Armitage K B, Schwartz O A
Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045-2106, USA.
Proc Natl Acad Sci U S A. 2000 Oct 24;97(22):12149-52. doi: 10.1073/pnas.200196097.
The yellow-bellied marmot (Marmota flaviventris) is a social, ground-dwelling squirrel that lives either individually or in kin groups of from two to five adult females. Philopatry and daughter recruitment lead to the formation and persistence of matrilines at habitat sites. By using 37 years of demographic data for 12 habitat sites, we could determine long-term trends in the effects of group size on two measures of fitness, survivorship and net reproductive rate, which otherwise are obscured by annual fluctuations in these measures. Both size and number of matrilines varied among sites and survivorship and net reproductive rate varied among sites and among matriline sizes. The role of social organization was explored further by examining the effect of matriline size, averaged over all years and sites, on fitness. For both survivorship and net reproductive rate the relationship with matriline size was curvilinear. Fitness increased with the increase in matriline size and then decreased in the largest groups. Decreased fitness in matrilines of four or five was associated with agonistic behavior, a large number of 2-year-old females in the social group, and reproductive suppression. There is no evidence that females acted to increase their fitness by increasing indirect fitness; i.e., by assisting relatives, but attempted to increase direct fitness. Direct fitness increased when mortality and fission of large matrilines reduced group size and the surviving females increased reproduction.
黄腹土拨鼠（Marmota flaviventris）是一种群居的地松鼠，它们要么单独生活，要么以由两到五只成年雌性组成的亲属群体生活。留居习性和雌性后代的加入导致了在栖息地形成母系群体并长期存在。通过使用12个栖息地37年的种群统计学数据，我们能够确定群体大小对两个适合度指标（存活率和净繁殖率）影响的长期趋势，否则这些指标会被每年的波动所掩盖。母系群体的大小和数量在不同栖息地之间有所不同，存活率和净繁殖率在不同栖息地以及不同母系群体大小之间也有所不同。通过研究在所有年份和栖息地平均的母系群体大小对适合度的影响，进一步探讨了社会组织的作用。对于存活率和净繁殖率而言，与母系群体大小的关系都是曲线的。适合度随着母系群体大小的增加而增加，然后在最大的群体中下降。四或五个个体的母系群体适合度下降与攻击行为、群体中大量两岁雌性以及繁殖抑制有关。没有证据表明雌性通过增加间接适合度（即通过帮助亲属）来提高自身适合度，而是试图增加直接适合度。当大型母系群体的死亡率和分裂导致群体大小减少，幸存的雌性增加繁殖时，直接适合度增加。
