Negotiation over investment in biparental birds

当父母双方都照顾同一个后代时，从进化的角度来看，如果父母双方都能以牺牲伴侣为代价减少自己的努力，那么他们的表现就会更好。因此，在双亲共同抚养的物种中，父母双方在哺育幼崽方面应该付出多大的努力存在着性冲突。理论研究假设父母协商（在进化或行为时间尺度上）来决定父母双方的努力，这些模型得出的结论是，只有当父母双方都不完全补偿伴侣努力的任何变化时（即，如果父母一方减少了努力，伴侣应该增加自己的努力，但不足以弥补不足），才能实现稳定的双亲照顾。在大多数鸟类物种中，父母双方都照顾它们的幼雏，因此鸟类被证明是两性冲突理论的肥沃试验场。许多实验研究减少了对父母一方的照顾，并监测了其伴侣的行为反应，预测了不完全补偿。然而，这些研究的结果是模棱两可的。在某些情况下，伴侣不完全补偿，但在其他许多情况下，研究人员发现要么没有反应，要么完全补偿，甚至是一个匹配的反应（即与被操纵的父母相同的方向改变）。为什么实证研究为这一强有力的理论体系提供了如此不完整的支持？最近的一个模型为这个问题提供了一个可能的答案。以前的谈判模式假设父母可以评估被喂养后代的需求或价值；例如，一方努力的增加会导致一窝雏鸟乞讨的减少，从而向伴侣发出需求降低的信号。然而，如果父母不能直接评估后代的需求，他们可能会关注后代需求的其他潜在信号，例如伴侣的工作率。父母对后代需求或价值的不确定性最近在“信息模型”中被形式化了。该模型预测伴侣可能会表现出广泛的适应性反应，从补偿（当有关于雏鸟需求的准确信息时）到匹配反应（当有部分信息时），再到在某些中间信息水平上没有净反应。个体父母评估雏鸟需求的能力可能在物种之间、在物种内或在父母之间有所不同。本研究的目的是使用长尾山雀作为模型系统来测试信息模型的预测。当雏鸟独自待在巢里时，通过向这只雏鸟广播乞讨的叫声来增加一只雏鸟的照顾力度，并监测它的伴侣的反应。该模型的预测将通过在以下情况下进行实验来检验：父母双方都可以评估雏鸟的需求，父母双方都有部分信息，以及信息不对称，即一方有良好的信息，另一方有不良的信息。从后代和来自伴侣的信息整合的过程也将进行实验研究。“这将是对一个假设的第一次实证检验，该假设扩展了我们对进化理论的一个重要组成部分的理解，即亲代投资的性别冲突。”这些发现将引起研究进化生物学、动物行为和生态学的研究人员的普遍兴趣。

When both parents provide care for the same offspring, each parent does better, in evolutionary terms, if it can reduce its own effort at the expense of its partner. Thus, in species with biparental care there is sexual conflict over how hard each parent should work at feeding their young. Theoretical studies assume that parents negotiate (over evolutionary or behavioural time scales) to determine each parent's effort, and such models conclude that stable biparental care is achieved only when each parent compensates incompletely for any change in effort by their partner (i.e. if one parent reduces its effort, the partner should increase its own effort but not sufficiently to make up the shortfall). In most bird species both parents care for their chicks and birds have therefore proved to be a fertile testing ground for theories of sexual conflict over biparental care. Many studies have experimentally reduced the care of one parent and monitored the behavioural response of its partner, predicting incomplete compensation. However, the results of such studies are equivocal. In some cases, the partner has compensated incompletely, but in many others researchers have found either no response, complete compensation, or even a matching response (i.e. a change in the same direction as the manipulated parent). Why have empirical studies provided such a patchy support for this robust body of theory? A recent model provides a potential answer to this question. Previous negotiation models have assumed that parents can assess the need or value of the offspring being fed; for example, an increase in the effort of one parent will result in a brood of nestlings begging less and hence signaling lower demand to the partner. However, if parents are not able to directly assess the need of their brood, they may pay attention to other potential signals of offspring need, such as the work rate of their partner. This idea that there is parental uncertainty over offspring need or value has recently been formalized in the 'information model'. This model predicts that a partner may exhibit a wide range of adaptive responses from compensation (when accurate information on nestling need is available) through to a matching response (when partial information is available) via no net response at some intermediate level of information. The ability of individual parents to assess nestling need may vary between species, within species through the rearing period, or between parents. The aim of this study is to use the long-tailed tit as a model system to test predictions of the information model. The caring effort of one parent will be increased by broadcasting begging calls to just that parent when it is alone at the nest during the nestling period and the response of its partner will be monitored. The predictions of the model will be tested by conducting this experiment in situations where both parents can assess nestling need, when both parents have partial information and when information is asymmetric, i.e. one parent has good information and the other poor information. The process of integration of information from offspring and from a partner will also be investigated experimentally. `This will be the first empirical test of a hypothesis that extends our understanding of an important component of evolutionary theory, i.e. sexual conflict over parental investment. The findings will be of general interest to researchers studying evolutionary biology, animal behaviour and ecology.