Department of Evolution, Faculty of Health and Life Sciences, Institute of Integrative Biology, Ecology and Behaviour, Biosciences Building, University of Liverpool, Crown Street, Liverpool, UK.
Ecol Lett. 2012 Jun;15(6):576-83. doi: 10.1111/j.1461-0248.2012.01770.x. Epub 2012 Apr 4.
A central explanation for group living across animal taxa is the reduced rate of attack by predators. However, many field observations show a weak or non-existent effect of group size on per capita mortality rates. Herein we resolve this apparent paradox. We found that Pieris brassicae larvae defended themselves less readily when in groups than when alone, in that they were more reluctant to regurgitate in response to simulated attacks and produced less regurgitant. Furthermore, a simple model demonstrates that this reluctance was sufficient to cancel out the benefit from being in a group. This conditional strategy can be understood in terms of the costs and benefits of defences. For grouped individuals, defence is less often required because attack rates are lower and the costs of defence may be higher due to competition for resources. These phenomena are likely to be widespread in facultatively gregarious species that utilise anti-predator defences.
跨动物类群的群体生活的一个主要解释是减少了被捕食者攻击的速度。然而，许多实地观察表明，群体大小对人均死亡率的影响很小或不存在。在此，我们解决了这个明显的悖论。我们发现，当被模拟攻击时，甘蓝菜粉蝶幼虫在群体中比单独时更不愿意反刍，并且它们反刍的量也更少。此外，一个简单的模型表明，这种不情愿足以抵消群体生活的好处。这种有条件的策略可以根据防御的成本和收益来理解。对于群体中的个体，由于攻击率较低，并且由于对资源的竞争，防御的成本可能更高，因此防御通常不需要。这些现象可能在利用防御性逃避策略的兼性群居物种中很普遍。
