Birth control in Syrian hamsters is no problem. As adults must be housed singly, every litter is a planned event. Dwarf hamsters, however, are housed in pairs, trios or colonies and so births may occur at any time. A number of steps can be taken to encourage contraception. A varied and nutritious diet is the most important aid to hamster health and a healthy hamster is far more likely to breed successfully. The easiest good diet to provide is a basic hamster mix supplemented with fresh apple (use the core when you have eaten one), greens, a sprinkling of wheat germ and a drop of cod liver oil on the food two or three times per week. You can also add a little cooked meat and other leftovers, and perhaps dog meal for variety.
Step two is to keep the cage clean to minimise the risk of disease. As a minimum, clean the "wet corner" every week, removing any damp or perishable foods. Every two to three weeks, depending on circumstances, give the cage a thorough clean out and disinfect. I have found that dwarf hamsters are not upset by this, no matter what stage of pregnancy or age of litter, so long as you are gentle and give plenty of new bedding and a supply of fresh food to keep them occupied.
Step three is to keep the bedding and sawdust plentiful and dry. Nothing is worse for hamster health than damp, especially when it is cold. The problem I am most asked about, however, is how to stop them breeding. Campbell's Russian Dwarf hamsters can produce three litters in the space of five weeks. The female is "on heat" soon after a litter is born, so litter can follow litter in quick succession.
One way to prevent overbreeding is to remove the male once pregnancy is obvious. It may, however, be difficult to reintroduce the male at a later date, although methods I have mentioned in previous articles should be successful. Partitioning the cage with netting can be a better option, allowing contact between the pair without danger of mating taking place.
In the wild, breeding is less likely to occur in Winter, a time when food is less plentiful and less varied. I have found that reducing the variety of foodstuffs given does reduce the number of litters produced. Feeding only one food, dog meal (e.g. Webbox, Morrison's or Safeway's dog meal) can prevent births altogether. Other single foods may also work, but I stumbled upon the use of dog meal in this way quite by accident.
I use dog meal to supplement my hamster mix as a matter of course, partly because it is large enough to be retained by the netting in my hamster cage food hoppers. This helps to keep a proportion of the food clean and dry and out of the sawdust. It also gives the hamsters something to do as they have to nibble at it to get it through the netting. Also when I go away for a few days, I fill up the food hoppers with dog meal and know that they won't run out of clean, dry food.
After being away for a week and having to continue feeding dog meal until I could get a fresh 20kg bag of hamster mix, I noticed a very sharp fall-off in hamster births. The food, I surmised, was the culprit, and I have since successfully used diet in this way, as a form of hamster contraception. You may find it helpful to do the same. I would not recommend continued use of such a restricted, monotonous diet. It could, in the long run, lead to problems associated with vitamin deficiency e.g. fur loss. After all, variety is the spice of life!