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I recently got into a debate with the family about why do Greys and Red Squirrels not hybridise.
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I've never come across any records of hybrids between these two species, although -- as aeshna5 said -- the fact that one's American and the other is European in origin doesn't preclude hybridization. They certainly have the same number of chromosomes (2n = 40) and, although this is not the only necessary requisite, this does suggest that any hybrids could be fertile. Personally, I'm inclined to think that the reason they don't interbreed is probably related to the observation that the two species generally do not occur sympatrically (i.e. their ranges tend not to overlap, producing a form of geographical isolation). So, the opportunity to interbreed probably doesn't raise itself very often, if at all. Of course, even if their ranges did overlap, merely being in the the same place at the same time doesn't mean that you'd necessarily
expect hybridization.
Cheers,
Marc.