Ticked Off

Predators are not the only creatures to feed off other animals. Certainly, if you're an antelope, the obvious things to fear are lions, crocodiles, or whatever else might seek to ambush and eat you. But virtually all animals, and certainly all mammals, also suffer from parasites, a rather more insidious threat.

Very broadly speaking, parasites employ one of two tactics. Endoparasites live in inside an animal, often in the gut if they're any larger than single-celled organisms, although they can infect other organs. These are typically parasitic "worms" of one kind or another, and fighting them off is more a matter of an animal's immune system than of any behavioural traits (at least, once it's already infected). Ectoparasites, on the other hand, live on the outside of an animal, clinging to the skin and perhaps hiding among the fur. Fleas are an obvious example of this sort of parasite and grooming can be at least a partial defence against them.

Ticks are another common ectoparasite, feeding on a wide range of mammals, and often transmitting disease in the process. Ticks are arachnids, but it's worth pointing out that "arachnid" is quite a broad term, with the same taxonomic rank as "mammal" itself. (Such ranks are, of course, arbitrary, and not directly equivalent across different types of animal, but it gives a rough indication of how similar - or not - we consider the members of a group to be from a human perspective). In fact, ticks are not especially closely related to spiders or scorpions, belonging to a different branch of the arachnid family tree that otherwise seems to consist largely of mites - although the exact relationships remain contentious.

The winter tick (Dermacentor albipictus) is a typical example. A close relative of the American dog tick, this is found across much of North America, where it feeds on many of the local species of deer. These hatch from eggs laid on the ground in the spring, and spend the summer as essentially inactive six-legged larvae before climbing up tall vegetation in the autumn and detaching onto passing deer. About ten days later they moult into their eight-legged form and, after feeding on sufficient blood and then enjoying another multi-month rest, transform into their final adult stage some time around January. The sexually active females gorge themselves on blood again and, after mating, drop off the deer to die and lay their eggs, starting the cycle again.

Clearly, this isn't any fun for the deer, but how harmful is it really? Most studies of the effects of winter tick infestation have been performed on moose, one of their preferred prey animals, and a particularly visible and distinctive deer species. The main effect that the ticks have on moose is to simply make them unwell, presumably as a side-effect of the resulting anaemia requiring the animal to create more blood to replace that which has been lost. This results in calves growing more slowly and in less fat reserves being laid down, which can be a particular problem for pregnant mothers. 

An additional effect may be hair loss from the affected body parts, although it is unclear whether this is due to the ticks themselves, or the deer trying to rub or scratch them off using their hooves or available tree trunks or the like. Such hair loss might lead to the animals finding it harder to keep warm, but the effort of grooming to try and remove the ticks is just a nuisance in itself. A deer can hardly spend all its time on such activities when it also needs to be feeding, resting, and, for that matter, looking out for full-sized predators.

If this is so, we might expect deer to modify their behaviour depending on the number of ticks they are infested with and the urgency of whatever tasks they might need to perform. This does seem to be true of moose, but it's less clear whether it's also true for the other species that the ticks feed on. To try and evaluate this, a recent study looked at the elk (Cervus canadensis) population living in the 3,945 ha (15 square mile) Ya Ha Tinda ranch just outside the Banff National Park in Alberta, Canada. This involved counting the number of ticks on captured elk before fitting them with radio collars, releasing them, and seeing what they did over the following few months.

As it turned out, the degree of infestation with ticks made remarkably little difference to the elk in the study. In part, this seems to be due to the fact that the elk mostly scratched themselves when they would otherwise have been resting so that it wasn't cutting into the time needed to find or digest food... although it might well have been a bit exhausting, cutting into their rest. On the other hand, it did apparently cut into the time spend watching out for predators (wolves, mainly, in this part of the world). But that wasn't by very much, and the fact that elk, unlike moose, travel in herds where at least one individual is likely to be watching out for trouble at any given time may mean that this isn't really much of a risk for them.

Since they also didn't seem to lose much hair, and were, in general, less bothered by the ticks than moose normally are. Moose can lose a significant amount of hair if they are infested with ticks, but that may be partly because the ticks really seem to like them, and are found in much greater numbers on moose than they are on elk. A moose is, of course, larger than an elk, so we'd expect a typical infestation to involve more ticks per animal, but not to the extent that we actually see it - it has been estimated that there are an average of 1,200 ticks per elk, but a whopping 37,000 per moose. 

This isn't because the ticks like the smell of moose (or whatever other signal they might be using) and are more likely to climb onto them in the autumn. Back in the 1990s, experimenters took moose, elk, and the two local species of smaller deer (white-tailed and mule) and added approximately one tick larva to them per square centimetre of skin. When they came back to the animals in the spring, the moose had far more engorged and sexually mature female ticks on them than did the other animals and they were, on average, much plumper.

So there's something about moose that helps the ticks to grow and mature on them... or, perhaps more likely, there's something about the elk (and other deer) that doesn't. They can survive, certainly, and it's likely that they cause discomfort and general irritation, but the elk have evolved some kind of resistance to them that makes it harder for the ticks to reach maturity and to drink the amount of blood that they'd really like.

Why haven't the moose done the same, if all other three affected deer species have? In the case of the smaller species, the answer is probably that they've simply had longer. White-tailed and mule deer have lived in North America ever since they first evolved, and it's worth noting that the winter tick is unique to that continent. Moose, on the other hand, first arrived in the Americas less than 15,000 years ago, crossing over from Siberia when the Ice Ages lowered the sea level. They simply haven't had long enough for their immune systems to evolve the sort of resistance that smaller local deer managed to achieve.

It's less clear that the same argument would work for elk, since they may well have entered the continent at around the same time, also being descended from an original Siberian population which, like that of Eurasian moose, still survives today. Perhaps they have been luckier, or perhaps the different habitat requirements and/or herding behaviour of the two species mean that they first encountered the ticks at a later date. 

Either way, moose may have it bad, but elk seem to have found a biological way to discourage the unwanted parasites.

[Photo by Bernd Thaller, from Wikimedia Commons.]