Bilde T, Coates K S, Birkhofer K, Bird T, Maklakov A A, Lubin Y, Avilés L
Department of Biological Sciences, Ecology and Genetics, University of Aarhus, Aarhus, Denmark.
J Evol Biol. 2007 Nov;20(6):2412-26. doi: 10.1111/j.1420-9101.2007.01407.x.
The evolution of cooperation requires benefits of group living to exceed costs. Hence, some components of fitness are expected to increase with increasing group size, whereas others may decrease because of competition among group members. The social spiders provide an excellent system to investigate the costs and benefits of group living: they occur in groups of various sizes and individuals are relatively short-lived, therefore life history traits and Lifetime Reproductive Success (LRS) can be estimated as a function of group size. Sociality in spiders has originated repeatedly in phylogenetically distant families and appears to be accompanied by a transition to a system of continuous intra-colony mating and extreme inbreeding. The benefits of group living in such systems should therefore be substantial. We investigated the effect of group size on fitness components of reproduction and survival in the social spider Stegodyphus dumicola in two populations in Namibia. In both populations, the major benefit of group living was improved survival of colonies and late-instar juveniles with increasing colony size. By contrast, female fecundity, female body size and early juvenile survival decreased with increasing group size. Mean individual fitness, estimated as LRS and calculated from five components of reproduction and survival, was maximized for intermediate- to large-sized colonies. Group living in these spiders thus entails a net reproductive cost, presumably because of an increase in intra-colony competition with group size. This cost is traded off against survival benefits at the colony level, which appear to be the major factor favouring group living. In the field, many colonies occur at smaller size than expected from the fitness curve, suggesting ecological or life history constraints on colony persistence which results in a transient population of relatively small colonies.
合作的进化需要群体生活的益处超过成本。因此，一些适合度成分预计会随着群体规模的增加而增加，而其他一些成分可能会因为群体成员之间的竞争而减少。群居蜘蛛提供了一个绝佳的系统来研究群体生活的成本和益处：它们以各种规模的群体出现，且个体寿命相对较短，因此生活史特征和终生繁殖成功率（LRS）可以作为群体规模的函数来估计。蜘蛛的社会性在系统发育上相距甚远的科中反复出现，并且似乎伴随着向群体内持续交配和极端近亲繁殖系统的转变。因此，在这样的系统中群体生活的益处应该是巨大的。我们在纳米比亚的两个种群中研究了群体规模对群居蜘蛛杜氏盘腹蛛繁殖和生存的适合度成分的影响。在这两个种群中，群体生活的主要益处是随着群体规模的增加，群体和大龄幼蛛的存活率提高。相比之下，雌性繁殖力、雌性体型和幼蛛早期存活率随着群体规模的增加而下降。根据LRS估计并从繁殖和生存的五个成分计算得出的平均个体适合度，在中大型群体中达到最大值。因此，这些蜘蛛的群体生活带来了净繁殖成本，大概是因为群体内竞争随着群体规模的增加而加剧。这种成本与群体层面的生存益处相互权衡，而生存益处似乎是有利于群体生活的主要因素。在野外，许多群体的规模比适合度曲线预期的要小，这表明存在对群体持续存在的生态或生活史限制，从而导致了相对较小群体的短暂种群。
