Most species of hamsters will not interbreed and are not able to produce young. The only exception amongst the species kept as pets is the the Dwarf Winter White Russian and Dwarf Campbells Russian Hamsters which can be interbred and produce young.
Although the two species have many similarities they do not occupy the same ranges in the wild and so interbreeding of the two species does not occur in the wild. Therefore interbreeding the two in captivity is unnatural. Interbreeding can also present problems and can threaten the existence of both species in captivity.
Unfortunately many owners and pet shops simply have problems distinguishing between the two species and so much interbreeding is by accident. To distinguish between the two pure species and how to identify a hybrid, see the page on Dwarf Russian Hybrids.
Breeding a Winter White female to a Campbells male particularly can lead to the female being unable to give birth, resulting in the loss of both the babies and female. This is due to the Campbells being larger than the Winter White and the female having problems giving birth to the unusually large babies from the mating. Although the breeding of Campbells female to Winter White male does not present the same problems there are many other reasons why the two species should not be interbred.
The hybrids produced from breeding Campbells and Winter Whites together can suffer from low fertility or be infertile and even if fertile, the fertility in future generations often declines. Therefore it is often not possible to sustain an indefinite line of hybrids. If the breeding of hybrids were to become widespread this could threaten the survival of each pure species in captivity and ultimately threaten the existence of the Russian Hamster as a pet altogether. Once hybrids are produced it is not possible to use them to produce "pure Campbells" or "pure Winter Whites" by continued mating through generations to one of the species - hybrids can only ever produce more hybrids.
Although the Campbells and Winter Whites have the same number of chromosomes there are physical differences between the chromosomes in the two species. One such chromosome is the sex chromosome and this may well be the reason for the effect on fertility in hybrid offspring. Differences in the other chromosomes could possibly lead to other problems.
The reason often given to breed hybrids is the wider range of colours in the Campbells and much smaller range of colours in the Winter Whites. Often breeders mistakenly think that if they breed a coloured Campbells to a Winter White they will obtain a Winter White in a that colour. However, the offspring produced are hybrids in that colour and not a Winter White in that colour and all that will ever produced from those offspring are more hybrids, not Winter White even if future generations are bred repeatedly to Winter Whites. Therefore the breeding of hybrids with the goal of obtaining "new colour Winter Whites" is unachievable.
Although hamster enthusiasts no doubt would like to see a wider range of colours appear in the Winter White Russian Hamster it is not possible to hurry nature along in this desire by interbreeding the two species of Russian Hamster. If a new colour were to appear in the Winter White it will do so in its own good time naturally.
Inbreeding the two species of Russian Hamster can also have the effect of preventing or delaying colours from emerging in either of the pure species naturally due to decreased numbers of pure species being bred. Of course when a totally new colour emerges by natural mutation in a hybrid this colour does not exist in either pure species but only in the hybrid and so can never be recognised as a colour of either pure species unless and until it emerges by natural mutation again in either a pure line of Campbells or Winter Whites. New colours such as the Yellow/Pudding/Orange have already emerged in hybrid lines and it could have equally emerged in one or other of the pure species, had hybrid lines not existed.
With an increasing number of hybrids being bred and the tendency of decreasing fertility in hybrid lines of course any colours occurring in hybrid lines may not be sustainable indefinitely. If the breeding of hybrids became extensive this could reduce the number of pure Winter Whites and Campbells in circulation and being bred from, and this then would decrease the chances of natural mutations occurring in each pure species. Therefore in a mistaken effort to increase the colours available in Winter Whites by breeding hybrids breeders are actually decreasing the chances of new colours occurring and surviving in the pure Winter White species.