Cuckoo-host co-evolution : portfolios of offence and defence

在自然界中，生物体的进化不仅是为了应对物理环境的变化，也是为了应对竞争对手、捕食者和寄生虫的变化。普通杜鹃（Cuculus canorus）与其宿主之间的进化之战为研究这些野生进化过程提供了一个很好的模型系统。布谷鸟是一个著名的骗子：它欺骗其他种类的鸟（宿主）孵化它的蛋和抚养它的孩子。布谷鸟在每个寄主巢只产一个蛋。孵化后不久，布谷鸟雏鸟就把寄主的蛋和雏鸟从巢中驱逐出去。然后，它操纵宿主通过一种引人注目的乞讨叫声来喂养它，这听起来像许多饥饿的宿主年轻。考虑到宿主自身繁殖的这些成本，宿主进化出防御机制也就不足为奇了，比如排斥与自己不同的卵。作为回应，布谷鸟已经进化成几个不同的遗传种族，每个种族都专门针对一个宿主物种，并产下与其特定宿主物种的卵相匹配的卵类型。布谷鸟与宿主的互动不太可能只涉及这一条防线和进攻。有可能在几个阶段的相互作用，导致'组合'的适应和反适应。例如，成年杜鹃的围攻可以成为宿主防御的有效前线，这首先降低了巢穴被寄生的机会。布谷鸟鹰一样的外表很可能是一种反击，这使得近距离接触对宿主来说可能是危险的。布谷鸟反击的结果是宿主的防御是昂贵的。排斥卵可能会弥补宿主的生殖投资，但也会带来宿主排斥自己的卵而不是寄生虫卵的风险，而围攻成年杜鹃则会带来潜在的危险。我们以前已经表明，主机不同的防御关系到当地的寄生风险。该提案解决了两个新问题：第一，宿主防御的连续防线之间是否存在相互作用？第二，宿主如何评估当地的寄生风险？我们将研究沼泽地杜鹃的主要寄主苇莺。利用成年杜鹃和模型杜鹃蛋进行的野外实验将测试具有更好的监视或更强的围攻反应的夫妇是否更有可能或更少地拒绝杜鹃蛋。最近的理论表明，一个更强大的前线防御减少了后续防御的需要，就像前门上的坚固锁排除了卧室门上的锁一样。我们还将研究作为一种潜在的防御巢建筑。从理论上讲，杜鹃应该选择更坚固的宿主巢，因为杜鹃雏鸟比宿主巢长得更大，而且雏鸟期也更长。作为回应，它可以支付苇莺建造更多脆弱的巢穴，特别是在寄生风险高的地方。我们将测试如何巢设计影响成功的寄生和unparasitised巢，并在不同的寄生风险的网站不同的巢设计的成本和效益模型。为了测试苇莺是否改变巢防御反应的直接线索，从成年杜鹃，或间接线索（如。距离潜在的布谷鸟监视栖息地），我们将比较有和没有布谷鸟的网站的防御。实验还将测试是否天真，年轻的芦苇莺可以通过观察有经验的邻居的反应来学习加强他们对成年杜鹃的识别，作为一个可以安全围攻的敌人。这项研究将提高我们对共同进化过程的理解，揭示相互作用的一个阶段的适应如何影响其他阶段的适应。它还将提高我们对个人如何利用当地信息迅速适应环境变化的理解。它将增加我们对杜鹃的了解，杜鹃是我们乡村的象征之一。

In Nature, organisms evolve in response to changes not only in the physical environment but also in their competitors, predators and parasites. The evolutionary battle between the common cuckoo, Cuculus canorus, and its hosts provide a good model system for studying these evolutionary processes in the wild. The cuckoo is a famous cheat: it tricks other species of birds (the hosts) into incubating its eggs and raising its young. The cuckoo lays just one egg per host nest. Soon after hatching, the cuckoo chick ejects the host's eggs and young out of the nest. It then manipulates the hosts to feed it by a remarkable begging call, which sounds like many hungry host young. Given these costs to the host's own reproduction, it is not surprising that hosts have evolved defences, such as rejecting eggs that differ from their own. In response, the cuckoo has evolved into several distinct genetic races, each specialising on one host species and laying an egg type that matches the egg of its particular host species. Cuckoo-host interactions are unlikely to involve just this one line of defence and offence. There is the potential for interactions at several stages, leading to 'portfolios' of adaptations and counteradaptations. For example, mobbing of adult cuckoos can be an effective front-line of host defence, which reduces the chance that the nest is parasitized in the first place. The hawk-like appearance of cuckoos is likely to be a counter-response, which makes close approach potentially risky for the hosts. The outcome of cuckoo counter-responses is that host defences are costly. Egg rejection may redeem the host's reproductive investment but entails the risk that the host rejects its own eggs rather than the parasite egg, while mobbing an adult cuckoo entails potentially dangerous enemy inspection. We have previously shown that hosts vary their defences in relation to local parasitism risk. This proposal addresses two new questions: First, are there interactions between successive lines of host defence? Second, how do hosts assess local parasitism risk? We will study reed warblers, Acrocephalus scirpaceus, the main cuckoo host in marshland. Field experiments, using mounts of adult cuckoos and model cuckoo eggs, will test whether pairs with better surveillance or stronger mobbing responses are more or less likely to reject cuckoo eggs. Recent theory suggests that a stronger front-line of defence reduces the need for subsequent defences, just as a strong lock on the front door precludes the need for a lock on the bedroom door too. We will also investigate nest architecture as a potential defence. In theory, cuckoos should select stronger host nests because the cuckoo chick grows to be more massive than a host brood and also has a more prolonged nestling period. In response, it could pay reed warblers to build more flimsy nests, particularly in sites at high risk of parasitism. We will test how nest design influences the success of parasitized and unparasitised nests, and model the costs and benefits of different nest design at sites of varying parasitism risk. To test whether reed warblers vary nest defences in response to direct cues from adult cuckoos, or indirect cues (eg. distance from potential cuckoo surveillance perches), we will compare defences at sites with and without cuckoos. Experiments will also test whether naïve, young reed warblers can learn to enhance their recognition of the adult cuckoo, as an enemy that can be safely mobbed, by observing the responses of experienced neighbours. This research will improve our understanding of co-evolutionary processes, revealing how adaptations at one stage of an interaction influence those at other stages. It will also enhance our understanding of how individuals use local information to adapt rapidly to environmental change. And it will add to our knowledge of the cuckoo, one of the icons of our countryside.