Division of Biological Sciences, University of Montana, Missoula, Montana, United States of America.
PLoS One. 2018 Nov 7;13(11):e0206997. doi: 10.1371/journal.pone.0206997. eCollection 2018.
The size of sexually selected weapons and their performance in battle are both critical to reproductive success, yet these traits are often in opposition. Bigger weapons make better signals. However, due to the mechanical properties of weapons as lever systems, increases in size may inhibit other metrics of performance as different components of the weapon grow out of proportion with one another. Here, using direct force measurements, we investigated the relationship between weapon size and weapon force production in two hindleg weapon systems, frog-legged beetles (Sagra femorata) and leaf-footed cactus bugs (Narnia femorata), to test for performance tradeoffs associated with increased weapon size. In male frog-legged beetles, relative force production decreased as weapon size increased. Yet, absolute force production was maintained across weapon sizes. Surprisingly, mechanical advantage was constant across weapon sizes and large weaponed males had disproportionately large leg muscles. In male leaf-footed cactus bugs, on the other hand, there was no relationship between weapon size and force production, likely reflecting the importance of their hindlegs as signals rather than force-producing structures of male-male competition. Overall, our results suggest that when weapon force production is important for reproductive success, large weaponed animals may overcome mechanical challenges by maintaining proportional lever components and investing in (potentially costly) compensatory mechanisms.
性选择武器的大小及其在战斗中的表现对繁殖成功都至关重要，但这些特征往往相互矛盾。更大的武器能发出更好的信号。然而，由于武器作为杠杆系统的机械特性，尺寸的增加可能会抑制其他性能指标，因为武器的不同组件彼此不成比例地生长。在这里，我们使用直接力测量，研究了两种后肢武器系统（蛙腿甲虫（Sagra femorata）和叶足仙人掌虫（Narnia femorata））中武器大小与武器力产生之间的关系，以测试与武器尺寸增加相关的性能权衡。在雄性蛙腿甲虫中，相对力产生随着武器尺寸的增加而减小。然而，绝对力产生在整个武器尺寸范围内得以维持。令人惊讶的是，机械优势在整个武器尺寸范围内保持不变，并且大型武器的雄性具有不成比例的大型腿部肌肉。另一方面，在雄性叶足仙人掌虫中，武器尺寸与力产生之间没有关系，这可能反映出它们的后腿作为信号而不是雄性间竞争的力产生结构的重要性。总的来说，我们的结果表明，当武器力产生对繁殖成功很重要时，大型武器的动物可能通过维持比例杠杆组件和投资（可能昂贵的）补偿机制来克服机械挑战。
