[mbaldw]

If red and grey ranges don't overlap, I'd be interested to know how the grey passes the parapoxy virus on to the reds.

You're right, I should've worded my response more carefully - sorry about that! Just to clarify, I should've said that their home ranges generally don't overlap significantly, even though the species' distributions might. The populations may be directly adjacent and there may well be some range overlap at the periphery, especially where mutually accessible food is prolific. Nonetheless, I think it's fair to say that for the most part, the two species typically do not intermingle. Assuming that's correct, and given that squirrels tend to focus most of their activity within their core territory, even where there are neighbouring populations of Greys and Reds, the two species probably don't come into direct contact -- and hence don't have the opportunity to mate -- with one another very often (if at all these days, thanks to human intervention). Hence it seems to me that this is a possible reason for the lack of hybridization.

However, as you rightly point out, the fact that the two probably don't bump into each other with any degree of regularity doesn't mean that the two species would never come into contact with each other, or -- more likely, in my opinion -- use the same feeding stations. Additionally, most populations have their share of itinerant individuals, which may pass through the territory/home range of individuals of both species - if one such individual were infectious, this provides opportunity for the virus to spread. Anyway, itinerants aside, Greys have been observed to pilfer Red caches (suggesting they do sometimes enter into Red territories) and, although I've not heard of any recently, there are reports of Red and Greys feeding together. So, with this in mind, it makes sense that one of the leading theories of squirrel poxvirus transmission is that the two species may come into contact with the same contaminated feeders. Given that the virus is very resistant to dessication, it seems logical that the virus could also be passed to other hard surfaces that the two might also come into contact with (e.g. branches where stripping or scent-marking has taken place, or pilfered caches) and this may play a role in its epidemiology. As you point out, there are other species (e.g. some ruminants) that can carry the virus and they may play a role in aiding the spread of the disease within and between populations.

Cheers,
Marc.