Bats and moths in the real world: neuronal responses as adaptations to predation

蝙蝠和昆虫之间的相互作用一直令进化生物学家着迷。蝙蝠利用回声定位来探测和追踪夜间活动的昆虫，世界上大约70%的蝙蝠以昆虫为食。为了防御，至少有7目昆虫进化出了耳朵，可以接收蝙蝠回声定位发出的超声波频率。这些耳朵通常结构简单，但在触发逃跑行为方面非常有效，从而降低了昆虫被吃掉的风险。对耳朵的研究最多的是在飞蛾身上，1-4个感觉细胞向中枢神经系统发送信号，然后触发一系列行为反应，从逃离信号源到不可预测的复杂循环操作。蝙蝠和飞蛾之间的相互作用通常被视为一场进化上的军备竞赛，蝙蝠对回声定位的适应驱动了昆虫对听觉反应的适应，而听觉反应反过来又影响了蝙蝠对回声定位信号的进一步进化。迄今为止，大多数关于蝙蝠和飞蛾之间相互作用的研究都是在实验室进行的。我们的目标是在自然界中研究这些相互作用，这很重要，因为蝙蝠的回声定位呼叫在野外和实验室条件下有很大的不同。因此，我们将使用飞蛾作为生物麦克风，记录蝙蝠飞行路线上的听觉神经元的反应。我们将测试飞蛾探测蝙蝠回声定位呼叫的距离（具有不同的频率，时间和强度参数）是否取决于信号设计。我们可以准确地量化探测距离，因为我们可以通过测量一组麦克风的到达时间差异，在三维空间中精确地确定蝙蝠的位置。我们还可以通过使用测量麦克风来计算已知位置蝙蝠叫声的强度，这样我们就可以测量触发飞蛾神经反应的声压水平。我们亦会用两台摄影机及红外线照明灯记录飞蛾在黑暗中的飞行轨迹，以量化飞蛾的躲避动作。然后，我们可以对飞蛾使用的逃跑策略进行分类，并将它们与蝙蝠使用的信号设计联系起来。这些方法将使我们能够验证这样一个假设，即飞蛾会从远处的蝙蝠那里飞走，只有在蝙蝠靠近时才会做出不可预测的逃跑动作（因此会发出更强烈的信号）。我们的第一个视频记录了一只飞蛾躲避蝙蝠的攻击，显示它使用了一种以前被描述为在飞机缠斗中避免攻击的方法！我们将把我们的知识从现场带到实验室，在更可控的条件下测试我们的预测。我们将回放一些蝙蝠对飞蛾准备发出的攻击序列。我们最近发表在《当代生物学》上的研究表明，飞蛾可以根据声源的强度改变它们的听觉反应。在低声强下，蛾的耳膜对低频敏感，在高声强下，它们对高频更加敏感。这种听觉敏感度的变化是完全出乎意料的。然而，从适应性的角度来看，这些变化是完全有道理的/蝙蝠在捕捉昆虫时使用的频率通常比在寻找昆虫时使用的频率更高。我们的现场记录将准确地描述捕获猎物时信号设计的变化，并且通过监测鼓膜的反应（通过激光振动仪），我们可以通过从听神经记录来确定同时反应是否在神经水平上起作用。我们相信，通过在自然条件下进行研究，可以最好地推进对捕食者-猎物相互作用的理解。我们的工作将确定飞蛾的听觉如何对野外蝙蝠的回声定位做出反应，飞蛾如何对已知结构和强度的蝙蝠叫声做出行为反应，以及飞蛾是否可以调整他们的听觉反应以最好地主动探测蝙蝠。

Interactions between bats and insects have long fascinated evolutionary biologists. Bats use echolocation to detect and track nocturnal insects, and about 70% of bat species worldwide eat insects. In defence, insects in at least 7 orders have evolved ears that pick up the ultrasonic frequencies emitted by echolocating bats. These ears are often simple in structure, but highly effective for triggering escape behaviours that reduce the risk of the insect being eaten. Ears have been most studied in moths, where 1-4 sensory cells send signals to the central nervous system, which can then trigger a range of behavioural responses ranging from flight away from the signal source to unpredictable complex looping manoeuvres. Interactions between bats and moths are often viewed as an evolutionary arms race, with adaptations in the echolocation calls of bats driving adaptations in the hearing responses of insects, which in turn shape the further evolution of echolocation signals in bats. To date, most work on interactions between bats and moths has taken place in the laboratory. We aim to study these interactions in nature, and this is important because bat echolocation calls differ substantially in field and laboratory conditions. We will therefore use moths as biological microphones, recording responses of auditory neurones along the flight paths of bats. We will test whether the distance at which moths detect the echolocation calls of bat species (with differing frequency, time and intensity parameters) depends on signal design. We can quantify detection distances accurately because we can pinpoint the bat's position accurately in 3-dimensions by measuring time-of-arrival differences at an array of microphones. We can also calculate the intensity of the bat calls at known positions by using a measuring microphone, allowing us to measure the sound pressure level that triggers a neural response in the moth. We will also quantify the evasive manoeuvres used by moths in the dark by recording their flight paths using two video cameras and infrared lighting. We can then categorise the escape manoeuvres used by moths, and relate these to signal designs used by bats. These methods will allow us to test the hypothesis that moths fly away from distant bats, and only perform unpredictable escape manoeuvres when bats are close by (and hence emitting more intense signals). Our first video recording of a moth evading a bat attack has shown it to use a manoeuvre previously described as a method to avoid attack in dogfights by aircraft! We will take our knowledge from the field into the laboratory to test our predictions under more controlled conditions. We will play back some of the attack sequences emitted by bats to moth preparations. Our recent work, published in Current Biology, suggests that moths can change their hearing responses in relation to the intensity of the sound source. At low sound intensities, moth ear membranes are sensitive to low frequencies, at higher intensities they become more sensitive to higher frequencies. Such changes in hearing sensitivity were totally unexpected. The changes make perfect sense from an adaptive perspective however / bats often use higher frequencies when they home in on insects than when they are searching for them. Our field recordings will give an accurate picture of how signal design changes in prey capture, and by monitoring responses of the eardrum (by laser vibrometry) we can establish whether simultaneous responses operate at the neural level by recording from the auditory nerve. We believe that understanding predator-prey interactions can best advance by performing studies in natural conditions. Our work will determine how moth hearing responds to bat echolocation in the field, how moths respond behaviourally to bat calls of known structure and intensity, and whether moths can adjust their auditory responses to best detect bats in an active manner.