MacColl Andrew D C, Hatchwell Ben J
Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, United Kingdom.
Am Nat. 2002 Aug;160(2):186-94. doi: 10.1086/341013.
Cooperative breeding is paradoxical because some individuals forego independent reproduction and instead help others to reproduce. The ecological constraints model states that such behavior arises because of constraints on independent reproduction. Spatial variation in constraints has been shown to co-vary with the incidence of cooperative breeding in correlational and experimental studies. Here, we examine whether temporally variable ecological constraints can act in a similar way to promote cooperative breeding in the atypical system of long-tailed tits Aegithalos caudatus. In this species, individuals may switch reproductive tactics from breeding to helping within the same breeding season. Using 7 yr of field data, we show that reproductive success declined seasonally because of declines in brood size, nestling weight, and juvenile survival. The survival to breeding age of chicks from nests with helpers was higher than for chicks from nests without helpers, and since helpers usually helped at the nest of a close relative, they accrued inclusive fitness benefits. We used these data to model the expected fitness payoffs of breeding and helping at different times during the season. The model shows that late in the breeding season, the fitness payoff from a kin-directed helping tactic becomes greater than that from independent breeding. The behavioral switch predicted by the model is consistent with the observed switch from breeding to helping, which shows that cooperative breeding may evolve as a way of making the best of a bad job at the end of a temporally constrained breeding season.
合作繁殖是自相矛盾的，因为一些个体放弃独立繁殖，转而帮助其他个体繁殖。生态限制模型表明，这种行为的出现是由于独立繁殖受到限制。在相关性研究和实验研究中，限制因素的空间变化已被证明与合作繁殖的发生率共同变化。在这里，我们研究了随时间变化的生态限制是否能以类似的方式在长尾山雀（Aegithalos caudatus）这种非典型系统中促进合作繁殖。在这个物种中，个体可能在同一个繁殖季节内从繁殖策略转变为帮助策略。利用7年的野外数据，我们表明繁殖成功率随季节下降，原因是窝卵数、雏鸟体重和幼鸟存活率下降。有帮手的巢中雏鸟活到繁殖年龄的比例高于没有帮手的巢中的雏鸟，而且由于帮手通常在近亲的巢中帮忙，它们获得了广义适合度收益。我们利用这些数据来模拟在季节中不同时间繁殖和帮助的预期适合度回报。该模型表明，在繁殖季节后期，亲属导向的帮助策略的适合度回报大于独立繁殖的回报。模型预测的行为转变与观察到的从繁殖到帮助的转变一致，这表明合作繁殖可能作为一种在时间受限的繁殖季节结束时尽力应对不利情况的方式而进化。
