|Left: Female (top) and Male (bottom) Ixodes holocyclus, Right: Engorged female after feeding|
Photos by Alan R Walker from here and here
A group of researchers in Sydney conducted a study to look at the distribution of I. holocyclus on native and introduced mammals, in particular the long-nosed bandicoots and introduced black rats from areas around the Northern Beaches of Sydney, Australia. They captured these small mammals with cage traps, then briefly inspected them for ticks before letting them go free.
They found that on average, bandicoots had about three to four times as many I. holocyclus as rats, but most of those ticks were found on an unlucky few that were each infected with over 30 ticks. The ticks also distributed themselves different on the bodies of those animals. On the bandicoots, I. holocyclus spread themselves out pretty evenly across the host's body, clinging to the bandicoot's head, legs, belly, flanks, and there were even a few around the genital region. But on the rat they mostly hung around the head and neck region of the animal.
So even though I. holocyclus would happily drink blood from both bandicoots and rats, it seems they would much prefer a bandicoot. Compared with bandicoots which have co-evolved with I. holocyclus for a long time, rats are relatively recent interlopers. So while the ticks can infect them, rats are just not comparable to the native marsupials that they are more used to.
Ticks have specialised mouthparts for clinging to and feeding from their host, and even though I. holocyclus is a generalist that can drink blood from many different animals, its mouth part might not work equally well on them all. So whereas they can comfortably access all areas on the bandicoot, on a rat they stick to the sweet spot around the head to get their fill of blood.
This has important consequences when it comes to quantifying parasite abundance in a given environment. For example, if you are trying to find out about tick abundance in a given region, you might get vastly different results depending on which animals you decide to examine. Parasites are not evenly distributed across the landscape, across hosts, or even across different hosts' bodies. For a tick like I. holocyclus the host's body is an entire landscape in itself, and when in unfamiliar territory, it is better to stick to a well-trodden path.
Lydecker, H. W., Etheridge, B., Price, C., Banks, P. B., & Hochuli, D. F. (2019). Landscapes within landscapes: A parasite utilizes different ecological niches on the host landscapes of two host species. Acta Tropica 193: 60-65