Endocrinology of sociality: comparisons between sociable and solitary individuals within the same population of African striped mice.

The social organization of species ranges from solitary-living to complex social groups. While the evolutionary reasons of group-living are well studied, the physiological mechanisms underlying alternative social systems are poorly understood. By studying group-living and solitary individuals of the same species, we can determine hormonal correlates of sociality without the problem of confounding phylogenetic factors. The African striped mouse (Rhabdomys pumilio) is a socially flexible species, which can be solitary or alternatively form complex family groups, depending on population density and the extent of reproductive competition. We predicted group-living striped mice to show signs of reproductive suppression and social stress, resulting in higher corticosterone but lower testosterone levels when compared to solitary-living individuals. To determine whether differences in social organization correlated with hormonal differences, we collected blood samples from free-living striped mice during four breeding seasons when we experimentally induced solitary-living in philopatric individuals by locally reducing population density. Striped mice that were group-living did not change their corticosterone or estosterone levels during the study, indicating that there was no temporal effect during the breeding season. Striped mice of both sexes had significantly lower corticosterone levels after switching from group- to solitary-living. Solitary males - but not solitary females - had higher testosterone levels than group-living conspecifics. Our results suggest that group-living results in physiological stress and can induce reproductive suppression, at least in philopatric males. The switch to solitary-living may thus be a tactic to avoid reproductive competition within groups, and is associated with decreased stress hormone levels and onset of independent reproduction.

Schoepf I ，Schradin C 《-》

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

1. 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.

Schoepf I ，Schradin C 《-》

Staying put or leaving home: endocrine, neuroendocrine and behavioral consequences in male African striped mice.

Social flexibility occurs when individuals of both sexes can change their social and reproductive tactics, which in turn can influence the social system of an entire population. However, little is known regarding the extent to which individuals of socially flexible species vary in their social behavior and in the underlying physiological mechanisms that support different social tactics. The present study in African striped mice modeled in captivity three male tactics described from the field: (a) philopatric males remaining in the family; (b) solitary roamers; or (c) group-living breeding males. Sixteen pairs and their offspring were kept in captivity, while one male offspring from the family remained as singly housed after he reached 21 days of age. Differences in behavior, morphology, hormone and neuropeptide levels were tested, and physiological measurements were correlated with behavioral measurements. In standardized arena experiments group-living males (philopatrics and breeders) were significantly more aggressive than singly housed males, in agreement with previous data suggesting that group-living, but not roaming males, are territorial. Philopatric males showed signs of reproductive suppression, small testes, lower testosterone and higher corticosterone levels than their singly housed brothers. Higher levels of arginine vasopressin (AVP) were measured in the PVN and BNST of singly housed males compared to group-living males. Based on these findings we hypothesize that roamers are physiologically primed, and capable, if the opportunity to mate arises, to release AVP, form social bonds and become territorial, thus quickly adopting the tactic as breeding male which would yield a higher reproductive success.

Schradin C ，Kenkel W ，Krackow S ，Carter CS ... -  《-》

Alternative reproductive tactics in female striped mice: Solitary breeders have lower corticosterone levels than communal breeders.

Alternative reproductive tactics (ARTs), where members of the same sex and population show distinct reproductive phenotypes governed by decision-rules, have been well-documented in males of many species, but are less well understood in females. The relative plasticity hypothesis (RPH) predicts that switches between plastic ARTs are mediated by changes in steroid hormones. This has received much support in males, but little is known about the endocrine control of female ARTs. Here, using a free-living population of African striped mice (Rhabdomys pumilio) over five breeding seasons, we tested whether females following different tactics differed in corticosterone and testosterone levels, as reported for male striped mice using ARTs, and in progesterone and oestrogen, which are important in female reproduction. Female striped mice employ three ARTs: communal breeders give birth in a shared nest and provide alloparental care, returners leave the group temporarily to give birth, and solitary breeders leave to give birth and do not return. We expected communal breeders and returners to have higher corticosterone, owing to the social stress of group-living, and lower testosterone than solitary breeders, which must defend territories alone. Solitary breeders had lower corticosterone than returners and communal breeders, as predicted, but testosterone and progesterone did not differ between ARTs. Oestrogen levels were higher in returners (measured before leaving the group) than in communal and solitary breeders, consistent with a modulatory role. Our study demonstrates hormonal differences between females following (or about to follow) different tactics, and provides the first support for the RPH in females.

Hill DL ，Pillay N ，Schradin C 《-》

Reproductive competition favours solitary living while ecological constraints impose group-living in African striped mice.

Schradin C ，König B ，Pillay N 《-》