"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

April 8, 2019

Ixodes holocyclus

There are 14000 known living species of blood-sucking animals, but while drinking blood has become a staple in many different lineages of animals,  some of nature's vampire can be quite picky about which animals they feed on. Even for those that drink from a variety of different animals, they might have preference for certain bouquets of blood over others.
Left: Female (top) and Male (bottom) Ixodes holocyclus, Right: Engorged female after feeding
Photos by Alan R Walker from here and here
Ixodes holocyclus is a species of hard tick native to Australia. It can infect a wide range of different animals including various Australian native marsupials, bird and reptiles. But over the last two hundred years, many other species of mammals have been introduced to the Australian continent, and I. holocyclus has eagerly taken to those new hosts as well. But while I. holocyclus is capable of drinking from both Australian native marsupials and the more recently introduced placental mammals, that does not mean that they are equivalents from the tick's perspective.

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