Abstract Understanding the trajectory of host-parasite co-evolution requires knowledge of how hosts and parasites impact one another’s fitness, especially among the avian and insect social parasites. Host choice is an important first step in this process, but the principles guiding host choice are unresolved, especially for specialist parasites choosing among individual hosts with multiple traits. To determine how parasites weigh various host traits relative to others, we need to identify their costs and benefits. Here I use the slave-making ant, Temnothorax americanus , to investigate the payoffs from different host trait combinations. I measured the costs and benefits of raids at hosts that varied in their value (# brood), defensive power (# workers), or their ratios. Additionally, I investigated whether slave-maker fighting power influences which host trait combinations were optimal. Slave-makers performed best when hosts contained more brood but fewer workers. However, the ability to maximize this ratio is constrained by the correlation of brood and worker numbers in natural nests, making the optimal host moderately sized. Measures of costs reinforce this conclusion, since slave-maker mortality increased with the number of host workers. Additionally, I found that larger slave-maker colonies have higher payoffs at larger hosts, suggesting their optimal host trait profile differs from smaller colonies. This study shows that social parasites exercising force ought to balance a trade-off between host value and defensibility, rather than maximizing only value. Furthermore, the results highlight that host demography could play a larger role in insect social parasite arms races than previously appreciated. Significance statement A key to understanding the outcome of co-evolutionary arms races in social parasites is identifying the traits that contribute to the success of parasites and hosts. Host choice determines which traits experience selection, but the relative costs and benefits associated with different host traits, and how parasites ought to weigh them, remains unclear. I measure parasite success in relation to host and parasite demographic traits to distinguish the relative impacts of each using a slave-making ant. Maximizing host brood-to-worker ratio leads to higher payoffs, while the most populous hosts are impenetrable. These findings provide evidence that social parasites attacking by force seek a balance between a host’s value and defensibility, even if it means forgoing hosts with higher potential value. This in turn highlights colony demography and life history as important host traits under selection in co-evolutionary arms races in social insect parasites.

中文翻译：

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不要太大，也不要太小：对中等规模的主机进行突袭可以为制造奴隶的蚂蚁带来最佳结果

摘要 了解宿主-寄生虫共同进化的轨迹需要了解宿主和寄生虫如何影响彼此的健康，尤其是在鸟类和昆虫社会寄生虫中。宿主选择是这一过程中重要的第一步，但指导宿主选择的原则尚未解决，特别是对于在具有多种特征的单个宿主中进行选择的专业寄生虫。为了确定寄生虫如何权衡各种宿主特征相对于其他宿主，我们需要确定它们的成本和收益。在这里，我使用制造奴隶的蚂蚁 Temnothorax americanus 来研究不同宿主特征组合的收益。我测量了对主机进行突袭的成本和收益，这些主机的价值（# 窝）、防御能力（​​# 工人）或它们的比率各不相同。此外，我调查了奴隶制造者的战斗力是否会影响哪些宿主特征组合是最佳的。当宿主包含更多的巢穴但工人更少时，奴隶制造者的表现最好。然而，最大化这个比率的能力受到天然巢穴中育雏和工人数量的相关性的限制，使得最佳宿主的大小适中。成本的衡量加强了这一结论，因为奴隶制造者的死亡率随着寄宿工人数量的增加而增加。此外，我发现较大的奴隶制造者群体在较大的宿主上有更高的回报，这表明它们的最佳宿主特征特征与较小的群体不同。这项研究表明，行使武力的社会寄生虫应该平衡宿主价值和防御能力之间的权衡，而不是仅最大化价值。此外，结果突出表明，宿主人口统计学在昆虫社会寄生虫军备竞赛中可能发挥比以前认为的更大的作用。意义声明 了解社会寄生虫共同进化军备竞赛结果的关键是确定有助于寄生虫和宿主成功的特征。宿主选择决定了哪些特征会经历选择，但与不同宿主特征相关的相对成本和收益，以及寄生虫应该如何衡量它们，仍不清楚。我测量了与宿主和寄生虫人口特征相关的寄生虫成功率，以区分每个使用奴隶制蚂蚁的相对影响。最大化宿主育雏与工人的比率会导致更高的回报，而人口最多的宿主是不可穿透的。这些发现提供了证据，证明通过武力攻击的社会寄生虫在宿主的价值和防御能力之间寻求平衡，即使这意味着放弃具有更高潜在价值的宿主。这反过来又强调了群体人口学和生活史是社会昆虫寄生虫共同进化军备竞赛中选择的重要宿主特征。