Cooperative breeding behavior in birds ranges from inducible to obligate strategies and has evolved across diverse taxa, in species that display a wide range of social and reproductive behavior. It is often thought to evolve when independent breeding is constrained, and cooperation increases fitness. Yet many systems show variable, even maladaptive, fitness effects. This observation, together with the wide range in the form and frequency of cooperative breeding, raises the question of how the recurrent appearance of cooperative breeding and its extensive variation across species—from inducible to obligate—can be explained. Here, we take a proximate perspective on the evolution of cooperative breeding to argue that cooperative strategies are delineated by the history of prior adaptations and emerge through the rearrangement of preexisting neuroendocrine mechanisms underlying social, dispersal, and parental behaviors. Natural selection sorts among the resultant variants to alter regulation of cooperation, producing stabilization through either greater developmental entrenchment or greater reliance on environmental cues. Thus, species showing inducible cooperative behavior may be at a transitory stage in this process. To assess this possibility, we first evaluate evidence that the components of cooperative breeding are common across taxa. Then, we review the neuroendocrine mechanisms that regulate the pathways underlying cooperative strategies. Finally, we assess the evidence for neuroendocrine linkages during development that may channel coexpression of some components of cooperative breeding and facilitate its evolution. We conclude that understanding the mechanistic bases of the behaviors comprising cooperative breeding strategies may provide novel insight into the recurrent emergence of this strategy across disparate environments and avian taxa.

中文翻译：

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合作育种起源与演化的神经内分泌视角

鸟类的合作繁殖行为范围从诱导策略到强制策略，并且已经跨越不同的分类群，在表现出广泛的社会和生殖行为的物种中进化。当独立育种受到限制并且合作增加适应性时，它通常被认为是进化的。然而，许多系统显示出可变的，甚至是适应不良的健身效果。这一观察结果，连同合作育种形式和频率的广泛变化，提出了如何解释合作育种的反复出现及其在物种间的广泛变异——从诱导型到专性型——的问题。这里，我们对合作育种的演变采取了一种近似的观点，认为合作策略是由先前适应的历史所描述的，并通过重新排列社会、分散和父母行为背后的预先存在的神经内分泌机制而出现。自然选择在产生的变体中进行分类以改变合作的调节，通过更大的发展巩固或更多地依赖环境线索来产生稳定。因此，表现出可诱导合作行为的物种可能处于该过程的过渡阶段。为了评估这种可能性，我们首先评估合作育种的组成部分在分类群中是常见的证据。然后，我们回顾了调节潜在合作策略途径的神经内分泌机制。最后，我们评估了发育过程中神经内分泌联系的证据，这些联系可能会引导合作育种的某些成分的共表达并促进其进化。我们得出的结论是，了解构成合作育种策略的行为的机制基础可以为这种策略在不同环境和鸟类分类群中反复出现提供新的见解。