What Parasites Can Tell Us About Pika Evolution

The Bering Land Bridge, a temporary Pleistocene corridor between Siberia and North America, is well known to mammalogists. It was responsible for the dispersal of some mammalian species from Asia into North America and others from North America into Asia (including ancient horses and camelids). The traditional view is that species entering North America from Asia eventually made their way southward to colonize lower latitudes. However, there is little information about the dispersal of small mammals across Beringia.

One small mammal might shed light on such dispersal events, is the pika. These small lagomorphs (
Ochotona spp.) colonized the Nearctic via Beringia. Today there are two North American species; the collared pika (Ochotona collaris) occurs in Alaska and nearby Canadian Provinces, and the American pika (Ochotona princeps) inhabits the Intermountain West of North America (Figure 1).

pika
Figure 1. An American pika (Ochotona princeps). (From Peter Stevens/Flickr)

Mammalogists assumed that the more northern collard pika is directly ancestral to the original Beringian colonizers, while the more southern American pika (
O. princeps) evolved later as collared pikas expanded southward along the Coastal and Rocky Mountains. If true, this would be another example of the typical north-to-south dispersal into North America. However, it is also possible that the ancestors of the American pika moved northward along the mountain ranges to establish a new species, the collard pika, in Alaska and northwestern Canada (Figure 2).

pikamap
Figure 2. Pika distributions and dispersal predictions of North American pikas. (a) grey indicates distributions for Ochotona princeps (filled circles) and
O. collaris (filled stars). Phylogenetic trees below show predicted phylogenetic hypotheses for north-to-south dispersal, equivocal, and south-to-north dispersal patterns. (From Galbreath and Hoberg, 2011)

One way to test these completing hypotheses is to use pika parasites. This approach is called a “host–parasite comparative phylogeographical (HPCP) approach. “ It allows scientists to evaluate competing biogeographic hypotheses for groups of host-specific parasites shared by the two North American pika species. Using molecular phylogenetic methods on pika parasites, Galbreath and Hoberg (2011) were able to identify centers of origin for the living North American pikas.

As the authors state,” Because the parasites have obligate associations with pikas, dispersal and colonization must be mediated by the hosts.” As predicted, all parasite lineages originated in the Old World and later crossed Beringia into the New World prior to the split between the two New World pika species. However, once in the New World, three of the five parasite lineages shared by both pika species support a south-to north dispersal hypothesis. Thus it appears that there was an initial colonization of North America via the Bering Land Bridge, the spread of ancestral pikas southward across the Intermountain West, the later extirpation of northern pika populations, and the subsequent recolonization of the north by a southern North American pika source population, which later speciated into the collard pika.


References

Galbreath, K., & Hoberg, E. (2011). Return to Beringia: parasites reveal cryptic biogeographic history of North American pikas Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.0482