibility might thus arise from the absence of flexibility in social behaviour, making solitary-living the only adaptive alternative to groupliving when inter-individual conflict increases to a certain threshold level.…”

Absence of reproductive suppression in young adult female striped mice living in their natal family

“…For 232 this, individuals were removed by hand from their tank, inspected visually and 233 returned within 2 minutes. Female rodents, including female striped mice, are 234 regarded as being sexually mature and ready to mate when their vagina is perforate 235 (open) (Brooks, 1982; this study), which we also use in field studies to determine 236 sexual maturity and readiness to mate (Schoepf & Schradin, 2012a). When a singly 237 housed female showed a perforate vagina before her sister did, a blood sample was 238 taken from each to compare their corticosterone levels.…”

“…We therefore used a paired data design by randomly 227 assigning one sister to being family housed and the other sister being singly housed. 228Some of the other individuals were used for a similar study in males (Schradin et al (open) (Brooks, 1982; this study), which we also use in field studies to determine 236 sexual maturity and readiness to mate (Schoepf & Schradin, 2012a). When a singly 237 housed female showed a perforate vagina before her sister did, a blood sample was 238 taken from each to compare their corticosterone levels.…”