Deer topping: Preferences for large hosts increase mating efficiency and reduce Allee threshold densities in tick vectors

Finding a mate is a key challenge faced by sexually reproducing organisms and decreased mate-finding efficiency at low density often leads to an Allee effect. Parasites that mate on/in their adult-stage hosts depend on the host to accumulate potential mates for them, and thus mating efficiency should increase with parasite burden. A striking yet unexplained pattern across most disease-vectoring tick species is that, while juveniles are host generalists, adults specialize on larger host species. We argue that adult large-host preference is a strategy to maximize tick mating success. Using observed allometric scaling between mammalian body mass and movement distance, we demonstrate that tick mating efficiency increases dramatically with host size because larger hosts move more and tend to accumulate more potential mates. A novel consequence of this relationship is that the Allee density threshold below which the parasite population collapses depends on host size. Parameterizing our model for Ixodes scapularis suggests that large hosts are required for low-density tick populations to persist, but smaller hosts may suffice at higher densities. More generally, our hypothesis sheds new light on tick range expansion, suggests alternative management strategies, and provides novel insights on the role of large hosts in parasite lifecycles.