Density-dependence is an important mechanism in the population regulation of small mammals. Stressors induced by high-density (e.g., crowding and aggression) can cause physiological and neurological disorders, and are hypothesized to be associated with alterations in gut microbiota, which may in turn reduce the fitness of animals by increasing stress- or disease-associated microbes. In this study, we examined the effects of housing density on the hormone levels, immunity, and composition of gut microbiota in male Brandt's voles (Lasiopodomys brandtii) by conducting two specific housing density experiments with or without physical contact between voles. Voles in high density groups exhibited higher serum corticosterone (CORT), serotonin (5-HT), and immunoglobulin G (IgG) levels, as well as higher testosterone (T) levels only in the experiment with physical contact. Meanwhile, high-density treatments induced significant changes in the composition of gut microbiota by increasing disease-associated microbes. The levels of hormones and immunity (i.e., CORT, 5-HT, and IgG) elevated by the high density treatment were significantly correlated with some specific microbes. These results imply that high-density-induced stress may shape the fitness of animals under natural conditions by altering their gut microbiota. Our study provides novel insights into the potential roles of gut microbiota in the density-dependent population regulation of small rodents as well as the potential mechanisms underlying psychological disorders in humans and animals under crowded conditions.

密度依赖性是小型哺乳动物种群调节的重要机制。高密度（例如拥挤和侵略）诱发的应激源可能会导致生理和神经疾病，并被认为与肠道菌群的改变有关，这反过来可能会通过增加与压力或疾病相关的微生物而降低动物的适应性。在这项研究中，我们检查了住房密度对雄性布氏田鼠（Lasiopodomys brandtii）的激素水平，免疫力和肠道菌群组成的影响。），在田鼠之间进行或不进行物理接触的情况下，进行两个特定的房屋密度实验。仅在有物理接触的实验中，高密度组的田鼠表现出较高的血清皮质激素（CORT），血清素（5-HT）和免疫球蛋白G（IgG）水平，以及较高的睾丸激素（T）水平。同时，高密度治疗通过增加与疾病相关的微生物，引起肠道菌群组成的显着变化。通过高密度处理提高的激素和免疫水平（即CORT，5-HT和IgG）与某些特定微生物显着相关。这些结果表明，高密度诱导的压力可能会通过改变动物的肠道菌群来影响动物在自然条件下的适应能力。