Department of Animal Behaviour, Institute of Evolutionary Biology and Environmental Studies, University of Zurich, Zurich, Switzerland.
Horm Behav. 2013 Jun;64(1):89-94. doi: 10.1016/j.yhbeh.2013.04.011. Epub 2013 May 3.
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.
物种的社会组织形式从独居到复杂的社会群体不等。虽然群居的进化原因已经得到了很好的研究，但支持替代社会系统的生理机制还知之甚少。通过研究同一物种的群居和独居个体，我们可以确定社会性的激素相关性，而不会受到混淆进化因素的问题。非洲条纹鼠（Rhabdomys pumilio）是一种具有社会灵活性的物种，它可以独居，也可以根据种群密度和繁殖竞争的程度，形成复杂的家庭群体。我们预测群居的条纹鼠会表现出繁殖抑制和社会压力的迹象，导致皮质酮水平升高，但睾丸激素水平降低，与独居个体相比。为了确定社会组织形式的差异是否与激素差异相关，我们在四个繁殖季节从自由生活的条纹鼠身上采集了血液样本，在这些季节中，我们通过局部降低种群密度，在亲代个体中实验性地诱导独居。群居的条纹鼠在研究期间没有改变它们的皮质酮或雌激素水平，这表明繁殖季节期间没有时间效应。从群居转变为独居后，雌雄条纹鼠的皮质酮水平都显著降低。独居雄性——而不是独居雌性——的睾丸激素水平高于群居的同种动物。我们的结果表明，群居会导致生理压力，并可能导致繁殖抑制，至少在亲代雄性中是这样。因此，独居的转变可能是一种避免群体内繁殖竞争的策略，并且与应激激素水平降低和独立繁殖的开始有关。
