Some books change the way you see the world. Parasite Rex, by Carl Zimmer, was one of those for me—an account of life and evolution from a parasite’s perspective. (Marlene Zuk’s more recent Riddled with Life is also recommended.) One of the things I took away from it was how finely tuned host-parasite relationships can be. Many parasites, like the brown pelican pouch louse, are found only on a specific body part of a single host species.
Because of that, the evolutionary history of parasites tends to mirror that of their hosts. A few years ago, in the process of researching an article for the Academy of Science’s magazine California Wild (just before it went extinct), I learned that pocket gophers and their lice were considered a model system for host-parasite coevolution. When a lineage of gophers split, so did that of its lice. You wind up with each species of gopher having its unique species of louse, with mouthparts finely calibrated to the thickness of the rodent’s hairs.
That was supposed to be how it worked in most host-parasite systems, with the occasional lateral jump (for example, when predators acquired parasites from their prey). But beaver mites don’t seem to play by those rules.
Beavers, like the rest of us, are ambulatory ecosystems. They’re inhabited by mites–tiny arachnids, related to ticks—of the genus Schizocarpus, which are beaver specialists. The mites live in the beaver’s thick fur, feeding on skin and hair secretions, and are preyed upon by a unique species of beetle.
With one possible exception, Eurasian and North American beavers harbor different species of Schizocarpus mites. Unlike the pocket gophers and their lice, there’s not just one mite species for each of the two beaver species. Seventeen different mite species have been identified from North American beavers, and another 25 from Eurasian beavers. A single beaver may host multiple species of mite.
Different mite species seem to have laid claim to different parts of the host. Four species inhabit the beaver’s head, shoulders, and front legs. Another six are found only on the back and the outside of the hind legs; six more only on the belly and inside of the hind legs. There’s also a species that can turn up anywhere on the animal but is relatively rare. No single beaver would have the full set of, say, head mites, but some have more than one.
You might think these clusters of specialized parasite species had adapted to some physical feature of the host’s head, or back, or belly, as the gopher lice adapted to the thickness of their hosts’ hair. That microhabitat notion was the first hypothesis of the scientists who described the mites.
That would jibe with the idea that most new species evolve in isolation from their nearest relatives–classically, on separate islands in an archipelago, like the Galapagos or Hawai’i. While natural barriers keep them apart, each population goes its own way, developing its unique suite of physical traits and its own genetic signature, diverging so far that the two populations couldn’t interbreed if the barriers were removed.
But that’s apparently not how it happened. There are no obvious barriers between microhabitats. And the only thing that distinguishes one mite species from another is the size and position of the sucker plates of the adult males, which play a role in courtship. The mites don’t differ in how they make their living. (As far as I know, no one has tried to work out their genetics. It’s anyone’s guess as to whether all the head-and-shoulder mites, for instance, are closely related.)
In a 1988 paper, two acarologists—mite researchers—named Alex Fain and John O. Whitaker, Jr. proposed that the physical environments on the different parts of the beaver couldn’t have functioned as isolation mechanisms. They continued: “About the only way we could see speciation of the mites occurring within beaver populations is if a genetic change occurred (macromutation, if you will) in which the changes were large enough to immediately serve as secondary as well as primary isolation mechanisms.” From then on, sexual selection would drive divergences in the sucker plates to ensure that the mites found genetically compatible mates.
Natura non facit saltum, Darwin famously observed: Nature does not make leaps. But if Fain and Whitaker were right, beaver mite evolution involved multiple leaps. Although beaver mites are probably a low priority for research funding, I’d like to see someone try to sort this thing out—if it’s not too late for that.
At various times wildlife agencies have gone in for stocking vacant beaver habitat with animals from different geographic areas—transplanting Michigan beavers to trapped-out Massachusetts, or Oregon and Idaho beavers to the beaverless Sierra. Both beaver and mite stocks may have become hopelessly entangled by now.