|MadSci Network: General Biology|
Septuplets may be a lot for a human, but it's nothing compared to what some mammals can produce. The record for the number of young produced at one time is 31, by the tailless tenrec (it's a little animal that looks like a hedgehog and lives in Madagascar). The next largest litter size is about 20, produced by some opossums (the Virginia opossum, which is the species common in the U.S., and the woolly opossum, a South American species that looks much like ours but has softer fur). Ermine have been know to have up to 18 kits in a single litter. All the rest of the top 10 (actually, I'll list 11) are rodents. Multimammate mice and rats can have as many as 15, lemmings and some species of voles can have up to 13, cotton rats may have 12 and hamsters and naked mole rats have litters of up to 11 pups.
A lot of the variation in litter size is related to variation in teat number. Primates typically only have one or two offspring at a time because that's all they can nurse. In fact, I feel like I'm cheating to say that the Virginia opossum can give birth to 20 offspring; it's true, but the mother only has 13 nipples, so no more than 13 of those pups can survive. Average litter size at weaning is actually around 7. Multimammate rats get their name from the unusually large number of nipples they have--12 to 24, depending on the species. However, this rule is not hard-and-fast.
Environmentally, availability of food and other resources has a lot to do with number of offspring produced. Many times litter size will be reported as "usually x, but as many as y in good years." And of course, if we're talking about a mammal that hasn't been well studied, we may not have a clue as to what the litter size is, or we may not have been around in a "good year" to see how large the litter might be.
By the way, I got most of the information to answer your question from Grzimek's Encyclopedia of Mammals. It's a great reference, and it has photos, range maps and more detailed information for all the animals I've mentioned.
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