冠羽畫眉之群成員數與群穩定度

Abstract

在有關社會演化(social evolution)的研究中，影響社會結構(social structure)的生態因子與社會因子是目前研究的焦點，透過分析群體生活的利與弊，可以瞭解群體成員的適存度(fitness)與群成員數之間的關聯。群內成員與加入者衝突模型(insider-outsider conflict model, IOM)提供了一個可供測試之架構，作為研究前述群成員數與群成員的適存度與群成員之間利益衝突的基礎。群內成員與加入者衝突模型是在描述群體中的群內成員(insiders)與潛在的加入者(outsiders)之間的衝突。我們於1997至2007年在梅峰地區進行冠羽畫眉的合作生殖行為研究，透過記錄影響冠羽畫眉社會結構的因子，如群成員數(group size)、群組成(group composition)、穩定度(stability)，來測試群內成員與加入者衝突模型。我們發現冠羽畫眉最常見的群成員數(4隻一群)為最穩定的群成員數(穩定度83.7%)。大群(群成員數6-8)的群成員變動主要是群成員數減少(83.3%)，而小群(群成員數2-3)大多消失在研究地區(41.7%)，沒有消失的群以傾向變成較大的群成員數(54.2%)。親戚群體出現在大群的比例(67.3%)高於在中群(群成員數4-5)的比例(32.7%)，而小群的冠羽畫眉沒有發現任何的親戚群體。本研究是第一個在合作生殖的脊椎動物測試IOM,結果完全符合根據IOM所提出的預測，確立了IOM可以作為研究社會群體的群成員數、群組成、穩定度等社會結構的基礎框架。

Ecological and social determinants of social structure are one of the focuses in study of social evolution. The cost-benefit analysis of group living helps understand the outline of the function relating individual fitness to group size. Building on the functions of individual fitness to group size, the insider-outsider conflict model (IOM) describes the tension between group members (the insiders) and potential joiners (the outsiders). We study factors affect social structure (group size, group composition, and stability) by testing the predictions derived from IOM in joint-nesting Taiwan Yuhinas (Yuhina brunneiceps). I found that the commonest observed group size, i.e. two pairs, is the most stable (stability 83.7%) group size in Yuhinas. And the Yuhina groups tended to converge to the commonest group size. The size of large groups (6-8 individuals) tended to decrease (83.3%), and that of small groups (2-3 individuals) tended to increase (54.2%). And part of small groups disappeared from study site (41.7%). We also found the observed group size of kin group were larger than that of non-kin group in Yuhinas. All of the predictions derived from IOM are supported by our results, confirming the application of insider-outsider conflict model in cooperatively breeding species.